Cell.Cell Cycle.

Lecture by Prof Dr Marina Kapitonova for the Year 1 Medical Students

OBJECTIVES:OBJECTIVES:

1. To describe cytoplasm, cell membrane, nucleus of the cells.

2. To characterize cell cycle.

I. DEFINITION OF THE CELL.

- Cells are the basic functional units of complex organisms.

- Cells that are related to or are similar to each other as well as cells that function in a particular manner or serve a common purpose are grouped together to form tissues.

II. HISTORY OF THE CELL THEORY

In 1665 Robert Hooke, curator of the Royal Society of London, examined razor-thin slices of cork under very simple microscope. He discovered small boxlike spaces which he named cells.

Three-dimensional schematic diagram of an idealized cell as visualized by TEM. Various organelles and cytoskeletal elements are displayed.

Purkinje cell of the cerebellum.

4. Number of types of cell – more than 200.

IV. IV. VARIATIONS IN CELL STRUCTURE:VARIATIONS IN CELL STRUCTURE:

1. Size (from 5 to 200 micrometers).

2. Shape (flat-cubic-cylindric-round-oval-fusiform, pyramidal; smooth-with processes or other cell membrane specializations).

3. Number of cells in the human body - 1013-1014.

The Main Constituents of the Cell and Their Distribution

The living component of the cell is protoplasm which is subdivided into cytoplasm and the nucleoplasm.

3. cytoplasmic inclusions (e.g., stored glycogen, lipid and pigment) not consistently present in cells.

V. V. BASIC ORGANIZATION OF CELL: BASIC ORGANIZATION OF CELL:

Almost all nucleated cells share the same basic organization:

1. A central nucleus,

2. Surrounding cytoplasm,

3. Peripheral cell membrane (plasmalemma).

The three basic components of the cell cytoplasm are:

1. The cytoplasmic organelles, each kind individually designed to carry out certain functions,

2. the cytosol (cytoplasmic matrix) – a macromolecular complex that envelops the various cytoplasmic organelles,

Motor Neurons, Light Microscopy, Nissl Staining, x 400

Cell Membrane

Cell Membrane, TEM, x 100,000

Under EM you may see cell membrane as a trilaminar structure of two thin dense lines with an intervening light area. Each layer is about 2.5 nm in width. The entire structure is known as unit membrane with outer leaflet (outer dense line) and inner leaflet (inner dense line). The cell membrane is composed of a lipid bilayer (fluid mosaic model).

Cell Membrane Structure

In a lipid bilayer phospholipid hydrophobic groups face inward, and the hydrophylic groups face outward. Protein molecules (integral proteins) float within this basic structure with projecting carbohydrate groups being attached to glycolipids or proteins.

The protein components either span the entire lipid bilayer as integral proteins (transmembrane proteins) or are attached to the cytoplasmic aspect of the lipid bilayer as peripheral proteins (intrinsic proteins). Transmembrane proteins frequently form ion channels and carrier proteins that facilitate the passage of specific ions and molecules.

V. V. CELL MEMBRANECELL MEMBRANE

Membrane proteins:

Attach cytoskeletal filaments to cell membrane

Attach cells to extracellular matrix (adhesion molecules)

Transport molecules in or out of the cell (carrier proteins, membrane pump proteins, channel proteins)

Act as receptors for chemical signaling between cells (e.g. hormone receptors).

Possess specific enzymatic activity.

Three dimensional diagramatic representation of the fluid mosaic model of the cell membrane

Other amphipathic molecules such as glycolipids and cholesterol are also present in the cell membrane. Cholesterol limits the lateral fluid movement of adjacent phospholipids and makes the membrane less fluid and more mechanically stable. Glycolipids are involved in intercellular communication.

Establishing transport systems for specific molecules (glucose, for example). 

