MWENGE UNIVERSITY COLLEGE OF EDUCATION

(MWUCE)

CYTOLOGY AND CELL PHYSIOLOGY

BIO 103

Unit Module Instructor: Martin Muthee

2014/ 2015

CYTOLOGY AND CELL PHYSIOLOGY

What is cytology?

- Cytology is the branch of biology that deals with the formation, structure, function,

multiplication and life history of cells. Cells are complex systems of molecules capable of

carrying out all the interactions of life including growth, reproduction, response to outside

stimuli and movement.

- A cell is an organized independent mass of protoplasm (nucleus and cytoplasm) which makes

the basic primary structure of an organism.

What is cell physiology?

The term ―physiology‖ refers to all the normal functions that take place in a living organism.

Physiology is therefore the study of how organisms and their parts work or function. There are

many branches of physiology, e.g. cell physiology which is the scope of this course, plant

physiology, invertebrate physiology, neurophysiology (working of nervous system) cardiac

physiology (working of the heart) etc.

Cell physiology: Is the biological study of the cell‘s mechanism and interaction in its

environment. Cellular physiology include the study of the mechanism division and self-regulation,

transport of nutrients, ions, and water into and out of the cell, as well as how cells communicate with

each other through signaling pathways, or respond to external cues.

It is the ―inner life of a cell‖.

- Absorption of water by roots, production of food in the leaves, and growth of shoots towards

light are examples of plant physiology

- The heterotrophic metabolism of food derived from plants and animals and the use of

movement to obtain nutrients are characteristics of animal physiology.

Some of the cell physiological processes

Movement of proteins - the movement of proteins about the cell for use in structure and

enzymatic processes

Active and passive transport - the processes facilitating the movement of molecules into

and out of the cell

Autophagy – the process whereby cells ―eat‖ their own internal components or microbial

invaders

Adhesion - the chemical processes whereby cells and other tissues are held together

Cell division - a eukaryotic cell process resulting in formation of daughter cells. There are

two major types; mitosis and meiosis.

Cell movement – Chemotaxis (is chemically prompted taxis, in which somatic cells,

bacteria, and other single-cell or multicellular organisms direct their movements according

to certain chemicals in their environment.), contraction, use of cilia and flagella.

Cell signaling - regulation of cell behavior by signals from outside, such as the use of

hormones and neurotransmitters.

Metabolism- Glycolysis, respiration and photosynthesis- processes whereby energy is stored

and/or liberated for use by cell.

CELL CONCEPT

One of the most important concepts in biology is that a cell is the basic structural and

functional unit of living organism. All organisms are composed of structural and functional units

of life called ‗cells‘. The body of some organisms like bacteria, protozoan and some algae is

made up of a single cell while the body of fungi, plants and animals are composed of many cells.

Human body is built of about one trillion cells. Cells vary in size and structure as they are

specialized to perform different functions. But the basic components of the cell are common to

all cells.

The cell theory

In 1838 M.J. Schleiden a Belgian Botanist and T. Schwann a German Zoologist formulated the

―cell theory.‖ Also known as the cell doctrine. The cell theory states that:

All organisms are composed of cells.

Cell is the structural and functional unit of life, and

Cells arise from pre-existing cells.

The cell theory, in its modern form, includes the following additional three principles:

Cells contain hereditary information which is passed from cell to cell during cell division

All cells are basically the same in chemical composition

All energy flow (metabolism and biochemistry) of life occur within cells

THE CELL

A cell may be defined as a unit of protoplasm bounded by a plasma membrane, cell membrane or

plasmalemma. Protoplasm is the life giving substance and includes the cytoplasm and a nucleus.

The cytoplasm has in it organelles such as ribosomes, mitochondria, golgi bodies plastids,

lysosomes and endoplasmic reticulum. Plant cells have in their cytoplasm large vacuoles

containing non-living inclusions like crystals, pigments etc. The bacteria have neither organelles

nor a well formed nucleus. But every cell has three major components:

Plasma membrane

cytoplasm

DNA (naked in bacteria and covered by a membrane in all other organisms)

Why Aren’t Cells Larger?

Cells are, with a few notable exceptions small with length measured in micrometers (μm, where

1000 μm =1mm) read about the discovery of cell and the people involved in pg. 4. Most cells are

not large for practical reasons. The most important of these is communication. The different

regions of a cell need to communicate with one another in order for the cell as a whole to

function effectively. Proteins and organelles are being synthesized, and materials are continually

entering and leaving the cell. All of these processes involve the diffusion of substances at some

point, and the larger a cell is, the longer it takes for substances to diffuse from the plasma

membrane to the center of the cell. For this reason, an organism made up of many relatively

small cells has an advantage over one composed of fewer, larger cells.

The advantage of small cell size is readily visualized in terms of the surface area-to-volume

ratio. As a cell‘s size increases, its volume increases much more rapidly than its surface area.

For a spherical cell, the increase in surface area is equal to the square of the increase in diameter,

while the increase in volume is equal to the cube of the increase in diameter. Thus, if two cells

differ by a factor of 10 cm in diameter, the larger cell will have 102, or 100 times, the surface

area, but 103, or 1000 times, the volume, of the smaller cell (figure 5.4).

Surface area-to-volume ratio: As a cell gets larger, its volume increases at a faster rate than its surface

area. If the cell radius increases by10 times, the surface area increases by 100 times, but the volume

increases by 1000 times. A cell’s surface area must be large enough to meet the needs of its volume.

A cell‘s surface provides its only opportunity for interaction with the environment, as all

substances enter and exit a cell via the plasma membrane. This membrane plays a key role in

controlling cell function, and because small cells have more surface area per unit of volume than

large ones, the control is more effective when cells are relatively small. Although most cells are

small, some cells are nonetheless quite large and have apparently overcome the surface area-to-

volume problem by one or more adaptive mechanisms. For example, some cells have more than

one nucleus, allowing genetic information to be spread around a large cell. Also, some large cells

actively move material around their cytoplasm so that diffusion is not a limiting factor. Lastly,

some large cells, like your own neurons, are long and skinny so that any given point in the

cytoplasm is close to the plasma membrane, and thus diffusion between the inside and outside of

the cell can still be rapid.

Slowing down metabolism: An e.g. in unfertilized chicken egg is another mechanism of

overcoming this problem.

TWO BASIC TYPES OF CELLS

Cytologists recognize two basic types of cells; prokaryotic (pro = early/primitive, karyon=

nucleus) and eukaryotic (eu = true, karyon = nucleus) cells. Prokaryotic cells consist of a single

closed compartment that is surrounded by the plasma membrane, lacks a true nucleus; their

genetic material lies in a free region known as a nucleoid. They have a relatively simple internal

organization. All prokaryotes have cells of this type. Bacteria, the most numerous prokaryotes,

are single-celled organisms.

Their differences have been tabulated below in on pg. 13. Organisms which do not possess a

well formed nucleus and membrane-bound organelles are called prokaryote such as the bacteria.

All others organisms possess a well defined nucleus, covered by a nuclear membrane and

membrane-bound organelles. They are called eukaryotes.

PROKARYOTIC CELLS (Bacteria Are Simple Cells)

Prokaryotes, the bacteria, are the simplest organisms. Prokaryotic cells are small, consisting of

cytoplasm surrounded by a plasma membrane and encased within a rigid cell wall, with no

distinct interior compartments (figure below). A prokaryotic cell is like a one room cabin in

which eating, sleeping, and watching TV all occur in the same room. Bacteria are very important

in the economy of living organisms. They harvest light in photosynthesis, break down dead

organisms and recycle their components, cause disease, and are involved in many important

industrial processes.

Strong cell wall

Most bacteria are encased by a strong cell wall composed of peptidoglycan,(also known as

murein) which consists of a carbohydrate matrix (polymers of sugars) that is cross-linked by

short polypeptide units. No eukaryotes possess cell walls with this type of chemical composition.

With a few exceptions like TB and leprosy-causing bacteria, all bacteria may be classified into

two types based on differences in their cell walls detected by the Gram staining procedure. The

name refers to the Danish microbiologist Hans Christian Gram, who developed the procedure to

detect the presence of certain disease-causing bacteria. Gram-positive bacteria have a thick,

single-layered cell wall that retains a violet dye from the Gram stain procedure, causing the

stained cells to appear purple/violet under a microscope.

