Cell Growth & Division

Limits to Cell Growth •All cells are derived from pre-existing cells There are two main reasons why cell divide rather continuing to grow indefinitely:

1. The larger the cell becomes, the more demands the cell places on its DNA. 2. The cell has more trouble moving enough nutrients and wastes across the

cell membrane.

Reason #1 DNA “Overload”•DNA: The information that controls a cell’s function.

•In Eukaryotic cells, DNA is found in the nucleus of the cell.

•The information stored in that DNA is able to meet all of the cell’s needs.

Reason #1 DNA “Overload”•As a cell increases in size, it usually does not make extra copies of DNA.

•If a cell were to grow without limit, an “information crisis” would occur.

•In time, the cell’s DNA would no longer be able to serve the increasing needs of the growing cell.

Reason #2 Exchanging Materials •Food, oxygen, and water enter a cell through its cell membrane.

•The rate at which this exchange takes place depends on the surface area of the cell.

•The rate at which food and oxygen are used up and waste products are produced depends on the cell’s volume.

Ratio of Surface Area to Volume

Cell size is limited by a cell's surface area to volume ratio.

◦ Surface area: refers to the outside area of an object.

◦ Volume: refers to the amount of space inside of the object.

Ratio of Surface Area to Volume PROBLEM:

As the surface area increases, its volume increases at a faster rate.

CONSEQUENCE: Cells have a more difficult time to move needed materials in and waste products out.

Let’s think about it…

Cell Division

Cell Division •The process by which a cell divides into two new “daughter” cells.

•Before cell division occurs, the cell replicates, or copies, all of its DNA.

•This replication of DNA, solves the problem of information storage because each daughter cell gets one complete set of genetic information.

Cell Division •Cell division also solves the problem of increasing size by reducing cell volume.

•This allows efficient exchange of materials with the environment.

Why Do We Need Cells to Divide? For:•Growth•Repair•Reproduction

Chromosomes •In Eukaryotic cells, the genetic information that is passed on from one generation of cells to the next is carried by chromosomes.

•Chromosomes are made up of DNA

•The cells of every organism have a specific number of chromosomes.

Chromosomes •Well before the cell divides, each chromosome is replicated or copied.

•Each chromosome consists of two identical “sister” chromatids.

•When the cell divides, the “sister” chromatids separates from each other other.

Chromosomes •One chromatid goes to each of the two new cells. •Each pair of chromatids is attached at an area called the centromere. •Centromeres are usually located near the middle of the chromatids.◦ Some do lie near the ends.

The Cell Cycle •Series of events that cells go through as they grow and divide.

•During the cell cycle a cell:• Grows • Prepares for Division• Divides to form two daughter cells

•Consist of four phases.

The Cell Cycle Consist of Four Phases G1 Phase

• Cell Growth

S Phase • DNA Replication

G2 Phase ◦ Preparation for Mitosis

M Phase• Mitosis and Cytokinesis

Known as Interphase

Events of the Cell Cycle

M Phase MITOSIS & CYTOKINESIS

Mitosis Biologist divide the events of mitosis into four phases:

1. Prophase 2. Metaphase 3. Anaphase 4. Telophase

Prophase The first and longest phase if mitosis.

• Can take as much as 50 to 60 percent of the total time required to complete mitosis.

During Prophase: ◦ Chromosomes become visible. ◦ Centrioles separate and take up positions on

opposite sides of the nucleus.

The centrioles lie in a region called the centrosome that helps to organize the spindle.

◦ Spindle: Fanlike microtubule structure that helps septate the chromosomes.

Prophase During Prophase:

◦ The condensed chromosomes become attached to fibers in the spindle at the point near the centromere of each chromatid.

Near the end of Prophase:◦ The chromosomes coil more tightly. ◦ The nucleus disappears and the nuclear

envelope breaks down.

Metaphase The second phase of mitosis.

During Metaphase: • The chromosomes line up across the center

of the cell.

• Microtubules connect the centromere of each chromosome to the two poles of the spindle.

Meta meaning MIDDLE

Anaphase The third phase of mitosis.

During Anaphase: • The centromeres that join the sister

chromatids split, allowing the sister chromatids to separate and become individual chromosomes. • The chromosomes continue to move until

they have separated into two groups near the poles of the spindle.

Anaphase ends when the chromosomes stop moving.

Telophase The fourth and final phase of mitosis.

During Telophase: • The chromosomes, which were distinct and

condensed, begin to disperse into a tangle of dense material. • A nuclear envelope re-forms around each

cluster of chromosomes. • The spindle begins to break apart, and a

nucleolus becomes visible in each daughter nucleus.

Mitosis is complete.

Cytokinesis •As a result of Mitosis, two nuclei-each with a duplicate set of chromosomes-are formed, usually within the cytoplasm of a single cell. • Cytoplasm: material inside the cell membrane-

not including the nucleus.

•All that remains to complete the M phase of the cycle is cytokinesis. • Cytokinesis: Division of the cytoplasm itself.

•Cytokinesis usually occurs at the same time as telophase.

Regulating the Cell Cycle

Controls on Cell Division Not all cells move through the cell cycle at the same rate.

In the human body:•Most muscle cells and nerve

cells do not divide at all once they have developed.

Controls on Cell Division In contrast, the cells of the skin and digestive tract, and cells in the bone marrow that make blood cells:• Grow and divide rapidly throughout life.