V. CELL MEMBRANEV. CELL MEMBRANE

1. FUNCTIONS:

Maintaining the structural integrity of the cell

Controlling movements of substance in and out of the cell (selective permeability)

Regulation cell-cell interactions

Recognition, via receptors, antigens, altered cells, foreign cell

Transducting extracellular physical or chemical signals into intracellular event

Acting as an interface between the cytoplasm and the external milieu

A fine filamentous network also radiates from plasma membrane’s external leaflet and represents the cell coat or glycocalyx composed of carbohydrate chains. It plays an important role in determining the immunological properties of the cell and its relationships and interaction with other cells.

GLYCOCALIX.GLYCOCALIX.

Cell Cortex, Diagram

The cell cortex is composed of a stiff cross-linked meshwork of actin and actin-linking proteins, the most abundant being filamin. It forms a layer that lines the cytosolic face of the cell membrane.

Transport across the cell membrane may be active and passive. Passive transport occurs without any input of energy other than

that inherent in the concentration gradient:1)simple diffusion - a few nonpolar molecules (benzene, oxygen, nitrogen) and uncharged polar molecules (water, glycerol) can move across the cell membrane down their concentration gradients.2)facilitated diffusion – most ions and small molecules move across a membrane even when driven by a concentration gradient, require aid of channel proteins (ion channel-mediated diffusion: voltage-gated, ligand-gated, mechanically gated, G-protein-gated and ungated) or carrier proteins (carrier-mediated diffusion).

TRANSMEMBRANE TRANSPORT. Passive Transport

TRANSMEMBRANE TRANSPORT - Active Transport

Active transport requires expending of energy, by it ions and small molecules may be transported against their concentration gradients. Only carrier proteins can mediate such energy-requiring active transport. It may be uniport or  coupled, with two different molecules moving in the same (symport) or opposite (antiport) directions.

Transport of larger molecules in and out of the cell may be through the active role of plasmalemma in bringing macromolecular materials into the cell (endocytosis) and out of it (exocytosis).

Pinocytosis (cell drinking) is used when cell takes up fluid and small molecules to form small vesicles about 100 nm in diameter. The term endocytosis and phagocytosis (cell eating) are used when cells ingest large particles to form endosomes more than 250 nm in diameter.

Types of pinocytosis

-        -fluid-phase (non-selective)

-        -receptor-mediated (selective).

Stages of endocytosis: the invaginated cell membrane fuses to form an endocytotic vesicle or endosome which move into cytoplasm as small smooth-surfaced vesicles. The membrane and any material incorporated into such a vesicle can then be processed within the cell.

Several proteins have been defined which mediate the process of membrane fusion. NSF (N-ethylmaleimide sensitive fusion protein) interacts with SNAPS (NSF attachment protein) to form an active fusogenic complex.

Endocytosis (non-selective)

A highly selective uptake of specific proteins, such as hormones, is accomplished by receptor-mediated pinocytosis. Its selectivity depends on randomly distributed receptors in the plasmalemma.

a) A coated pit is braced by a coat protein molecules & bears surface receptors that bind specific extracellular ligand. In most cases the coat protein is clathrin, which forms a hexagonal lattice around the pit membrane.

b),c),d) Assembly of the coat protein lattice drives progressive invagination of the pit to form a coated vesicle.

e) Once internalized, the coat protein is shed and returns to the cell surface to form new coated pit.

SelectiveEndocytosis

CYTOPLASM = Cytosol (cytoplasmic matrix) + organelles + inclusions

Organelles are subcellular components, the specialized units of the cell that perform specific functions and constitute part of the living substance of the cell.Many diseases are caused by molecular alterations in specific cell components. Several of these diseases present morphologic characteristics that can be determined by LM or EM or cytochemistry.

CLASSIFICATION OF THE ORGANELLES:1. Membranous organelles:- Mitochondria- RER- SER- Golgi apparatus- Lysosomes- Lysosome-like bodies (secretory granules,

endosomes, coated vesicles)- Peroxisomes.