More complex cell walls have evolved in other groups of bacteria. In them, the wall is

multilayered and does not retain the purple dye after Gram staining; such bacteria exhibit the

background red dye and are characterized as gram negative. The susceptibility of bacteria to

antibiotics often depends on the structure of their cell walls. Penicillin and vancomycin, for

example, interfere with the ability of bacteria to cross-link the peptide units that hold the

carbohydrate chains of the wall together. Like removing all the nails from a wooden house, this

destroys the integrity of the matrix, which can no longer prevent water from rushing in, swelling

the cell to bursting. Cell walls protect the cell, maintain its shape, and prevent excessive uptake

of water. Plants (cellulose), fungi (chitin), and most protists also have cell walls of a different

chemical structure.

Rotating Flagella

Some bacteria use a flagellum (plural, flagella) to move. Flagella are long, threadlike structures

protruding from the surface of a cell that are used in locomotion and feeding. Bacterial flagella

are protein fibers that extend out from a bacterial cell. There may be one or more per cell, or

none, depending on the species. Bacteria can swim at speeds up to 20 cell diameters per second

by rotating their flagella like screws (figure below). A ―motor‖ unique to bacteria that is

embedded within their cell walls and membranes powers the rotation. Only a few eukaryotic

cells have structures that truly rotate.

Simple Interior Organization

If you were to look at an electron micrograph of a bacterial cell, you would be struck by the

cell‘s simple organization. There are few, if any, internal compartments, and while they contain

simple structures like ribosomes, most have no membrane-bounded organelles, the kinds so

characteristic of eukaryotic cells. Nor do bacteria have a true nucleus. The entire cytoplasm of a

bacterial cell is one unit with no internal support structure. Consequently the strength of the cell

comes primarily from its rigid wall. The plasma membrane of a bacterial cell carries out some of

the functions organelles perform in eukaryotic cells. When a bacterial cell divides, for example,

the bacterial chromosome, a simple circle of DNA, replicates before the cell divides. The two

DNA molecules that result from the replication attach to the plasma membrane at different

points, ensuring that each daughter cell will contain one of the identical units of DNA. Moreover,

some photosynthetic bacteria, such as cyanobacteria and Prochloron have an extensively folded

plasma membrane, with the folds extending into the cell‘s interior. These membrane folds

contain the bacterial pigments connected with photosynthesis. Because a bacterial cell contains

no membrane-bounded organelles, the DNA, enzymes, and other cytoplasmic constituents have

access to all parts of the cell. Reactions are not compartmentalized as they are in eukaryotic cells,

and the whole bacterium operates as a single unit.

EUKARYOTIC CELLS

(have complex interiors)

Eukaryotic cells are far more complex than prokaryotic cells. Eukaryotic cells contain

membrane-bounded organelles that carry out specialized functions. The hallmark of the

eukaryotic cell is compartmentalization. The interiors of eukaryotic cells contain numerous

organelles, membrane-bounded structures that close off compartments within which multiple

biochemical processes can proceed simultaneously and independently.

Plant cells often have a large membrane-bounded sac called a central vacuole, which stores

proteins, pigments, and waste materials. Both plant and animal cells contain vesicles, smaller

sacs that store and transport a variety of materials. Inside the nucleus, the DNA is wound tightly

around proteins and packaged into compact units called chromosomes. All eukaryotic cells are

supported by an internal protein scaffold, the cytoskeleton. While the cells of animals and some

protists lack cell walls, the cells of fungi, plants, and many protists have strong cell walls

composed of cellulose or chitin fibers embedded in a matrix of other polysaccharides and

proteins. This composition is very different from the peptidoglycan that makes up bacterial cell

walls.

The differences between prokaryotic and eukaryotic cell

CHARACTERISTIC PROKARYOTIC CELL EUKARYOTIC CELL

Nucleus Distinct with well formed nuclear

membrane

Absent , it is in the form of

a nuclear zone called a

nucleoid nuclear membrane

absent

DNA Circular DNA in cytoplasm

(nucleoid)

Linear DNA bounded by a

nuclear envelop

Ribosomes 80s 70s

Organelles Few organelles in cytoplasm

No membrane-bound

organelles such chloroplast and

mitochondria.

Many organelles in

cytoplasm

Membrane-bound

organelles present

Flagella Lack 9-2 fibrillar structure if

present.

Has 9-2 fibrillar structure if

present

Cytoplasmic streaming Does not occur May occur

Cell wall Made of peptidoglycan When present made up of

other component but not

peptidoglycan

Cell division Occurs by binary fission Occurs by mitosis or

meiosis

Cytoplasmic streaming

(Protoplasmic streaming)

Absent May occur

Cytoskeleton Absent Present

Nucleolus Absent Present

Photosynthetic apparatus May contain chlorophyll but not

in chloroplast

Chlorophyll when present

contained in chloroplast

Organisms Bacteria, blue-green algae,

cyanobacteria.

Protists, fungi, plants and

animals

Svedberg unit: When the cell is fractionated or broken down into its components by rotating in

an ultracentrifuge at different speeds the ribosomes of eukaryotic and prokaryotic sediment

(settle down) at different speeds. The coefficient of sedimentation is represented in Svedberg unit

and depicted as S.

Let’s now examine the structure and function of the internal components of eukaryotic cells

in more detail.

CELL MEMBRANE.

Each cell has a limiting boundary, the cell membrane, plasma membrane or plasmalemma. It is a

living membrane, outermost in animal cells but next to cell wall in plant cells. It is flexible and

can fold in (as in food vacuoles of Amoeba) or fold out (as in the formation of pseudopodia of

Amoeba). The plasma membrane is made of proteins and lipids and several models were

proposed regarding the arrangement of proteins and lipids. The lipid layer that forms the

foundation of a cell membrane is composed of molecules called phospholipids. (Next chapter).

THE NUCLEUS:

(Information center for the cell)

Nucleus is the largest and most easily seen organelle within a eukaryotic cell that is not dividing.

First described by the English botanist Robert Brown in 1831, nuclei are roughly spherical in

shape and, in animal cells; they are typically located in the central region of the cell. In some

cells, a network of fine cytoplasm filaments seems to cradle the nucleus in this position. The

nucleus is the repository of the genetic information that directs all of the activities of a living

cell. Most eukaryotic cells possess a single nucleus (uninucleate), although the cells of fungi and

some other groups may have several to many nuclei (multinucleate) Mammalian erythrocytes

(red blood cells) lose their nuclei when they mature. Many nuclei exhibit a dark staining zone

called the nucleolus, which is a region where intensive synthesis of ribosomal RNA is taking

place. A nucleus contain the following; chromosomes, nucleoli, nucleoplasm, (The fluid found

inside the nucleus of eukaryotic cells made up of water and other dissolved substances) and

nuclear membrane.

Functions:

Maintains the cell in a working order.

Co-ordinates the activities of organelles.

Takes care of repair work. Participates directly in cell division to produce genetically

identical daughter cells, this division is called mitosis.

Participates in production of gametes through another type of cell division called meiosis.

The Nuclear Envelope: Getting In and Out

The surface of the nucleus is bounded by two phospholipid bilayer membranes, which together

make up the nuclear envelope. The outer membrane of the nuclear envelope is continuous with

the cytoplasm‘s interior membrane system, called the endoplasmic reticulum. Scattered over the

surface of the nuclear envelope, like craters on the moon, are shallow depressions called nuclear

pores. These pores form 50 to 80 nanometers apart at locations where the two membrane layers

of the nuclear envelope pinch together. Rather than being empty, nuclear pores are filled with

proteins that act as molecular channels, permitting certain molecules to pass into and out of the

nucleus. Passage is restricted primarily to two kinds of molecules: (1) proteins moving into the

nucleus to be incorporated into nuclear structures or to catalyze nuclear activities; and (2) RNA

and protein-RNA complexes formed in the nucleus and exported to the cytoplasm.

THE ENDOPLASMIC RETICULUM:

(Compartmentalizing the cell)

The interior of a eukaryotic cell is packed with membranes. So thin that they are invisible under

the low resolving power of light microscopes, this endomembrane system fills the cell, dividing

it into compartments, channeling the passage of molecules through the interior of the cell, and

providing surfaces for the synthesis of lipids and some proteins. The presence of these

membranes in eukaryotic cells constitutes one of the most fundamental distinctions between

eukaryotes and prokaryotes.

The largest of the internal membranes is called the endoplasmic reticulum (ER). The term

endoplasmic means ―within the cytoplasm,‖ and the term reticulum is Latin for ―a little net.‖

Like the plasma membrane, the ER is composed of a lipid bilayer embedded with proteins. It

weaves in sheets through the interior of the cell, creating a series of channels between its folds.

Of the many compartments in eukaryotic cells, the two largest are the inner region of the ER,

called the cisternal space, and the region exterior to it, the cytosol.

