Chapter 9Cell Division-Proliferation and

Reproduction

Cell Division

Cell Division• Cell Division is where one cell becomes two cells.

– Asexual Reproduction is where one parent cell divides into two identical cells.

– Sexual Reproduction is where genetic information from two parent cells to create a unique individual cell.

– Three Types of Cell Division• Binary Fission

– Asexual Reproduction

• Mitosis– Asexual Reproduction

• Meiosis– Sexual Reproduction

Binary Fission• Binary Fission is form of asexual reproduction

and cell division used by all prokaryotic and some eukaryotic organisms.

1. The parent cell DNA duplicates, and the cell elongates2. Cell wall and plasma membrane begin to divide3. Cross-wall forms completely around divided DNA4. Cells separate, there are now two daughter cells that are

identical

Binary Fission

Human Genome• Humans have 23 pairs of chromosomes. This makes

a total of 46 chromosomes.• A karotype is a description of the number, size, and

shape of chromosomes.

Somatic and Sex Cells• In humans, somatic (body) cells have two sets

of chromosomes, a total of 46.• These cells are called diploid or 2N.

• In humans, gametes (sperm or eggs) have one set of chromosomes, a total of 23. These are also known as sex or germ cells.

• Gametes have exactly one copy of each chromosome.

• These are known as haploid or N.

Chromosomes• What is a chromosome?

– A chromosome is double-stranded DNA molecules coiled into a short, compact unit containing genetic material.

• Chromatin is DNA wrapped around histone proteins. This is to keep them from getting tangled.

Chromosomes

• Uncoiled, each of your chromosomes is about two inches long.• So, imagine all 46 of your chromosomes into a

cell’s nucleus that is 5 microns in diameter.• This is the equivalent of fitting 46 strings, each the

length of a football field into a baseball.

Chromsomes

• A chromatid is one of two parallel parts of a chromosome.

• Sister chromadtids are identical copies of a chromosome, attached by a centromere.

Cell Cycle• The cell cycle consists of all of the stages of

growth and division for eukaryotic cells.– It is divided into Interphase and Mitosis.

Interphase• Interphase is where cells will continue in normal

metabolic activities and begins to prepare for cell division. Interphase is the longest stage of the cell cycle.– G1 Stage

– S Stage (Synthesis)– G2 Stage

G1 Stage

• In the G1 stage of interphase, the cell will gather nutrients and other resources from its environment.– G1 is the “first growth” phase during the cell’s

primary growth phase– If a cell will remain in the G1 phase for an

extended period of time, it may be also called the G0 stage. This is because it is not moving forward in the cell cycle.

S & G2 Stage• In the S Stage of Interphase, DNA replication

occurs in the cell.– This leaves a cell with two identical copies of DNA

for later on in the cell cycle.

• The final part of interphase is the G2 Stage.• Final preparations are made before going into

mitosis.• This includes making the proteins used for moving the

chromosomes.

Mitosis• After interphase, mitosis is the process of the cell cycle

where one parent cell will divide into two daughter cells. During mitosis, all cellular activity ceases.

• Mitosis is divided into four parts.• Prophase• Metaphase• Anaphase• Telophase

• Cytokinesis (division ofcytoplasm) typically follows telophase.

Prophase• The first stage of mitosis is prophase. There

are three important parts of prophase.• Chromosomes condense• Spindle and spindle fibers form• Nuclear membrane disassembles.

Prophase• In early prophase, two sets of microtubules,

known as centrioles, begin to separate and move to opposite poles of the cell.• Attached to the centrioles, spindle fibers will help

move the chromosomes later in mitosis.

Prophase• In later prophase, chromosomes begin to appear as 2

chromatids connected at the centromere. • The nucleolus and nuclear membrane have

disintegrated. • The spindle fibers have moved farther apart. • Chromosomes attach to the spindle fibers.

Metaphase• Metaphase, the second

stage of mitosis, is where chromosomes align at the equatorial plane of the cell.