2. Nonmembranous organelles:- Ribosomes (free ribosomes & polysomes)- Centrioles- Microtubules (cilia and flagella assembled from

microtubules)- Filaments (cytoskeleton)

Hepatocyte, TEM, x48,176.Typical orthodox

mitochondria with double membrane showing lamellar cristae and small granules in the matrix. Mitochondria are flexible, rod-shaped organelles about 0.5 to 1mcm in girth & sometimes as much as 7 mcm in length. Most animal cells possess a large number of mitochondria (as many as 2000 in each liver cell). Via oxidative phosphorilation they produce ATP, a stable form of energy that can be used by the cell for various energy-requiring activities.

Pineal Gland.Bright mitochondrion of crista type

Suprarenal gland.Mitochonria of tubular type

Suprarenal Gland, zona fasciculata.Mitochondria of saccular type

Heart, dark mitochondriaof crista type

Observations: self-duplicating, can divide & join together; contain DNA & RNA; are the “powerhouse” of the cell.

Mitochondria were described for the first time by Swiss histologist Kölliker* in 1857 in striated muscle.

All mitochondria possess a smooth outer membrane and folded inner membrane with a number of cristae to increase the surface of the membrane. The number of cristae is directly related to the energy requirement of the cell (high in cardiomyocytes).

The narrow space between the inner and outer membranes is called intermembranous space whereas the large space enclosed by the inner membrane is termed the matrix (intercristal) space.

Mitochondrion,TEM, 120,000x

Matrix is filled with dense fluid containing 50% protein responsible for the stepwise degradation of fatty acids and pyruvate to the metabolic intermediate acetyl CoA & the subsequent oxidation of this intermediate in the tricarboxylic acid (Krebs) cycle. Dense spherical matrix granules are also present. The are composed of phospholipoprotein and may bind Ca2+. In injured cells whose cytosolic Ca2+ levels are dangerously high, matrix granules may sequster calcium to protect cell from calcium toxity.

Distribution of Functionsbetween the MitochondrialCompartments

Golgi Apparatus, TEM. X 20,318.

The Golgi complex functions in the synthesis of carbohydrates & in the sorting, post-translational modification and packaging of proteins manufactured on the RER.

Typical Golgi apparatus is composed of one or more series of flattened, slightly curved membrane-bound cisternae, the Golgi stack. The periphery of each cisterna is dilated and is rimmed with vesicles that are in the process of either fusing with or budding off that particular compartment.

Golgi has three functional parts: the nuclear facing cis-face receives transport (shuttle) vesicles from SER & phophorilates certain proteins; the central medial Golgi compartment adds sugar residues to both lipids and peptides to form complex oligosaccharides; the trans Golgi network performs proteolytic steps, adds sugar residues and sorts different macromolecules into specific vesicles which bud off the trans face.

Sorting is performed by specific membrane receptor proteins, which recognize signal groups on macromolecules and direct them into correct vesicles.

New membrane lipid synthesized in the smooth ER makes its way into the cell membrane via the Golgi.

Golgi Apparatus, Diagram

Structure: stacks of flattened membranes (saccules) enclosing cisternae; many small membranous vesicles. Each stack contains forming face (cis = near) which receives newly synthesized proteins via transition (shuttle) vesicles from RER, and maturing (trans=beyond) face which releases sorted and packed products in secretory vesicles.

Function: packages secretory products and transports them to cell surface for release; synthesizes carbohydrate portion of some glycoproteins; also forms lysosomes.

Golgi Apparatus,Diagram

Cis-face (entry face) is convex in shape while trans-face (exit face) is concave. Vesicles associated with Golgi and RER possess a protein coat as well as surface markers (clathrin, coatomer I and II (COPI & COP II).

Golgi Apparatus,TEM

It is especially developed in secretory cells; appears to be polarized, i.e., certain cisternae have special function (secreting lysosomal enzymes, membrane proteins and secretory proteins).

Stains by reducing silver and osmium salts; also PAS-positive.

Schematic Diagram of the Golgi Apparatus and Packing in the Trans-Golgi Network

Endoplasmatic reticulum is the largest membranous system of the cell, comprising approximately half of the total membrane volume. It is a system of interconnected tubules and vesicles whose lumen is referred to as the cistern.