Rough ER: Manufacturing Proteins for Export

The ER surface regions that are devoted to protein synthesis are heavily studded with ribosomes,

large molecular aggregates of protein and ribonucleic acid (RNA) that translate RNA copies of

genes into protein (we will examine ribosomes in detail later in this chapter). Through the

electron microscope, these ribosome-rich regions of the ER appear pebbly, like the surface of

sandpaper, and they are therefore called rough ER. The proteins synthesized on the surface of

the rough ER are destined to be exported from the cell. Proteins to be exported contain special

amino acid sequences called signal sequences. As a new protein is made by a free ribosome (one

not attached to a membrane), the signal sequence of the growing polypeptide attaches to a

recognition factor that carries the ribosome and its partially completed protein to a ―docking site‖

on the surface of the ER. As the protein is assembled it passes through the ER membrane into the

interior ER compartment, the cisternal space, from which it is transported by vesicles to the

Golgi apparatus. The protein then travels within vesicles to the inner surface of the plasma

membrane, where it is released to the outside of the cell.

Smooth ER: Organizing Internal Activities

Regions of the ER with relatively few bound ribosomes are referred to as smooth ER. The

membranes of the smooth ER contain many embedded enzymes, most of them active only when

associated with a membrane. Enzymes anchored within the ER, for example, catalyze the

synthesis of a variety of carbohydrates and lipids. In cells that carry out extensive lipid synthesis,

such as those in the testes, intestine, and brain, smooth ER is particularly abundant. In the liver,

the enzymes of the smooth ER are involved in the detoxification of drugs including

amphetamines, morphine, codeine, and phenobarbital. Some vesicles form at the plasma

membrane by budding inward, a process called endocytosis. Some then move into the cytoplasm

and fuse with the smooth endoplasmic reticulum. Others form secondary lysosomes or other

interior vesicles.

THE GOLGI APPARATUS:

(Delivery System of the Cell)

At various locations within the endomembrane system, flattened stacks of membranes called

Golgi bodies occur, often interconnected with one another. These structures are named for

Camillo Golgi, the nineteenth-century Italian physician who first called attention to them. The

numbers of Golgi bodies a cell contains ranges from 1 or a few in protists, to 20 or more in

animal cells and several hundred in plant cells. They are especially abundant in glandular cells,

which manufacture and secrete substances. Collectively the Golgi bodies are referred to as the

Golgi apparatus.

The Golgi apparatus functions in the collection, packaging, and distribution of molecules

synthesized at one place in the cell and utilized at another location in the cell. A Golgi body has a

front and a back, with distinctly different membrane compositions at the opposite ends. The

front, or receiving end, is called the cis face, and is usually located near ER. Materials move to

the cis face in transport vesicles that bud off of the ER. These vesicles fuse with the cis face,

emptying their contents into the interior, or lumen, of the Golgi apparatus. These ER-synthesized

molecules then pass through the channels of the Golgi apparatus until they reach the back, or

discharging end, called the trans face, where they are discharged in secretory vesicles (figure

5.16).

Proteins and lipids manufactured on the rough and smooth ER membranes are transported into

the Golgi apparatus and modified as they pass through it. The most common alteration is the

addition or modification of short sugar chains, forming a glycoprotein when sugars are

complexed to a protein and a glycolipid when sugars are bound to a lipid. In many instances,

enzymes in the Golgi apparatus modify existing glycoproteins and glycolipids made in the ER by

cleaving a sugar from their sugar chain or modifying one or more of the sugars.

The newly formed or altered glycoproteins and glycolipids collect at the ends of the Golgi

bodies, in flattened stacked membrane folds called cisternae (Latin, ―collecting vessels‖).

Periodically, the membranes of the cisternae push together, pinching off small, membrane

bounded secretory vesicles containing the glycoprotein and glycolipid molecules. These vesicles

then move to other locations in the cell, distributing the newly synthesized molecules to their

appropriate destinations. Liposomes are synthetically manufactured vesicles that contain any

variety of desirable substances (such as drugs), and can be injected into the body. Because the

membrane of liposomes is similar to plasma and organellar membranes, these liposomes serve as

an effective and natural delivery system to cells and may prove to be of great therapeutic value.

MICROBODIES:

(Tiny but important)

Eukaryotic cells contain a variety of enzyme-bearing, membrane-enclosed vesicles called

Microbodies. Microbodies are found in the cells of plants, animals, fungi, and protists. The

distribution of enzymes into microbodies is one of the principal ways in which eukaryotic cells

organize their metabolism. Three main types of Microbodies are:

a) Lysosomes (lysis = breaking down; soma = body)

These are membrane-bounded digestive vesicles are also components of the endomembrane

system that arise from the Golgi apparatus. They perform intracellular digestion. They contain

high levels of degrading enzymes (sometimes 40 in number), which catalyze the rapid

breakdown of proteins, nucleic acids, lipids, and carbohydrates. Some examples of enzymes

present in the lysosomes include nucleases, proteases, lipases and carbohydrases. Throughout the

lives of eukaryotic cells, lysosomal enzymes break down old organelles, recycling their

component molecules and making room for newly formed organelles. For example,

mitochondria are replaced in some tissues every 10 days.

The digestive enzymes in lysosomes function best in an acidic environment. Lysosomes actively

engaged in digestion keep their battery of hydrolytic enzymes (enzymes that catalyze the

hydrolysis of molecules) fully active by pumping protons into their interiors and thereby

maintaining a low internal pH. Lysosomes that are not functioning actively do not maintain an

acidic internal pH and are called primary lysosomes. When a primary lysosome fuses with a food

vesicle or other organelle, its pH falls and its arsenal of hydrolytic enzymes is activated; it is then

called a secondary lysosome.

In addition to breaking down organelles and other structures within cells, lysosomes also

eliminate other cells that the cell has engulfed in a process called phagocytosis (see. also

Pinocytosis), a specific type of endocytosis (see chapter 6). When a white blood cell, for

example, phagocytizes a passing pathogen, lysosomes fuse with the resulting ―food vesicle,‖

releasing their enzymes into the vesicle and degrading the material within.

Lysosomes are called ―suicidal bags‖ as enzymes contained in them can digest the cell‘s own

material when damaged or dead. Lysosomes are common in the cells of Animals, Protoctista and

even Fungi, but rare in plants

NB: While lysosomes bud from the endomembrane system, other microbodies grow by

incorporating lipids and protein, then dividing.

Importance of intracellular digestion by the lysosomes:

(i) Help in nutrition of the cell by digesting food, as they are rich in various enzymes which

enable them to digest almost all major chemical constituents of the living cell.

(ii) Help in defense by digesting germs, as in white blood cells.

(iii) Help in cleaning up the cell by digesting damaged material of the cell.

(iv) Provide energy during cell starvation by digestion of the cells own parts (autophagic, auto :

self; phagos: eat up).

(v) Help sperm cells in entering the egg by breaking through (digesting) the egg membrane.

(vi) In plant cells, mature xylem cells lose all cellular contents by lysosome activity.

(vii) When cells are old, diseased or injured, lysosomes attack their cell organelles and digest

them. In other words lysosomes are autophagic, i.e. self devouring.

b) Peroxisomes.

Found both in plant and animal cells. Found in the green leaves of higher plants. They participate

in oxidation of substrates. One of the byproducts of the digestion is hydrogen peroxide (H2O2).

Peroxisomes have developed to a point where they are able to contain that hydrogen peroxide

and break it down into water (H2O) and oxygen (O2). The water is harmless to the cell and the

oxygen can be used in the next digestive reaction. They are very well known for digesting fatty

acids.

They often contain a central core of crystalline material called nucleoid composed of

urate oxidase crystals.

These bodies are mostly spherical or ovoid and about the size of mitochondria and

lysosomes.

They are usually closely associated with E.R.

They are involved in with photorespiration in plant cells.

They bring about fat metabolism in cells.

c) Glyoxysomes

These microbodies are present in plant cells (particularly in the fat storage tissues of germinating

seeds) and are morphologically similar to peroxisomes. Found in the cell of yeast and certain

fungi and oil rich seeds in plants. Functionally they contain enzyme of fatty acid metabolism

involved in the conversion of lipids to carbohydrates during germination.

RIBOSOMES:

(Sites of protein synthesis)

Although the DNA in a cell‘s nucleus encodes the amino acid sequence of each protein in the

cell, the proteins are not assembled there. A simple experiment demonstrates this: if a brief pulse

of radioactive amino acid is administered to a cell, the radioactivity shows up associated with

newly made protein, not in the nucleus, but in the cytoplasm. When investigators first carried out

these experiments, they found that protein synthesis was associated with large RNA protein

complexes they called ribosomes.