Such cells may pass through a complete cycle every few hours. • This process provides new cells to

replace those that wear out or break down.

Cell Cycle Regulators Discovered that cells in mitosis contained a protein that when injected into a nondiving cell, would cause a spindle to form.

This protein is called: cyclin

Cyclin: regulate the timing of the cell cycle in eukaryotic cells.

Since this discovery, dozens of other proteins have been discovered that also help to regulate the cell cycle.

Cell Cycle Regulators There are two types of regulatory proteins:

1. Internal Regulators: Proteins that respond to events inside the cell.• Allow the cell cycle to proceed only

when certain processes have happened inside the cell. • For example, makes sure that the cell

does not go into mitosis until all the chromosomes have been replicated.

Cell Cycle Regulators 2. External Regulators: Proteins that respond to events outside the cell are called external regulators. • Direct cells to speed up or slow down the

cell cycle. • Growth factors are among the most

important external regulators. • They stimulate the growth and division of

cells.

Why is cell growth regulated so carefully?

Uncontrolled Cell Growth •The consequences of uncontrolled cell growth in a multicellular organism are very severe.

•Cancer: is a disorder in which some of the body’s own cells lose the ability to control growth.

•Cancer cells do not respond to the signals that regulate the growth of most cells.

•As a result, they divide uncontrollably and form masses of cells called tumors that can damage the surrounding tissues.

Uncontrolled Cell Growth •Cancer cells may break loose from tumors and spread throughout the body, disrupting normal activities and causing serious medical problems.

•What causes the loss of growth control that characterizes cancer?

•The various forms of cancer have many causes, including: • Smoking tobacco • Radiation exposure • Viral Infections

Uncontrolled Cell Growth •All cancers, have one thing in common: • The control over the cell cycle has

broken down.

•Some cancer cells will no longer respond to external growth regulators, while others fail to produce the internal regulators that ensure orderly growth.

DNADeoxyribonucleic acid (DNA): nucleic acid that contains the sugar ribose.

Present in nearly all living organisms• Main constituent of chromosomes. • It is the carrier of genetic information

Is a long molecule made up of units called nucleotides.

Each nucleotide is made up three basic components:

1. 5-carbon sugar called deoxyribose2. A phosphate group3. A nitrogenous base

DNAThere are four kinds of nitrogenous bases in DNA. • Adenine• Guanine • Cytosine• Thymine

The backbone of a DNA chain is formed by a sugar (deoxyribose) and phosphate groups of each nucleotide.

The nitrogenous bases stick out sideways meaning that any sequence of bases is possible.

The Double Helix Francis Crick and James Watson were trying to

understand the structure of DNA by building three dimensional models.

British scientist, Rosalind Franklin showed Watson a X-ray pattern of DNA, and immediately Watson and Crick knew how to construct their 3D imagine of DNA.

Watson and Crick published their model of DNA as a double helix, in which two strands wound around each other.

The Double Helix A double helix looks like a twisted ladder or a spiral staircase. Watson and Crick did not explain what held the two strands together. They later discovered that hydrogen bonds could form between certain nitrogenous bases and provide just enough force to hold the strands together.

DNA Hydrogen bonds can form only between certain base pairs:• Adenine and thymine • Guanine and cytosine

This principle is called base pairing:• [A] = [T]• [G] = [C]

For every adenine there has to be one thymine molecule, for each cytosine molecule, there has to be a guanine molecule.

RNAoRNA= Ribonucleic acidoRNA, like DNA consists of a long chain of nucleotides. oIts principal role is to act as a messenger carrying instructions from DNA.

RNA There are three main differences between DNA and RNA:

1. The sugar in RNA is ribose instead of deoxyribose.

2. RNA is typically single stranded.

3. RNA contain uracil instead of thymine.

RNA There are three main types of RNA:

1. Messenger RNA 2. Ribosomal RNA 3. Transfer RNA

How does DNA code for our traits?

DNA MUST CODE FOR PROTEINS!

Example: Eye Color oWe know that our DNA carries the genetic information that says what colors our eyes will be.

oEye color=pigments

oIn order to have that pigment, your DNA must code for proteins that will help make that pigment.

Proteins (Polypeptides)oAmino acids (20 different kinds of aa) bonded together by peptide bonds (polypeptides).

oCombinations that can be formed from the 20 different amino acids.

aa1 aa2 aa3 aa4 aa5 aa6

Peptide Bonds

Amino Acids (aa)

Help make the

pigments!

Protein Synthesis oRNA molecules have many functions, but in the majority of cells most RNA molecules are involved in just one job: PROTEIN SYNTHESIS.

oThe assembly of amino acids into proteins is controlled by RNA.

How are we going to get the DNA out of the nucleus to start making the proteins we need?

Transcription oWe are going to transcribe the DNA into a message. oMolecules that carry copies of these instructions are known as “messengers” from DNA to the rest of the cell: mRNA. oWe make MESSAGES out of RNA. oThis mRNA, leaves the nucleus

Ribosomal RNAoProteins ae assembled on ribosomes, which are made up of many proteins as well as ribosomal RNA.

TranslationoWhere we use the message to build a protein. oWe are translating our message into a protein.oThis transfer RNA comes in. They have an amino acid on them.

TranslationoAll of these transfer RNA are looking for the matching bases on the mRNA. oThe transfer RNA will “drop off” the amino acid it is carrying on the mRNA.