The metabolic processes that occur on the surface of and within the ER are protein synthesis and modification, detoxication of certain toxic compounds, and manufacture of all membranes of the cell. It has two components: RER and SER.

DIAGRAM OF ROUGH & SMOOTH ENDOPLASMATIC RETICULUM

rough endoplasmic reticulum is the site of synthesis of proteins that are to be packed

smooth endoplasmic reticulum functions in synthesis of cholesterol-based lipids

Liver. Mouse. Electron microscopy. X 20,318.

The rough endoplasmic reticulum (rER) is obvious due to its ribosomes (R), whereas the smooth endoplasmic reticulum (sER) is less obvious.

Cells that function in the synthesis of proteins that are to be exported are richly endowed with RER. The membranes of RER possess integral proteins that function in recognizing and binding ribosomes to its cytosolic surface and also maintain flattened morphology of the RER (docking protein – signal recognition particle receptor; ribophorin I & II – ribosome receptor protein; pore protein.

Plasmatic Cell, TEM, x30,000

Diagram of the Relationship between ER & Golgi.

The lumen of RER is continuous with perinuclear space and with the lumen of SER, while Golgi forms a separate membrane system.

Communication between ER and Golgi is mediated by small vesicles of ER, which break off, move through the cytosol and fuse with Golgi membrane.

RER,TEM.

A system of anastomosing tubules and occasional flattened membrane-bound vesicles constitute the SER. It is involved in synthesis of steroids, cholesterol, triglycerides as well as synthesis & utilization of carbohydrates.

SMOOTH (AGRANULAR)ENDOPLASMATIC RETICULUM

Diagram to show spatial relationship between RER and SER.

Smooth ER cisternae are tubular. Most cells have only a relatively small quantity of SER with the exception of cells secreting or processing lipids and the cells that function in detoxication of toxic materials (e.g. alcohol, barbiturates).

SER is a site of processing synthesized protein and cell lipid synthesis, particularly membrane phospholipids. The lipid synthetic enzymes are located on its outer face with ready access to lipid precursors. Once synthesized and incorporated into the outer part of the SER membrane lipid bilayer; phospholipids are flipped over into the inner part by specific transport proteins colloquially termed flipases.

SER,TEM,X100,000

A lysosome is a membrane-bound organelle with a high content of hydrolytic enzymes (at least 40 types of acid hydrolases as sulphatases, proteases, lipases etc) operating in an acid pH.

“Primary Lysosome”, TEM, x 50,000

“Secondary lysosome”, TEM, x 100,000

It thus functions as an intercellular digestion system, processing either material ingested by the cell (macromolecules, microorganisms etc) or effete cellular components (senescent mitochondria, RER etc). This definition encompasses a variety of membrane-bound organelles derived form slightly different sources & with different functional roles.

Peroxisomes are small membrane-bound organelles containing enzymes involved in the oxidation of several substrates, particularly beta-oxidation of very long chain fatty acids (C18 & above). Their internal matrix often contains a crystalline structure (nucleotid). They provide breakdown of purines and contain enzymes involved in formation and destruction of hydrogen peroxide (catalase).

PEROXISOME, TEM, x 80,000

Free ribosomes in the cytosol: small electron-dense particles 20-30 nm in diameter,Present either singly or in chain called polyribosomes. Protein synthesis begins in the cytosol where messenger RNA attaches to free ribosomes and translation produces the new peptide.

Ribosomes, TEM, x 40,000

Polyribosomes consist of variable numbers of ribosomes joined by mRNA strand. Each ribosome has small and large subunits containing RNA and protein.Proteins destined to remain in the cytosol have a different signal sequence to those destined for entry into membranes or for secretion.

Polyribosomes, Diagram

small subunit

mRNA strand

large subunit

nascent polypeptide .

As mRNA enters the cytoplasm, it becomes associated with the small subunit of a ribosome. The small subunit has a binding site for mRNA as well as two binding sites (P & A) for tRNAs.

BIOSYNTHESISof PROTEIN

?