Ribosomes are made up of several molecules of a special form of RNA called ribosomal RNA,

or rRNA, bound within a complex of several dozen different proteins. Ribosomes are among the

most complex molecular assemblies found in cells. Each ribosome is composed of two subunits.

The subunits join to form a functional ribosome only when they attach to another kind of RNA,

called messenger RNA (mRNA) in the cytoplasm. To make proteins, the ribosome attaches to

the mRNA, which is a transcribed copy of a portion of DNA, and uses the information to direct

the synthesis of a protein.

Ribosomes are the site of protein synthesis in a cell. They are the most common organelles in

almost all cells. Some are free in the cytoplasm (Prokaryotes); others line the membranes of rough

endoplasmic reticulum (rough ER).

They exist in two sizes:

70s are found in all Prokaryotes, chloroplasts and mitochondria, suggesting that they have evolved

from ancestral Prokaryotic organisms. They are free-floating.

80s found in all eukaryotic cells – some free in the cytoplasm others attached to the rough ER (they

are rather larger).

So bacterial ribosomes are smaller than eukaryotic ribosomes. Also, a bacterial cell typically has

only a few thousand ribosomes, while a metabolically active eukaryotic cell, such as a human

liver cell, contains several million. Proteins that function in the cytoplasm are made by free

ribosomes suspended there, while proteins bound within membranes or destined for export from

the cell are assembled by ribosomes bound to rough ER.

NB: Unlike most other organelles, ribosomes are not surrounded by a membrane and because of this

some biologists prefer not to call them organelles.

MITOCHONDRIA:

(The cell’s chemical furnaces)

Mitochondria (singular, mitochondrion) are typically tubular or sausage-shaped organelles

about the size of bacteria and found in all types of eukaryotic cells. Mitochondria are bounded by

two membranes: a smooth outer membrane and an inner one folded into numerous contiguous

(adjacent) layers called cristae (singular, crista). The cristae partition the mitochondrion into two

compartments: a matrix, lying inside the inner membrane; and an outer compartment, or

intermembrane space, lying between the two mitochondrial membranes. On the surface of the

inner membrane, and also embedded within it, are proteins that carry out oxidative metabolism,

the oxygen-requiring process by which energy in macromolecules is stored in ATP.

Mitochondria are often referred to as the powerhouses of the cell, for it is within them that

energy is released from organic molecules by the process of respiration (to be discussed later).

They have their own DNA; this DNA contains several genes that produce proteins essential to

the mitochondrion‘s role in oxidative metabolism. All of these genes are copied into RNA and

used to make proteins within the mitochondrion. In this process, the mitochondria employ small

RNA molecules and ribosomal components that the mitochondrial DNA also encodes. However,

most of the genes that produce the enzymes used in oxidative metabolism are located in the

nucleus.

A eukaryotic cell does not produce brand new mitochondria each time the cell divides. Instead,

the mitochondria themselves divide in two, doubling in number, and these are partitioned

between the new cells. Most of the components required for mitochondrial division are encoded

by genes in the nucleus and translated into proteins by cytoplasmic ribosomes. Mitochondrial

replication is, therefore, impossible without nuclear participation, and mitochondria thus cannot

be grown in a cell-free culture.

Mitochondria are more numerous in cells that have a high energy requirement - our muscle cells

contain a large number of mitochondria, as do liver, heart and sperm cells. They are surrounded by

two membranes; hence the theory that they were once free-living organisms that have become

mutualistic and then a part of almost every eukaryotic cell with the exception of RBC and xylem

vessels. (Endosymbiotic hypothesis).

PLASTIDS

A characteristic feature of plant cells and some algae is the presence of plastids that make or store

food. Plastids are not present in animal cells. They occur in a variety of size, shape and colour.

Based on this fact, there are three types of plastids.

i. Chloroplast – green

ii. Chromoplast – blue, red, red, yellow etc.

iii. Leucoplast - white or colourless

i) CHLOROPLASTS: Where Photosynthesis Takes Place

Plants and other eukaryotic organisms that carry out photosynthesis typically contain from one to

several hundred chloroplasts. Chloroplasts bestow an obvious advantage on the organisms that

possess them: they can manufacture their own food. Chloroplasts contain the photosynthetic

pigment chlorophyll that gives most plants their green color.

The chloroplast body is enclosed, like the mitochondrion, within two membranes that resemble

those of mitochondria. However, chloroplasts are larger and more complex than mitochondria. In

addition to the outer and inner membranes, which lie in close association with each other,

chloroplasts have a closed compartment of stacked membranes called grana (singular, granum),

which lie internal to the inner membrane. A chloroplast may contain a hundred or more grana,

and each granum may contain from a few to several dozen disk-shaped structures called

thylakoids. On the surface of the thylakoids are the light-capturing photosynthetic pigments.

Surrounding the thylakoid is a fluid matrix called the stroma.

Like mitochondria, chloroplasts contain DNA, but many of the genes that specify chloroplast

components are also located in the nucleus. Some of the elements used in the photosynthetic

process, including the specific protein components necessary to accomplish the reaction, are

synthesized entirely within the chloroplast.

NB: Mitochondria and chloroplast are among the most interesting cell organelles because they

contain DNA (the genetic material) and ribosome (for protein synthesis) unlike all other

organelles. Since chloroplasts and mitochondria contain their own DNA the hereditary molecule

and also their own ribosomes, they are termed semi-autonomous organelles only because they

are incapable of independent existence though they have ribosomes and DNA.

ii) Chromoplasts:

These are a second type of plastids found in some cells of more complex. Although

chromoplasts are similar to chloroplasts in size, they vary considerably in shape, often being

angular. They are usually yellow, orange or red in colour due to the presence of carotenoid

pigments, which they synthesis and accumulate. They are most abundant in yellow, orange or

some red parts of plants such as ripe tomatoes, carrots or red pepper.

iii) Leucoplasts:

Leucoplasts are a third type of plastids common to cells of higher plants. They are essentially

colorless and include amyloplasts which synthesize and store starch and elaioplasts which

synthesize oils. If exposed to light some leucoplasts will develop into chloroplasts and vice

versa.

NB: All plastids come from the division of existing plastids.

CENTRIOLES:

(Microtubule Assembly Centers)

Centrioles are barrel-shaped organelles found in the cells of animals and most protists. They

occur in pairs, usually located at right angles to each other near the nuclear membranes; the

region surrounding the pair in almost all animal cells is referred to as a centrosome. Although

the matter is in some dispute, at least some centrioles seem to contain DNA, which apparently is

involved in producing their structural proteins. Centrioles help to assemble microtubules, long,

hollow cylinders of the protein tubulin. Microtubules influence cell shape, move the

chromosomes in cell division, and provide the functional internal structure of flagella and cilia,

as we will discuss later. Centrioles may be contained in areas called microtubule-organizing

centers (MTOCs). The cells of plants and fungi lack centrioles, and cell biologists are still in the

process of characterizing their MTOCs.

A centriole has 9 sets of peripheral tubules but none in the centre. Each set has three tubules

arranged at definite angles (Fig. below). It has its own DNA and RNA and therefore it is self

duplicating.

Read about:

Basal bodies.

Cilia and flagella (structure and differences)

CYTOPLASM:

An aqueous substance containing a variety of cell organelles and other structures such as

insoluble wastes and storage products. The soluble part of cytoplasm forms the ‗background

material‘ or ‗ground substances‘ between the cell organelles.

It contains about 90% water and forms a solution which contains all the fundamental

biochemical of life. Some of these are ions and small molecules in true solution; others are large

molecules such as proteins which form colloidal solutions.

VACUOLE

Vacuole is a fluid filled sac bounded by as single membrane. Animal cells contain relatively

small vacuoles, such as phagocytic vacuoles, food vacuoles, autophagic vacuoles and contractile

vacuoles. Typically plant cells have one or two large vacuoles filled with fluid known as cell sap

and surrounded by a membrane called tonoplast. The cell sap is a watery fluid containing water,

sugar, organic acid, mineral salts, pigments and toxic substances.

Functions:

1. Water generally enters the concentrated cell sap by osmosis. Osmotic uptake of water is

important in cell expansion during cell growth as well as in the normal water relations of

plants.

2. The vacuole sometimes contain pigments in solution e.g. anthocynins which are red, blue

and purple and other related compounds which are yellow and ivory.

3. Plant vacuole sometimes contains hydrolytic enzymes and act as lysosomes. After cell

death, the tonoplast loses its partial permeability and the enzymes escapes causing

autolysis.

4. Vacuoles contain waste products and certain secondary products of plant metabolism

such as calcium oxalate, alkaloid, and tannin which offers protection from consumption

by herbivores.

