2- cell division chapter 9. copyright © the mcgraw-hill companies, inc. permission required for...

2-

Cell Division

Chapter 9

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.9-2

The Importance of Cell Division

The ability to grow and reproduce are two fundamental qualities of life.

During cell division, one cell becomes two new cells.• Reproduction occurs as binary fission in prokaryotes.• Growth and some reproduction occurs as mitosis in

eukaryotes.• Reproduction often involves meiosis in eukaryotes.

All cell division is preceded by DNA replication.

Prokaryotic DNA

Prokaryotes typically have only one circular chromosome - a strand of DNA

This chromosome contains all of the genes required for this cell to survive

Prokaryotes are ‘Haploid’ – They have only one copy of each gene

Compare this to humans: Humans are Diploid – They have 2 copies of each gene arranged in 46 chromosome pairs (23 pairs of chromosomes)

Prokaryotic DNA

The prokaryotic chromosome is located in a region of the cytosol called the Nucleiod

Recall: The Nucleiod is not surrounded by a membrane like the nucleus of eukaryotic cell

Prokaryotic DNA

Eukaryotic DNA

Multiple linear chromosomes Every species has a different number of

chromosomes Nucleus contains chromatin – a complex of

DNA and proteins Not all of the cell’s DNA is used all at once

– heterochromatin – regions that are not expressed– euchromatin – expressed regions

Eukaryotic DNA

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Eukaryotic DNA

DNA Replication

Recall: DNA is molecule that stores the genetic information of a cell

The genetic information is stored in the order of the nitrogenous bases

DNA Replication

Before a cell can divide, the DNA must be copied

James Watson and Francis Crick were British scientists who discovered the structure of DNA in 1953

Their publication ended with this statement: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material”

DNA Replication

The complementary property of double stranded DNA allows each strand to serve as a template for DNA Replication

DNA Replication

This model for replication is known as Semi-Conservative Replication because in each copy, one old strand is conserved (saved) and paired with a newly made strand


Binary Fission and Mitosis

In single-celled organisms– Mitosis and binary fission are means of asexual

reproduction.

In multi-cellular organisms mitosis:– Causes growth by increasing the number of cells– Replaces lost cells– Repairs injuries


Meiosis

Sexual reproduction involves the donation of genetic information from two parents.– Each parent can only donate half of the genome.

Meiosis occurs prior to sexual reproduction.– Generates gametes (egg and sperm) with half of

a genome– The egg and sperm then join during fertilization to

make a unique offspring with a full complement of genetic information.


The Cell Cycle

Eukaryotic cells– Pass through different

stages between cell divisions

– A continuous process– Cell cycle:

Interphase Mitosis

The Cell Cycle

The eukaryotic cell cycle has 5 phases:1. 1. G1 (gap phase 1)

2. S (synthesis)

3. G2 (gap phase 2)

4. M (mitosis) - cell replication5. C (cytokinesis)

The length of a complete cell cycle varies greatly among cell types.

interphase


Interphase

During interphase, cells– Cells conduct life activities – Engage in metabolic activities– Prepare for the next cell division


Phases of Interphase-G1

The cell gathers nutrients, carries out its regular metabolic roles, and performs its normal function.– Commits to divide– Some cells never divide; they stay in G1, called Go..

– Prepares for DNA replications


Phases of Interphase-S

S phase– DNA replication occurs.– The DNA in chromosomes

condense and are copied– When S phase is complete

The identical chromosome copies are connected together.

Each is called a sister chromatid.

– Connected at the centromere

– Held


Phases of Interphase-G2

During G2

– Final preparations are made for mitosis.

– Proteins are made that will move and separate the chromosomes within the cell nucleus.


Mitosis-cell Replication

The two events of cell division– Mitosis

Separating the chromosome copies (sister chromatids) into two new nuclei

Occurs in four phases that are continuous with one another

– CytokinesisDividing the cytoplasm into two new cells that

will house the new nuclei

Mitosis

Mitosis is divided into 4 phases:1. Prophase

2. Metaphase

3. Anaphase

4. Telophase


1. Prophase

The thin, tangled chromatin gradually coils and thickens.

– Becomes visible as separate chromosomes, each with two sister chromatids

Nucleus disassembles. Nucleolus is no longer

visible.


Late Prophase

Spindle fibers attach to chromosomes at their centromeres.

– Spindles are made of microtubules.

– Will move chromosomes around within the cell.


2. Metaphase

The spindle fibers move the chromosomes so that they are all arranged at the middle of the cell.– This is called the equatorial plate.– Chromosomes complete this process at

metaphase.


Metaphase


3. Anaphase

Sister chromatids separate and move toward opposite poles.– Once the sister chromatids are separated, they

are known as daughter chromosomes.

Spindle fibers are used to pull the sister chromatids to opposite poles of the cell– The poles begin to move farther apart.– The kinetochore (proteins attached at the

centromere) pulls the chromatid along the spindle fiber.


Kinetochores


Anaphase


4. Telophase

Spindle fibers disassemble.

Nuclear membranes form around the two new sets of chromosomes.