A centriole is composed of a cylindrical bundle measuring 200 x 400 nm composed of 9 microtubule triplets arranged together by linking proteins. In most cells they exist in pairs arranged at right angles to each other. In EM one centriole is usually visible in cross-section, revealing the circular arrangement of tubules, while its partner is cut either longitudinally or oblique.

It serves as microtubule-organizing center involved in formation, organization and orientation of microtubules in mitotic and interphase cells; can become basal bodies & form cilia or flagella.

CENTRIOLES, DIAGRAM

TEM

Each microtubule is composed of 13 protofilaments of alternating alpha and beta tubulin subunits. Microtubules are polar, with polymerization occurring at one end and depolimerization at the other.In cross-section each microtubule is 25 nm in diameter. EM shows the circular profiles of microtubules and faint parallel lines of them in longitudinal section.

Cytoskeleton.

protofilaments

tubulin

Diagram of the Elements of Cytoskeleton and Centriole.

The cytoskeleton has 3 major components: thin filaments (microfilaments), intermediate filaments (microfibrils), microtubules. They form an intricate 3-dimensional meshwork of protein filaments that are responsible for maintenance of cell morphology, active in motion of cells and their organelles.

Thin filaments are actin filaments that interact with myosin to bring about intracellular or cellular movement.

Intermediate filamentsHave diameter 8 to 10 nm (between thin and thick filaments.

Intermediate filaments are made of proteins which may vary between different cell types and function to link separate cells into structural units:

Epithelial cell – cytokeratins,Connective tissue cells – vimentin,Muscle tissue – desminNervous tissue – neurofilament protein (neurons)

glial fibrillary acidic protein (astrocytes)

IntermediateFilaments,Epithelial Cell,X 100,000

Accumulations of product within certain cells may occur in the form of cytoplasmic inclusions.

zymogen granules

glycogen

lipofuscin

lipid droplets

SER

membrane-bound body that varies in size and may be irregular in shape

non-membrane-bound bodies of variable electron density which may be irregular in outline

contain neutral fat, may be a source of fatty acids.

may represent an residual body with indigestible material that is not exported from the cell and that may overtime transform into aging pigment

membrane-bound granules that contain a moderately electron-dense matrix, usually dischrged by exocytosis from apical end of secretory cell

may contain a variety of secretory products (proteins, glycoproteins) including enzymes and other important macromolecules

polymers of glucose, storage form of glucose in the liver and striated musc, exists in two morphological forms: alpha and beta (either particulate or rosette form)

a reserve food source, source of glucose

Accumulations of product within certain cellsmay occur in the form of cytoplasmic inclusions.

A reserve food source, source of glucose

?

cytoplasmic constituents

morphological features function

cell membrane

electron lucent layer (3.5-4.0 nm) separating two electron-dense layers (2.5-3.0 nm

selectively permeable membrane, cell-to-cell communication, receptors for hormones, cell recognition, ion opumps, generates messenger molecules

granular endoplasmic reticulum

network of membrane-bound tubules and cisternae, outer surface is covered with ribosomes

site of synthesis of protein to be discharged form the cell, lysosomal proteins

smooth endoplasmic reticulum

network of branching anastomosing membrane-bound tubules

varies with cell type, implicated in synthesis of steroid hormones, cholesterol, triglycerids, lipoproteins, detoxication of lipid-soluble drugs, acquisition or release of Ca++ ions

Golgi complex

stacked, parallel array of several flattened membrane-bound saccules (cisternae), convex surface in the forming face, concave surface in the maturing face

site of concentration, modification, and packaging of secretory products; limited synthetic activity for complex carbohydrates and glycoproteins

mito-chon-dria

size variable; usually round, oval, or rod-shaped; consists of two membranes: an outer smooth membrane and enveloping a thinner, highly folded inner membrane; lamellar or tubular cristae project into the interior (matrix), matrix granules prominent

provide energy in form of phosphate bonds (ATP) for cell activities; involved in synthesis of steroid hormones in specific cell types