5. Vacuole acts as a food storage organelle. It stores sucrose and mineral salts which can be

utilized by the cytoplasm when necessary.

THE CYTOSKELETON:

(Interior framework of the cell)

Cytoskeleton is a system of protein fibers (very thin strands, like threads) in the cytoplasm of a

eukaryotic cell that contributes to the structure and organization of a eukaryotic cell and gives it

the capacity for direct movement. Thus the cytoplasm of all eukaryotic cells is crisscrossed by a

network of protein fibers that supports the shape of the cell and anchors organelles to fixed

locations.

Cytoskeleton which is involved with in movement within a cell and in a cell‘s architecture, is an

intricate network constructed mainly of three kinds of cytoskeletal fibers, each formed from a

different kind of subunit namely; actin filaments, microtubules and intermediate filaments.

How are protein filaments/fibers formed?

Molecules of a single type of protein can interact to form geometrically regular assemblies. If a

protein has a binding site that is complementary to a region of its own surface, it will assemble

spontaneously to form a large structure. More often, an extended polymer of subunits will

results, and provided that each subunit is bound to its neighbor in an identical way, the subunits

in the polymer will be arranged in a helix that can be extended indefinitely. An actin filament for

example is a supermolecular helix formed from units of the globular protein actin (fibrous?).

Similarly, globular proteins can assemble into sheets, tubes, spheres or coils. Fibrous proteins

form coiled coils by pairing of two alpha helical subunits that have a repeating nonpolar side

chains.

1. Actin filaments

Actin filaments are long fibers about 7 nanometers in diameter. Each filament is composed of

two protein chains loosely twined together like two strands of pearls. Each ―pearl,‖ or subunit, on

the chains is the globular protein actin. Actin molecules spontaneously form these filaments,

even in a test tube; a cell regulates the rate of their formation through other proteins that act as

switches, turning on polymerization when appropriate. Actin filaments are responsible for

cellular movements such as contraction, crawling, ―pinching‖ during division, and formation of

cellular extensions.

2. Microtubules.

Microtubules are hollow tubes about 25 nanometers in diameter, each composed of a ring of 13

protein protofilaments. Globular proteins consisting of dimers of alpha and beta tubulin subunits

polymerize to form the 13 protofilaments. The protofilaments are arrayed side by side around a

central core, giving the microtubule its characteristic tube shape. In many cells, microtubules

form from MTOC (microtubule-organizing centers) nucleation centers near the center of the cell

and radiate toward the periphery. They are in a constant state of flux, continually polymerizing

and depolymerizing (the average half life of a microtubule ranges from 10 minutes in a

nondividing animal cell to as short as 20 seconds in a dividing animal cell), unless stabilized by

the binding of guanosine triphosphate (GTP) to the ends, which inhibits depolymerization. The

ends of the microtubule are designated as ―+‖ (away from the nucleation center) or ―−‖ (toward

the nucleation center).

Along with allowing for cellular movement, microtubules are responsible for moving materials

and organelles within the cell itself. Special motor proteins, (to be discussed later), move cellular

organelles around the cell on microtubular ―tracks.‖

Microtubules participate in a wide variety of cell activities. Most involve motion. The motion is

provided by special motor proteins that use the energy of ATP to move along the microtubule

(microtubular ―tracks‖).

Microtubule motors

There are two major groups of microtubule motors:

Kinesins Move toward the + end of the microtubules (i.e. towards the cell periphery)

Dyneins Move toward the - end (i.e. towards the cell center.)

3. Intermediate filaments.

Intermediate filaments have a diameter of about 10 nm, which is intermediate between the

diameters of the two other principal elements of the cytoskeleton, actin filaments (about 7 nm)

and microtubules (about 25 nm). - Which is why they are called intermediate filaments.

They are the most durable element of the cytoskeleton in animal cells found in a system of tough,

fibrous protein molecules twined together in an overlapping arrangement. Unlike microtubules

and actin filaments, intermediate filaments are polymers of fibrous protein. In contrast to actin

filaments and microtubules, the intermediate filaments are not directly involved in cell

movements. Instead, they appear to play basically a structural role by providing mechanical

strength to cells and tissues.

Once formed, intermediate filaments are stable and usually do not break down. Intermediate

filaments constitute a heterogeneous group of cytoskeletal fibers. Whereas actin filaments and

microtubules are polymers of single types of proteins (actin and tubulin, respectively),

intermediate filaments are composed of a variety of proteins that are expressed in different types

of cells. More than 50 different intermediate filament proteins have been identified and classified

into six groups based on similarities between their amino acid sequences.

The most common type, composed of protein subunits called vimentin, provides structural

stability for many kinds of cells. Keratin, another class of intermediate filament, is found in

epithelial cells (cells that line organs and body cavities) and associated structures such as hair

and fingernails. The intermediate filaments of nerve cells are called neurofilaments.

The cytoskeleton provides an interior framework that supports the shape of the cell, stretching

the plasma membrane much as the poles of a circus tent. Changing the relative length of

cytoskeleton filaments allows cells to rapidly alter their shape, extending projections out or

folding inward. Within the cell, the framework of filaments provides a molecular highway along

which molecules can be transported.

FIGURE

Molecules that make up the cytoskeleton. (a) Actin filaments. Actin filaments are made of two strands of the fibrous protein actin twisted together and usually occur in bundles. Actin filaments are ubiquitous, although they are concentrated below the plasma membrane in bundles known as stress fibers, which may have a contractile function. (b) Microtubules. Microtubules are composed of 13 stacks of tubulin protein subunits arranged side by side to form a tube. Microtubules are comparatively stiff cytoskeletal elements that serve to organize metabolism and intracellular transport in the nondividing cell. (c) Intermediate filaments. Intermediate filaments are composed of overlapping staggered tetramers of protein. This molecular arrangement allows for a ropelike structure that imparts tremendous mechanical strength to the cell

CELL SURFACE SPECIALIZATION: MATRIX BETWEEN ANIMAL CELLS, CELL

TO CELL JUNCTION

Communication between cells:

Communication between cells is common in nature. Cell signaling occurs in all multicellular

organisms, providing an indispensable mechanism for cells to influence one another. The cells of

multicellular organisms use a variety of molecules as signals, including not only peptides, but

also large proteins, individual amino acids, nucleotides, steroids and other lipids. Some of these

molecules are attached to the surface of the signaling cell; others are secreted through the plasma

membrane or released by exocytosis.

Any given cell of a multicellular organism is exposed to a constant stream of signals. At any

time, hundreds of different chemical signals may be in the environment surrounding the cell.

However, each cell responds only to certain signals and ignores the rest (figure below), like a

person following the conversation of one or two individuals in a noisy, crowded room. How does

a cell ―choose‖ which signals to respond to? Located on or within the cell are receptor proteins,

each with a three-dimensional shape that fits the shape of a specific signal molecule. When a

signal molecule approaches a receptor protein of the right shape, the two can bind. This binding

induces a change in the receptor protein‘s shape, ultimately producing a response in the cell.

Hence, a given cell responds to the signal molecules that fit the particular set of receptor

proteins it

Cell Signaling:

Four kinds of cell signaling. Cells communicate in several ways. (a) Two cells in direct contact

with each other may send signals across gap junctions. (b) In paracrine signaling, secretions

from one cell have an effect only on cells in the immediate area. (c) In endocrine signaling,

hormones are released into the circulatory system, which carries them to the target cells. (d)

Chemical synaptic signaling involves transmission of signal molecules, called

neurotransmitters, from a neuron over a small synaptic gap to the target cell.

The area of cell surface receptors and signaling is a wide one which will not be dealt with in this

course; instead our focus will be on the junction between cells. Multicellular organisms have

many advantages over single celled organisms, but certainly one of the major advantages is that

in a co-operative “family” of cells, each is free to specialize in ways that would be impossible if

each cell had to live alone.

It is customary for groups of specialized cells to be organized into tissues, which can, in turn, be

further organized in to organs and organ systems. This kind of association and co-operativity

requires that similar cells be held together in close and direct physical contact with one another.

Neighbors must not only work together, they must be joined together.

There are two major ways in which cells in tissues can be held together; an extracellular matrix

of macromolecules can form a lattice-work that can then be used by the associated cells to move,

change position and a framework in which cells can interact with one another, and cell junctions

can create firm, direct, specialized points of fusion between two cells in direct physical contact.

Junctions between Cells:

A cell junction is a structure that exists within the tissue of many multicellular organisms,

including humans. Cell junctions consist of protein complexes that provide contact between

neighboring cells or between a cell and the extracellular matrix. The space between cells created

by cell junctions is known as the paracellular space and serves an important role in numerous

forms of cell-cell communication and transport. Cell junctions are especially abundant in

epithelial tissues, like skin.