Chromatin uncoils. Nucleolus reforms. The daughter cells

enter interphase again.


Cytokinesis

Separates the two new nuclei into new cells

Roughly divides the cytoplasm and its contents in half

Animal cells– Membrane forms a

cleavage furrow– Cell pinches into two

Plant cells– Cell plate is formed.– A new cell wall is built,

separating the nuclei.

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http://www.youtube.com/watch?v=DD3IQknCEdc&feature=related


Controlling Cell Division

Cell division must be tightly regulated. Cells gather information about themselves and their

environment in order to decide whether or not to divide.

A cell decides whether to proceed through the cell cycle at checkpoints during interphase.

– Cells evaluate their genetic health, their location in the body and the body’s need for more cells.

Poor genetic health, wrong location in the body or over-crowding will cause the cell to wait before dividing.

The opposite signals will trigger the cell to proceed with division.


Genes Regulate the Cell Cycle

Cells use checkpoint proteins make the decision to proceed through the cell cycle or to stop.

Two classes of genes that code for checkpoint proteins

1. Proto-oncogenes Code for proteins that encourage cell division

2. Tumor-suppressor genes Code for proteins that discourage cell division

The balance of these two types of proteins tells the cell whether or not to proceed with cell division.


p53, a Tumor-suppressor Gene

Near the end of G1, the p53 protein identifies if the cell’s DNA is damaged.

– If the DNA is healthy, the p53 allows the cell to divide.– If the DNA is damaged, p53 activates other proteins that will

repair the DNA.– If the damage is too severe, p53 will trigger the events of apoptosis

(cell suicide).

Mutations in the p53 gene– Lead to cells that will proceed through the cell cycle with

damaged DNA– Lead to an accumulation of mutations– If the mutations occur in proto-oncogenes or tumor-suppressor

genes, then cancer will result.


p53, a Tumor-suppressor Gene


Cancer

Cancer is caused by a failure to control cell division.– Leads to cells that divide too frequently– Cell masses (tumors) interfere with normal body

functions. Benign tumors Malignant (metastatic) tumors

– p53 is mutated in 40% of all cancers. Leads to other mutations that result in cancer


Causes of Cancer

Mutagens are agents that damage DNA.– Chemicals– Radiation– Free radicals

Carcinogens are mutagens that cause mutations that lead to cancer.– Cigarette smoke

Has been linked directly to p53 mutations


Malignant Tumors Metastasize


Treatment Strategies < Surgery

Surgical removal– Once tumors are identified they can be

surgically removed.– Skin cancers and breast cancers are

frequently treated this way.– If the cancer is spread diffusely,

surgery is not an option.


Treatment Options < Chemotherapy and Radiation Therapy

Chemotherapy– Some drugs will target rapidly dividing cells.– Normal cells that divide rapidly will suffer as well.

Weakens the immune system Causes hair loss

Radiation therapy– Uses x-rays or gamma rays directed at the tumor

to kill the cancerous cells– Whole-body radiation is used to treat leukemia.

Can lead to radiation sickness– Nausea, hair loss, etc.

Meiosis

Meiosis is a form of cell division that leads to the production of gametes.– gametes are sex cells: egg cells and sperm cells

Gametes contain half the number of chromosomes of an adult body cell

Adult body cells (somatic cells) are diploid, containing 2 sets (pairs) of each chromosome.

Gametes are haploid, containing only 1 of every chromosomes.


Cell Division and Sexual Reproduction

Meiosis makes haploid gametes.– Eggs are made in ovaries (animals) and pistils (plants).– Sperm are made in testes (animals) and anthers (plants).

Egg and sperm only have half of the individual’s genetic information (haploid).

Fertilization - when egg and sperm join during sexual reproduction - restores the diploid genome– the zygote receives half of its chromosomes from

the egg and half from the sperm.

Cell Division and Sexual Reproduction

Sexual reproduction includes the fusion of gametes (fertilization) to produce a diploid zygote.

Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages.

Some life cycles include longer diploid phases, some include longer haploid phases.


Haploid and Diploid Cells


Life Cycles Involving Meiosis and Mitosis


Pairs of Chromosomes

Diploid cells have two sets of chromosomes.– One set from each parent

Homologous chromosomes– Have the same order of genes along their DNA– Have different alleles of the same genes– One chromosome in the pair came from mom; the

other came from dad. Non-homologous chromosomes

– Have different genes on their DNA


A Pair of Homologous Chromosomes


Different Species Have Different Numbers of Chromosomes


Meiosis

The production of gametes

Meiosis involves two successive cell divisions with no replication of genetic material between them

Results in a reduction of the chromosome number

Meiosis

Meiosis includes two rounds of division – Meiosis I and Meiosis II.

Meiosis I resembles mitosis– Meiosis II reduces the chromosome number from

diploid to haploid.

Meiosis II resembles mitosis, but without DNA replication


Meiosis


Meiosis I: Prophase I

Prophase I– Synapsis occurs

Homologous chromosomes move toward one another and associate with one another.