annulate lamellae

parallel stacks of membranes enclosing cisternal lumina 30-50 nm in diameter, become continuous at regular intervals forming fenestrations (pores) in cisternae, pores often lie in register

unknown

lyso-somes primary

demonstration of acid phosphotases needed for confirmation. Round membrane-bound dense bodies 0.25-0.55 mcm in diameter, interior variable in density

yet to begin digestive activity

lyso-somes secon-dary

membrane-bound bodies, ovoid or irregular in outline, size variable. Usually contain material of sime type (crystalloids, debris, organelle remnants, lipofuscin pigments

currently or have been involved in digestive activity

peroxi-somes

spherical membrane-bound bodies 0.2-0.4 mcm in diameter, matrix may contain dense crystalline structures (nucleotids)

oxidation of long-chain fatty acids

multi-ve-sicular bodies

numerous small vesicles limited by a thin smooth membrane

unknown

nonmembranous organelles

morphological features function

ribosomes dense granules (15-25 nm) comprised of a large and small subunit; polyribosomes are several ribosomes united by a strand of mRNA

protein synthesis

centrioles cylindrical structures (o.2x0.5 mcm) with an electron-dense wall containing nine triplet microtubules surrounding an electron-lucent center; occur in pairs oriented at right angles

essential for formation of microtubules of mitotic spindle, cilia, and flagella

components of cytoskeleton: microtubules

long slender tubules 25 nm in diameter, wall 5-7 nm in width surrounding an electron lucent lumen

maintain cell shape and participate in shape changes; forms axonemes of cilia and flagella; intracellular transport

microfilaments actin – 6 nm microfilament

myosin – 15-nm microfilament

cell movement, control focal movement of plasmalemma or in endocytosis

non-membra-nous organelles

morphological features function

Intermediate filaments

8-11 nm in diameter, subdivided IHC-ally into 5 classes: keratin filaments (in epithelia), desmin filaments (in 3 types of muscle), vimentin filaments (in connective tissue and other mesenchimal cell derivatives), neurofilaments – in neurons, glial filaments ( in glial cells)

stabilize cell shape and attachments

cytoplasmic inclusions

glycogen

beta particles (20-30 nm) occur singly and are irregular in outline, alpha particles are variously sized aggregates of the beta forms

storage form of glucose

lipid spheroidal droplets of varying size and density, lack a limiting membrane

primarily a storage form of fat (triglycerides)

pigments: melanin

dark brown, complexed in ellipsoidal melanosomes

inert

hemosiderin gold-brown, ultrastructurally appears as collections of 9nm particles

degradation product of hemoglobin, inert

lipofuscin coarse, irregularly shaped, brown-gold granules

end product of lysosomal activity, inert

Nucleus is the largest single membrane-bound compartment in the cell and contains nearly all of the DNA possessed by the cell as well as the mechanism for RNA synthesis.

The nucleus is limited by the nuclear envelope, and is filled with nucleoplasm (N) that contains genetic information encoded in a DNA molecules.

NLymphocyte, TEM, x20,000

HIGHLIGHTS:-the nuclear envelope is composed of two parallel unit membranes (inner & outer nuclear membranes) that enclose a perinuclear cysterna,

- the membranes fuse with each other at certain regions to form perforations known as nuclear pores;

-nuclear pores provide communication between the nucleus and the cytoplasm;

-the nuclear pore complex is composed of the nuclear pore and its associated glycoproteins;

-the nuclear pore functions in bidirectional nucleoplasmic transport;-the nuclear lamina is a scaffolding which maintains the shape of the nucleus.

The nuclear pore is surrounded by non-membranous structures embedded in its rim. These structures are called nuclear pore complex (NPC). It selectively guards passage through the pore. The pore complex contains granular and filamentous subunits.

Structurally pores are rimmed by 8 protein complexes to form the nuclear pore complex.