Junctions between cells most occur on or very near the cell's plasma membrane, but can also

involve the tiny space between cells and sometimes the layer of cytoplasm that lies just below

the plasma membrane. There are many way of classifying junctions, we can for example

categorize cell depending on the function they serve. i.e.; communicating junctions; these types

of junctions usually help small molecules pass from one cell to the next; Impermeable junctions;

these junctions hold cells in contact with their neighbors, but prevent them from sharing their

contents and adhering junctions; a simple, mechanical fastening between two cells.

Four kinds of junctions occur in vertebrates:

1. Tight junctions

Tight junctions, or zonula occludens/ occluding junctions, are structures joining the membranes

of two cells to form a barrier that is nearly impermeable to fluid. Tight junctions are composed

of a branching network of sealing strands, with each strand formed from a row of

transmembrane proteins embedded in the plasma membranes of adjacent cells, linking the cells

by way of extracellular domains. Although tight junctions are comprised of many proteins, the

major components are the claudins and the occludins. These associate with intracellular proteins

that anchor the sealing strands to the actin cytoskeleton.

Tight junctions connect the plasma membranes of adjacent cells in a sheet, preventing small

molecules from leaking between the cells and through the sheet (figure below). This allows the

sheet of cells to act as a wall within the organ, keeping molecules on one side or the other.

Tight junctions are common in epithelial tissues, e.g. the epithelial cells that line the small

intestine in humans are arranged into a sheet of cells that separate the contents of the guts and the

inner cavity of the organ that eventually empties into blood vessels. Epithelia are sheets of cells

that provide the interface between masses of cells and a cavity or space (a lumen). The portion of

the cell exposed to the lumen is called its apical surface. The rest of the cell (i.e., its sides and

base) make up the basolateral surface.

Tight junctions seal adjacent epithelial cells in a narrow band just beneath their apical surface.

Functions:

Tight Junctions (TJ)/occluding junctions. In simple epithelial and endothelial cells they were

shown to fulfil different functions, the most prominent being the paracellular “barrier function‖

and the ―fence function‖, which separates the apical membrane domain from the basolateral part.

Additionally, tight junctions perform other vital functions including providing structural support.

In addition, tight junctions act as both functional and protective barriers. As an example of a

functional barrier, tight junctions maintain the polarity of cells by preventing the lateral

diffusion of membrane proteins between the apical and basal surfaces, preserving the specialized

functions of each. As an example of a protective barrier, tight junctions prevent the passage of

molecules through the space between cells. Instead, materials must enter cells first in order to

pass through the tissue, providing control over the passage of substances. In this way, tight

junctions play a critical role in maintaining the blood-brain barrier (BBB).

2. Adherens junctions

An adherens junction (or zonula adherens, intermediate junction) is defined as a cell junction

whose cytoplasmic face is linked to the actin cytoskeleton. They are protein complexes that

occur at cell–cell junctions in epithelial and endothelial tissues. They are intercellular structures

that couple intercellular adhesion to the cytoskeleton thereby creating a transcellular network that

coordinate the behavior of a population of cells. Adherens junctions are built primarily from

cadherins, whose extracellular segments bind to each other and whose intracellular segments

bind to catenins. Catenins are connected to actin filaments.

They regulate cytoskeletal dynamics and cell polarity. As such, they regulate a diverse range of

other cellular processes next to adhesion, such as cell shape, division, growth, apoptosis and

barrier function. Adherens junctions provide additional mechanical structural support between

adjacent cells. For example, they hold cardiac muscle cells tightly together as the heart expands

and contracts.

3. Gap junctions

A gap junction (also called communicating junctions) is a specialized cell junction that directly

connects the cytoplasm of two cells. In this way, gap junctions allow various molecules and ions

to pass freely between cells. A single gap junction channel is composed of two opposing

connexons, also known as hemichannels, joined across the intercellular space. In vertebrates, most gap junction hemichannels are composed of a complex of six connexin

proteins, each characterized by four transmembrane domains. Six connexin sub-units assemble to

create one connexon, or hemichannel. Hemichannels of uniform connexin composition are

called homomeric, while those with differing connexins are heteromeric. Furthermore, gap

junction channels formed from two identical hemichannels are called homotypic, while those

with differing hemichannels are heterotypic. Channel composition, both at the level of the

individual connexin proteins and the hemichannel, influences the function of the gap junction.

Functions: Gap junctions serve a number of critical cellular functions. Most notably, gap

junctions provide cytoplasmic channels from one cell to an adjacent cell and thus allow for direct

electrical communication between cells. However, gap junctions also permit forms of chemical

communication between cells through the diffusion of small-sized material. Although different

connexin subunits can impart different pore sizes to a gap junction channel, large biomolecules,

such as nucleic acid and proteins, are too large to pass through gap junctions and are precluded

from cytoplasmic transfer between cells.

Gap junctions are expressed in virtually all tissues of the body, but the role of gap junctions is

most obvious for cell types that benefit from direct electrical communication, such as neurons

and cardiac muscle. Gap junctions in cardiac muscle allow the cells of the heart to contract in

tandem, and the smooth muscle cells responsible for the peristaltic movements of the intestine

for example

Also the normal development of ovarian follicles also depends on gap-junction-mediated

communication—in this case, between the oocyte and the surrounding granulosa cells. A

mutation in the gene that encodes the connexin that normally couples these two cell types causes

infertility.

4. Desmosomes

Desmosomes are localized patches that hold two cells tightly together. Desmosomes are attached

to intermediate filaments of keratin in the cytoplasm. They are characterized by a localized patch

that holds two cells tightly together. They are common in numerous epithelia including the skin

and are also found in muscle tissue where they bind muscle cells to one another. They are present

in tissues subject to shear or lateral stress and help tissues resist shearing forces.

The cell adhesion proteins of the desmosome - desmoglein and desmocollin - are members of the

cadherin family of cell adhesion molecules. They are transmembrane proteins that bridge the

space between adjacent epithelial cells by way of homophilic binding of their extracellular

domains to other desmosomal cadherins on the adjacent cell.

.

Summary of junctions in animals:

Read on:

1. Hemidesmosome 2. In many plant tissues, it turns out that the plasma membrane of each cell is continuous

with that of the adjacent cells. The membranes contact each other through openings in the

cell wall called Plasmodesmata.

CELL CYCLE AND CELL DIVISION

BACKGROUND:

Are you aware that all organisms, even the largest, start their life from a single cell? You may

wonder how a single cell then goes on to form such large organisms. Growth and reproduction

are characteristics of cells, indeed of all living organisms. All cells reproduce by dividing into

two, with each parental cell giving rise to two daughter cells each time they divide. These newly

formed daughter cells can themselves grow and divide; giving rise to a new cell population that

is formed by the growth and division of a single parental cell and its progeny. In other words,

such cycles of growth and division allow a single cell to form a structure consisting of millions

of cells (Refer to cell theory).

One of the most important characteristics of living things therefore is the ability to replicate and

pass on genetic information to the next generation. Cell division in individual bacteria and

Achaea usually occurs by binary fission. Mitochondria and chloroplasts also replicate by binary

fission, which is evidence of the evolutionary relationship between these organelles and

prokaryotes. Cell division in eukaryotes is more complex. It requires the cell to manage a

complicated process of duplicating the nucleus, other organelles and multiple chromosomes.

CELL CYCLE

Why do cells divide? There are many reasons for this. Cells absorb and release nutrients through

their membrane. The larger the cell, the harder it is to get rid of all the waste that is produced. So,

if there are many small cells (more surface area) rather than one large cell, the waste can be

disposed of more readily (Refer to: Why Aren’t Cells Larger?). The other 3 reasons are critical to

the survival of all organisms: Growth, repair and reproduction.

Growth: This is a result of mitosis. The more cells in an organism, the larger that organism is.

Humans start off as one single cell, and by the time they are adults, they have over 10 trillion

cells! This increase in the number of cells also allows for some of those cells to be specialized

for various functions. This is important to the survival of many organisms.

Repair: This too is a result of mitosis. If tissue is damaged, repair is extremely important. With

some organism, they are even able to regenerate lost limbs (such as arms or tails). For us, this is

important because it can repair skin, blood vessels and bones, for example. This also replaces

cells that have died. You have a ―new‖ skin every 28 days! That means that the old cells died

and the new ones took their place.

Reproduction: This is a result of mitosis or meiosis, depending on the type of reproduction.