While associated homologs experience crossing over– Homologs trade equivalent sections of DNA.– Shuffles the genes that are passed to the next

generation


Meiosis I: Metaphase I

Metaphase I:– The synapsed pairs

of homologous chromosomes are moved into position at the equatorial plate.


Meiosis I: Anaphase I

Homologous pairs segregate to opposite poles.– Sister chromatids do not separate at this point.

Chromosome number is reduced from diploid to haploid.

Homologous pairs line up randomly at the equatorial plate,– Each pair separates independently of the others.

This is called independent assortment.


Anaphase I


Meiosis I: Telophase I

Chromatin uncoils. Nuclear membrane

reforms. Nucleoli reappear. Cytokinesis divides the

two haploid nuclei into two daughter cells.

– Each chromosome still contains two sister chromatids.


Meiosis I


Meiosis II: Prophase II

Similar to prophase in mitosis

Nuclear membrane is disassembled.

Spindle begins to form.


Meiosis II: Metaphase II

Similar to metaphase in mitosis

Chromosomes are lined up at the equatorial plate.


Meiosis II: Anaphase II

Centromeres divide. Sister chromatids

separate.– Now called daughter

chromosomes


Meiosis II: Telophase II

Similar to telophase and cytokinesis in mitosis


Summary of Meiosis II

http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=related

Meiosis vs. Mitosis

Meiosis is characterized by 4 features:– 1. Synapsis and crossing over– 2. Sister chromatids remain joined at their

centromeres throughout meiosis I– 3. Kinetochores of sister chromatids attach to the

same pole in meiosis I– 4. DNA replication is suppressed between meiosis

I and meiosis II.

Meiosis vs. Mitosis

Meiosis produces haploid cells that are not identical to each other.

Genetic differences in these cells arise from:– crossing over– random alignment of homologues in metaphase I

(independent assortment)

Mitosis produces 2 cells identical to each other.


Mitosis vs. Meiosis


Genetic Diversity < The Advantage of Sex

Sexually reproducing organisms– Need two individuals to reproduce– Reproduce more slowly than asexually

reproducing organisms– Have large genetic diversity

When environmental conditions change, they are more adaptable and likely to survive

Genetic diversity is due to different alleles


Genetic Diversity < The Advantage of Sex

Five factors create genetic diversity by creating new alleles, or new combinations of alleles.1. Mutation

2. Crossing-over

3. Segregation

4. Independent assortment

5. Fertilization


Mutations

Mutations are changes in the nucleotide sequence of DNA.

This creates new alleles. New alleles lead to new forms of proteins. Increases genetic diversity


Crossing-over

The exchange of equivalent portions of DNA between homologous chromosomes

Occurs during prophase I when chromosomes are synapsed

Allows new combinations of genetic information to occur– Each gamete then receives some of your

mother’s and some of your father’s genes on each chromosome.


Synapsis and Crossing-over


The Results of Crossing-over


Crossing-over Separates Linked Genes

The closer genes are together on a chromosome, the less likely they will be separated by crossing-over.

– These genes will be inherited together.

The farther genes are apart on a chromosome, the more likely they will be separated by crossing-over.


Segregation

Alleles on homologous chromosomes separate during anaphase I.– Consider a person who has two alleles for insulin,

one allele produces functional insulin, the other is a mutation

Half of that person’s gametes would get the gene for functional insulin (I).

Half of the gametes would get the gene for nonfunctional gametes (i).

– If this gamete were used during fertilization, and was joined with another mutant insulin gamete, the offspring would not be able to make functional insulin.


Independent Assortment

The segregation of homologous chromosomes is independent of how other homologous pairs segregate.

Consider two pairs of chromosomes.– Given the two ways these pairs can line up on the

equatorial plate, there are four possible combinations of chromosomes in gametes.


Independent Assortment


Fertilization

Due to the large number of possible gametes resulting from independent assortment, segregation, mutation and crossing-over,– A large number of different offspring can be

generated from two parents.

Since gametes join randomly– The combinations of alleles is nearly infinite.


Nondisjunction and Chromosome Abnormalities

Nondisjunctions occur when homologous chromosomes do not separate during cell division.– Frequently results in the death of the cells– Some abnormal gametes live.

When these gametes participate in fertilization, the offspring will have an abnormal number of chromosomes.

– Monosomy describes a cell that has just one of a given pair of chromosomes.

– Trisomy describes a cell that has three copies of a given chromosome.


Nondisjunction During Gametogenesis


Karyotypes are a Picture of a Person’s Chromosomes


A Karyotype can Reveal Trisomy 21

Down syndrome– Three copies of

chromosome #21– Results in 47 chromosomes

instead of 46– Symptoms include:

Thickened eyelids Mental impairment Faulty speech


Determination and Differentiation

During sexual reproduction, fertilization of an egg by a sperm results in a single-celled zygote.

The zygote undergoes mitosis to develop into an adult.

As mitosis occurs, cells must become specific cell types.

– All body cells are genetically identical.– Cells only differ in the genes they express.– Determination is the process a cell goes through to select which

genes it will express, committing itself to becoming a certain cell type.

– When a cell is fully developed into a specific type of cell, it is said to be differentiated.


Determination and Differentiation of Skin Cells

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