NUCLEAR PORE COMPLEX

cytoplasmic ring subunit

scaffold

nucleoplasmic ring subunit

basket

inner nuclear membrane

outer nuclear membrane

transporter unit

thick filament

nucleus = nuclear envelope + nucleoplasm

The nucleoplasm:

-contains macromolecules & nuclear particles involved in the maintenance of the cell,

-houses two major components:

-chromatin, the genetic material of the cell;

-the nucleolus, the center for ribosomal RNA (rRNA) synthesis;

HIGHLIGHTS:-Nuclear DNA is organized around histones into nucleosomes. The nucleosomes are wound into a helix to form chromatin.

-Chromatin is a complex of DNA and proteins and represents relaxed uncoiled chromosomes of the interphase nucleus.

-Nucleus chromatin is divided into two parts: heterochromatin (H) is a dense- staining, while euchromatin is light-staining.

-Euchromatin represents actively transcribed cellular DNA.

-Heterochromatin is a highly condensed transcriptionally inactive form of DNA.

-Further looping of the chromatin results is formation of the supercoiled structures – chromosomes.

-Chromosomes are chromatin fibers that become so condensed and tightly coiled during mitosis and meiosis that they are visible under the light microscope.

NUCLEOLUS

The nucleolus presents a sponge-like appearance composed of electron-lucent and electron-dense materials, suspended free in the nucleoplasm. The electron-dense region is composed of pars granulosa and pars fibrosa, while the electron-lucent region is the nucleoplasm in which the nucleolus is suspended.

F A

G

NUCLEOLUS

The nucleolus is the deeply staining non-membranous-bound structure within the nucleus that is involved in rRNA synthesis and in the assembly of small and large ribosomal subunits.

It is observed only during interphase because it dissipates during cell division.

Densely staining regions are nucleolus-associated chromatin which is being transcribed into rRNA.

CHROMOSOMESKaryotype Human, peripheral lymphocyte culture

Analysis of male chromosomes seen in the previous slide. This arrangement of the chromosomes into groups is known as a karyo-typing. Note the dark and light areas (bands) that characterize each chromosome. Cells containing the full complement of chromosomes (46) are said to be diploid (2n). Germ cells are haploid (1n).

Note the 22 pairs of autosomes (Nos. 1 to 22) and the pair of sex chromosomes (XY in this case). The 22 pairs of chromosomes have been classified on morphological grounds into 7 groups (A to G). The chromosomes seen here are judged to be normal. Abnormal, duplicated, or missing chromosomes can be related to defective somatic and mental development in man.

68-hour peripheral lymphocyte culture, phytohemag-glutinin-stimulated to induce lymphocyte transformation and mitosis.

HIGHLIGHTS:

CELL NUCLEUS:

-bounded by a double nuclear membrane (nuclear envelope),

-contains the cellular DNA as chromatin,

-contains the nucleolus responsible for making ribosomes,

-moves substances in and out through nuclear pores.

COMPARATIVE CHARACTERISTICS OF THE NUCLEAR CONSTITUENTS

nuclear constituents

structure function

nuclear envelope

consists of inner and outer membranes that become continuous around nuclear pores; outer membrane studded with ribosomes, inner membrane smooth; are separated by a perinuclear cistern, nuclear pore complex is associated with each pore

specialized segment of endoplasmatic reticulum that bounds nucleus, nuclear pores permit communication between cytoplasm and nucleoplasm

nuclear lamina

thin network of interwoven filaments stabilizes inner nuclear membrane; attachment site for components of nucleoplasm

hetero-chromatin

dense staining, condensed chromatin inactive, part of genome not being expressed

euchromatin Light staining, dispersed chromatin active, part of genome being expressed

nucleolus conspicious round body in nucleus, nucleolonema consists of dense granules in a matrix of filaments; amorphous component may be present

synthesis of ribosomes

A diagram demonstrating the cell cycle in actively dividing cells. Nondividing cells , such as neurons, leave the cycle to enter the Go phase (resting stage). Other cells, such as lymphocytes, may return to the cell cycle. In most tissues only a small proportion of cells will be in the cell cycle, the majority being differentiated cells in a G0 phase. Stem cells may be in a G0 phase and only come to re-enter the cell cycle if there is a demand, for example following cell death.