There are two types of reproduction. The first type is asexual reproduction, and this is when

there is only one parent. This results from normal cell division i.e. mitosis. This occurs in

bacteria, protests, fungi, some plants and some animals. The offspring are genetically identical

to that of the parent. The other type of reproduction is sexual reproduction. This is when the

offspring have a combination of both parents DNA and is a result of meiosis.

Cell division is a very important process in all living organisms. During the division of a cell,

DNA replication and cell growth also take place. All these processes, i.e., cell division, DNA

replication, and cell growth, hence, have to take place in a coordinated way to ensure correct

division and formation of progeny cells containing intact genomes. The sequence of events by

which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually

divides into two daughter cells is termed as cell cycle. Although cell growth (in terms of

cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific

stage in the cell cycle. The replicated chromosomes (DNA) are then distributed to daughter

nuclei by a complex series of events during cell division. These events are themselves under

genetic control.

The increased size and more complex organization of eukaryotic genomes over those of bacteria

required radical changes in the process by which the two replicas of the genome are partitioned

into the daughter cells during cell division. This process called the cell cycle is divided into

three parts: interphase, mitosis and cytokinesis. The first part is further divided into three

phases thus cell cycle can be viewed as consisting of consisting of five phases:

Duration of the Cell Cycle:

The time it takes to complete a cell cycle varies greatly among organisms. Cells in growing

embryos can complete their cell cycle in less than 20 minutes; the shortest known animal nuclear

division cycles occur in fruit fly embryos (8 minutes). Cells such as these simply divide their

nuclei as quickly as they can replicate their DNA, without cell growth. Half of the cycle is taken

up by S, half by M, and essentially none by G1 or G2. Because mature cells require time to grow,

most of their cycles are much longer than those of embryonic tissue. Typically, a dividing

mammalian cell completes its cell cycle in about 24 hours, but some cells, like certain cells in the

human liver, have cell cycles lasting more than a year. During the cycle, growth occurs

throughout the G1 and G2 phases (referred to as ―gap‖ phases, as they separate S from M), as

well as during the S phase. The M phase takes only about an hour, a small fraction of the entire

cycle.

Most of the variation in the length of the cell cycle from one organism or tissue to the next

occurs in the G1 phase. Some cells in the adult animals do not appear to exhibit division (e.g.,

heart cells) and many other cells divide only occasionally, as needed to replace cells that have

been lost because of injury or cell death. These cells that do not divide further exit G1 phase to

enter an inactive stage called G0 phase (Quiescent stage); of the cell cycle. Cells in this stage

remain metabolically active but no longer proliferate unless called on to do so depending on the

requirement of the organism. They may remain in this phase for days to years before resuming

cell division. At any given time, most of the cells in an animal‘s body are in G0 phase (Cell cycle

arrest). Some, such as muscle and nerve cells, remain there permanently; others, such as liver

cells, can resume G1 phase in response to factors released during injury.

a) Interphase: (Preparing for Mitosis)

The interphase, though called the ‗resting phase’, is a period of great cellular activity. It is the

time during which the cell is preparing for division by undergoing both cell growth and DNA

replication in an orderly manner. The interphase which is very important for the successful

completion of mitosis is divided into three further phases: G1, S, and G2.

G1 phase (first gap): This phase corresponds to the interval between mitosis and initiation of

DNA replication. During G1 phase the cell is metabolically active and continuously growing but

does not replicate its DNA. The cell grows in volume as it produces tRNA, mRNA, ribosomes,

enzymes, and other cell component. This is therefore a period where the cell grows and develops.

Since some cells divide more actively than others, the time spent in the G1 phase will greatly

vary from cell to cell. There is no division that takes place in this phase; just growth and

development. Metabolic changes prepare the cell for division. At a certain point - the restriction

point, the cell is committed to division and moves into the S phase.

S phase (synthesis): The S phase, short for synthesis phase, is a period in the cell cycle during

interphase, between G1 phase and the G2 phase. In this phase DNA synthesis or replication

occurs. During this time the amount of DNA per cell doubles. If the initial amount of DNA is

denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number;

if the cell had diploid or 2n number of chromosomes at G1, even after S phase the number of

chromosomes remains the same, i.e., 2n.

Each chromosome replicates to produce two sister chromatids, which remain attached to each

other at the centromere. The centromere is a point of constriction on the chromosome,

containing a specific DNA sequence to which is bound a disk of protein called a kinetochore.

This disk functions as an attachment site for fibers that assist in cell division. Each

chromosome‘s centromere is located at a characteristic site. This replication is important,

because it allows there to be two full sets of DNA in each of the new cells, at the end of the

division.

G2 phase (second gap): During this phase, metabolic changes assemble the cytoplasmic

materials necessary for mitosis and cytokinesis. Organelles and other material required for cell

division are replicated or formed. For example, the centrioles in animal cells replicate

themselves, to form 2 pairs. During the G2 phase, proteins are synthesized in preparation for

mitosis while cell growth continues. An example is the proteins that will be used in the formation

of microtubules.

b) M Phase (Mitosis phase)

This is the most dramatic period of the cell cycle, involving a major reorganization of virtually

all components of the cell. Since the number of chromosomes in the parent and progeny cells is

the same, it is also called as equational division. Mitosis is a form of eukaryotic cell division that

produces two daughter cells with the same genetic component as the parent cell. Chromosomes

replicated during the S phase are divided in such a way as to ensure that each daughter cell

receives a copy of every chromosome.

The M Phase represents the phase when the actual cell division or mitosis occurs. It is significant

to note that in the 24 hour average duration of cell cycle of a human cell, cell division proper

lasts for only about an hour. The interphase lasts more than 95% of the duration of cell cycle. The M phase is broken down into 4 sub-phases: Prophase, Metaphase, Anaphase, and Telophase.

Though for convenience mitosis has been divided into four stages of nuclear division, it is very

essential to understand that cell division is a progressive process and very clear-cut lines cannot

be drawn between various stages

Prophase:

Prophase which is the first stage of mitosis follows the S and G2 phases of interphase. In the S

and G2 phases the new DNA molecules formed are not distinct but interwined. During prophase,

the nuclear envelope of the cell begins to break down. The centrioles, which are only present in

animal cells, separate and each moves to an opposite end of the cell. As they move apart, a

network of protein fibers that are made up of microtubules is left in their wake. These protein

fibers are known as spindle fibers. As the centrioles take their places at opposite sides of the cell,

they send out more spindle fibers. These spindle fibers seek out the sister chromatids that are

present in the cell. Spindle fibers from one side of the cell attach to one of the sister chromatids.

The spindle fibers from the other side of the cell attach to the other sister chromatids in the

chromosome. They attach at a point called the kinetochore, which is a disk or protein that is on

each side of the centromere. The spindle fibers will move the chromosomes until they are lined

up at the spindle equator.

At the end of this phase, the chromosomal material has condensed to form compact mitotic

chromosomes. Chromosomes are seen to be composed of two chromatids attached together at

the centromere.

Metaphase: (inclu’ Prometaphase)

The complete disintegration of the nuclear envelope marks the start of the second phase of

mitosis; Metaphase. By this stage, condensation of chromosomes is completed and they can be

observed clearly under the microscope. This then, is the stage at which morphology of

chromosomes is most easily studied. At this stage, metaphase chromosome is made up of two

sister chromatids, which are held together by the centromere. Small disc-shaped structures at the

surface of the centromeres are called kinetochores. These structures serve as the sites of

attachment of spindle fibres to the chromosomes that are moved into position at the centre of the

cell. Hence, the metaphase is characterized by all the chromosomes coming to lie at the equator

with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one

pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole

The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.

The key features of metaphase are:

Spindle fibres attach to kinetochores of chromosomes.

Chromosomes are moved to spindle equator and get aligned along metaphase plate

through spindle fibres to both poles.

Anaphase:

Of all the stages of mitosis, anaphase is the shortest and the most beautiful to watch. It starts

when the centromeres divide. Each centromere splits in two, freeing the two sister chromatids

from each other. The centromeres of all the chromosomes separate simultaneously, but the

mechanism that achieves this synchrony is not known. The sister chromatids of each

chromosome are pulled apart and moves away from the equatorial plate, to the opposite ends of

the cell, pulled by spindle fibres attached to the kinetochore regions. These separated sister

chromatids are known from this point forward as daughter chromosomes. This process is

critical, because it ensures that the soon to be daughter cells will each have full, identical sets of

chromosomes, also being identical to the parent cell.

(NB: It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.)

As each chromosome moves away from the equatorial plate, the centromere of each chromosome

is towards the pole and hence at the leading edge, with the arms of the chromosome trailing

behind. Thus, anaphase stage is characterized by the following key events:

Centromeres split and chromatids separate.

Chromatids move to opposite poles.

Telophase:

This is the final stage of mitosis, and a reversal of many of the processes observed during

prophase. The nuclear membrane reforms around each set of sister chromatids, which can now

be called chromosomes because each has its own centromere. The chromosomes soon begin to

uncoil into the more extended form that permits gene expression. One of the early group of genes

expressed are the rRNA genes, resulting in the reappearance of the nucleolus.

This is the stage which shows the following key events:

Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.

Nuclear envelope assembles around the chromosome clusters.

Nucleolus, golgi complex and ER reform.

c) Cytokinesis.

This is the final cellular division to form two new cells. Mitosis accomplishes not only the

segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is

divided into two daughter cells by a separate process called cytokinesis at the end of which cell

division is complete. In an animal cell, this is achieved by the appearance of a cleavage furrow

in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre

dividing the cell cytoplasm into two. Plant cells however, are enclosed by a relatively

inextensible cell wall; therefore they undergo cytokinesis by a different mechanism. Cytokinesis

in plant cells, which have cell walls, is markedly different. There is no cleavage furrow. Instead,

during telophase, vesicles derived from the Golgi apparatus move along microtubules to the

middle of the cell, where they coalesce, producing a cell plate that represents the middle lamella

between the walls of two adjacent cells. Cell wall materials carried in the vesicles collect in the

cell plate as it grows. The cell plate enlarges until its surrounding membrane fuses with the

plasma membrane along the perimeter of the cell. Two daughter cells result, each with its own

plasma membrane. Meanwhile, a new cell wall arising from the contents of the cell plate has

formed between the daughter cells. At the time of cytoplasmic division, organelles like

mitochondria and plastids get distributed between the two daughter cells. In some organisms

karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises

leading to the formation of syncytium (e.g., diagrammatic liquid endosperm in coconut).

Significance of Mitosis:

Mitosis or the equational division is usually restricted to the diploid cells only. However, in

some lower plants and in some social insects haploid cells also divide by mitosis. It is very

essential to understand the significance of this division in the life of an organism. Mitosis results

in the production of diploid daughter cells with identical genetic complement usually. The

growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio

between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to

restore the nucleo-cytoplasmic ratio. A very significant contribution of mitosis is cell repair. The

cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being

constantly replaced. Mitotic divisions in the meristematic tissues – the apical and the lateral

cambium, result in a continuous growth of plants throughout their life.

Meiosis (The Germ Cell Cycle/ reduction division)

The production of offspring by sexual reproduction includes the fusion of two gametes, each

with a complete haploid set of chromosomes. Gametes are formed from specialized diploid cells.

This specialized kind of cell division reduces the chromosome number by half results in the

production of haploid daughter cells (hence the name reduction division). This kind of division is

called meiosis or germ cell cycle. Meiosis ensures the production of haploid phase in the life

cycle of sexually reproducing organisms whereas fertilization restores the diploid phase.

We come across meiosis during gametogenesis in plants and animals. This leads to the formation

of haploid gametes. The key features of meiosis are as follows:

Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and

meiosis II but only a single cycle of DNA replication. The two are preceded by an interphase

phase which is much the same as interphase for somatic cells. Germ cells are growing;

chromosomes are replicating.

Meiosis I is initiated after the parental chromosomes have replicated to produce identical

sister chromatids at the S phase.

Meiosis involves pairing of homologous chromosomes and recombination between them.

Four haploid cells are formed at the end of meiosis II.

Meiotic events can be grouped under the following phases:

Meiosis I:

Prophase I: Prophase of the first meiotic division is typically longer and more complex when

compared to prophase of mitosis. It has been further subdivided into the following five phases

based on chromosomal behaviour i.e., Leptotene, Zygotene, Pachytene, Diplotene and

Diakinesis.

Leptotene: chromosomes start to condense and become gradually visible under the light

microscope. The compaction of chromosomes continues throughout leptotene.

Zygotene: Chromosomes start pairing together, such a paired chromosomes are called

homologous chromosomes and become closely associated (synapsis) to form pairs of

chromosomes (bivalents) consisting of four chromatids (tetrads).

Pachytene: This stage is characterized by the appearance of recombination nodules the site

at which crossing over between non-sister chromatids of the homologous chromosomes

occurs. Crossing over is the exchange of genetic material between two homologous

chromosomes. Crossing over leads to recombination of genetic material on the two

chromosomes. Recombination between homologous chromosomes is completed by the end

of pachytene, leaving the chromosomes linked at the sites of crossing over.

Diplotene: Homologous chromosomes start to separate but remain attached by chiasmata

(the sites of crossovers). In oocytes of some vertebrates, diplotene can last for months or

years.

Diakinesis: Homologous chromosomes continue to separate, and chiasmata move to the ends

of the chromosomes. During this phase the chromosomes are fully condensed and the meiotic

spindle is assembled to prepare the homologous chromosomes for separation. By the end of

diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis

represents transition to metaphase

Metaphase I: The bivalent chromosomes align on the equatorial plate. The spindle apparatus

forms from opposite ends of the cell. The spindle apparatus then sends out spindle fibers to

attach to the chromosomes. However, since the homologous chromosomes are lined up side by

side for crossing over, they are tightly held together. Therefore, the spindle fibers are only able to

make contact with the kinetochore on the sister chromatid that is facing outward in each

chromosome pair. Which chromosome is oriented toward which end of the cell is a matter of

chance; each pair is independent of one another.

Anaphase I: The attachment of the spindle fibers is complete. The homologous chromosomes

are pulled apart and move towards opposite ends of the cell. The homologous chromosomes

separate, while sister chromatids remain associated at their centromeres. All possible

combinations of genetic material are created from the independent assortment. At this point, the

sister chromatids are still attached to each other.

Telophase I: The chromosomes are now at opposite ends of the cell and begin to form two

distinct chromosome clusters. At this point, nuclear division begins, and the parent cell is divided

in half, forming 2 daughter cells. The nuclear membrane and nucleolus reappear, cytokinesis

follows and this is called as diad of cells. Each daughter cell will have half of the original

chromosomes. Each chromosome consists of 2 sister chromatids. The daughter cells now move

in to the third and final phase of meiosis: meiosis II. At the end of meiosis I there are two haploid

cells. Although in many cases the chromosomes do undergo some dispersion, they do not reach

the extremely extended state of the interphase nucleus. The stage between the two meiotic

divisions is called interkinesis and is generally short lived. Interkinesis is followed by prophase

II, a much simpler prophase than prophase I.

Meiosis II

Meiosis II is the second division of meiosis. It occurs in both of the newly formed daughter cells

simultaneously. In contrast to meiosis I, meiosis II resembles a normal mitosis in that the sister

chromatids are separated. It consists of 4 sub-phases: Prophase II, Metaphase II, Anaphase II,

Telophase II.

Prophase II: During prophase II, the nuclear membrane disappears by the end of prophase II.

The chromosomes begin to recondense and become compact and spindle fibers begin to form

once again. These spindle fibers seek out the sister chromatids that are present in the cell.

Metaphase II: At this stage the chromosomes align at the equator and the microtubules from

opposite poles of the spindle get attached to the kinetochores of sister chromatids.

Anaphase II: During anaphase II, the centromere splits, freeing the sister chromatids from each

other. At this point, spindle fibers begin to shorten, pulling the newly-separated sister chromatids

towards opposite ends of the cell. Unlike the sister chromatids in mitosis, the sister chromatids in

meiosis are not genetically identical due to crossing over.

Telophase II: During Telophase II, cell division begins again in each of the two daughter cells,

creating 4 daughter cells. Each of these 4 daughter cells contains 23 chromosomes, making them

haploid, and none of the 4 is exactly alike (due to crossing over and independent assortment).

Significance of meiosis:

Meiosis is the mechanism by which conservation of specific chromosome number of each

species is achieved across generations in sexually reproducing organisms, even though the

process, per se, paradoxically, results in reduction of chromosome number by half. It also

increases the genetic variability in the population of organisms from one generation to the next

as a result of crossing over. Variations are very important for the process of evolution. Meiosis

generates genetic diversity through:

The exchange of genetic material between homologous chromosomes during Meiosis I

The random alignment of maternal and paternal chromosomes in Meiosis I

The random alignment of the sister chromatids at Meiosis II

NB A checkpoint in the cell cycle is a control point where stop and go-ahead signals can regulate

the cycle. (The signals are transmitted within the cell by the kinds of signal transduction

pathways. Rhythmic fluctuations in the abundance and activity of cell cycle control molecules

pace the sequential events of the cell cycle. These regulatory molecules are mainly proteins of

two types: protein kinases and cyclins.