Cell cycle.

mitosis

interphase

division

Cell division for growth and renewal is achieved by the process of mitosis.

An essential feature of development is the ability of cells to divide and reproduce. In addition, death of mature cells in the adult needs to be compensated for by production of new cells.

Cells reproduce by duplicating their contents & dividing into 2 daughter cells. The phases involved in cell replication can be regarded as a cell cycle. The phases of cell division are visible histologically and involve duplication of cellular cytoplasmic contents, duplication of DNA, separation of cellular DNA into two separate areas of the cell and finally cell division (cytokinesis).

Cells can enter a phase of proliferation in which they divide. Cells which leave the cycle are said to be in the G0 phase.

CELLCYCLE

M

S

G2

GO

G1

cell cycle(dividing cell)

exit fromcell cycle(non-dividingcells)

The cell cycle is divided into two major events:Mitosis, the short period of time during which the cell divides its nucleus and cytoplasm, giving rise to two daughter cells, and interphase, a longer period of time during which the cell increases its size and content and replicates its genetic material.

CELL CYCLE

The DNA of cells is only replicated during certain phases of a cell’s growth pattern, which takes place in several stages. The cell cycle is divided into two main periods: mitosis and interphase which includes G1, S & G2

phases. G1 cells have just entered a phase of cellular growth. S phase cells actively synthesize DNA, G2 cells have a double complement of cellular DNA and are resting prior to cell division, and M phase cells are in mitosis which comprises 5 stages.

STAGES OF MITOSIS. PROPHASE.

The duplicated chromatin becomes condensed into parallel sister chromosomes imparting a coarse stippling to the nuclear region, which is associated with loss of the nucleolus.

The centriole replicates to form two microtubule organizing centers at opposite poles of the cell (the mitotic spindle).

microtubules of spindle

two sister chromosomesheld together at centromere

centromere

nuclear membrane centriole center of spindle

MITOSIS. Prometaphase.

The nuclear membrane breaks down to form small vesicles, allowing the microtubules of the spindle to interact with chromosomes. Each chromosome pair has an attachment site (kinetochore) which binds to spindle microtubules from each pole of the spindle (kinetochore tubules). The chromosome pairs move to the center of the spindle.

spindle pole

kinetochore microtubule

polar microtubulenuclear membrane vesicle

MITOSIS. Metaphase.

Movement of chromosomes along the microtubules lead to alignment of the chromosomes at the equator of the cell between the poles of the spindle.

cell equator

MITOSIS. Anaphase.

The kinetochore attachments to the paired chromosomes separate and the chromosomes move to opposite poles of the spindle. In late anaphase the spindle microtubules elongate causing elongation of the cell and further separation of the spindle poles.

nuclear envelope vesicles migrate towards poles

. elongation of polar microtubule

shortened kinetochore microtubule

chromatids pulled toward pole of spindle

MITOSIS. Telophase.

The separated chromatides become separated from the kinetochore microtubules and the nuclear membrane reforms around each group of chromosomes. The cell elongates further by elongation of the spindle microtubules. This phase signals the end of mitosis.

nuclear envelope reforms

chromosome decondense and lose microtubular attachment

MITOSIS. Cytokinesis.

Cleavage in two separate cells is produced by aggregation of an actin-myosin belt immediately beneath the equator of the telophase cell. The connecting region eventually separates with fusion of cell membranes to form two daughter cells. At this stage a nucleolus appears in the dense chromatin mass in the newly formed nucleus. The cell now enters the G1 phase of the cell cycle.

centriole

actin-myosin belt nuclear membrane

The Result of the Cytokinesis:

1) Cell organelles are evenly distributed between the daughter cells.

2) Immediately after division the daughter cells enter a phase of active RNA and protein synthesis resulting in an increase of the volume of both nucleus and cytoplasm.

3) The ER and Golgi are restored to their original concentrations.

4) Mitochondria reproduce by fission, centrioles replicate in the daughter cells just before next division.