• At the end of metaphase, the centromere holding the sister chromatids together divides.• Each chromatid is now

known as a daughter chromatid.

Anaphase• Anaphase, the third stage of mitosis, the sister

chromatids move toward opposite ends of the cell.• The kineochore is a

protein attached toeach chromatid at thecentromere.

Telophase• In telophase, the cell finishes mitosis

• Spindle fibers disassembles.• Nuclear membrane re-forms• Chromosomes uncoil• Nucleolus reforms

Cytokinesis• At the end of telophase, cytoskinesis occurs.

Cytokinesis is when the cytoplasm separate to form two separate cells.• For cytokinesis in animal cells, a cleavage furrow is

formed. A cleavage furrow is an indention of the plasma membrane that pinches to the center of the cell. This splits the cell in two.

Cytokinesis

• Yet, in plant cells, cytoplasm division occurs with a cell plate. The cell plate begins to form at the center of the cell and grows outward to the cell membrane.

• After cytokinesis,you have two daughter cells. Two

exact copies.

Controlling Mitosis• During cell division, there are checkpoints

which use proteins to evaluate the health of a cell.

• Proto-oncogenes will code for proteins that provide signals to the cell that will encourage cell division.

• Tumor-supressor genes will code for proteins to signal stopping cell division

p53 Protein• The p53 gene will code for the p53 protein, which is

a type of tumor-supressor gene.• If it detects damaged DNA at the end of the G1

phase, it will send out enzymes to fix the problem.• If the cell is deemed healthy afterwards, it is able to

undergo cell division.

p53 Protein• Yet, if the damage is too far beyond repair,

the p53 protein will cause the cell to digest itself from the inside out, known as apoptosis.

• The other healthy cells will undergo cell division to replace the lost cell.

p53 Protein• What is the p53 gene is mutated?

– It would code for a p53 protein that might not be able to monitor for damaged DNA.

• Mutations to the p53 gene appear in 40% of all cancers.

Cancer

• Cancer is a disease caused by the failure to control cell division.

– Mutagens are agents that mutate or chemically damage, DNA.

– Carcinogens are mutagens that cause cancer.• Tar in cigarette smoke is categorized as both a

mutagen and a carcinogen.

Cancer• There are several agents that gave been associated with

high risks of cancer:• Radiation

– X-Rays– UV-A from tanning lamps, UV-B

• Chemicals– Arsenic– Benzene– Asbestos– Food containing nitrates

• Some have a prediposition to cancer due to their genetic backgrounds. Mutated DNA may be passed from parent to offspring

Cancer• When uncontrollable mitotic division

occurs, a group of cells form called a tumor.– Tumor is a mass of cells not normally found

in a certain portion of the body.• Benign tumor is a cell mass that does not

fragment and spread beyond its original area of growth.

• Malignant tumors can spread through other parts of the body.

– Cells of malignant tumors metastasize, or move from the original site and begin to grow new tumors in other regions of the body.

Terminology

• Haploid is a cell with ONE set of chromsomes (N)• Diploid is a cell with TWO sets of chromosomes.

(2N)• Ultimately, one set comes from the haploid cells

provided by each parent (N + N = 2N)

Seed shape

lor

Seed shape

Leaf color

Terminology

• Homologous chromosomes have the same appearance and genes.

– Diploid cells will have many sets of homologous chromosomes

Flower color

Plant height

Flower color

Plant height

Terminology

• Non-homologous chromosomes have different genes on their DNA.

TerminologyDuring the preparations for meiosis, the

genetic information in a cell will be copied into sister chromatids.

At this point, sister chromatids are identical.

The purple sister chromatids on the right are homologous chromosomes to the green sister chromatids on the left because:

1. Same Genes

2. Similar Shapes

3. The centromeres holding sister chromatids are in the same location along the chromosomes.

Fig. 9.20, pg. 175

Sexual Reproduction• Meiosis transforms diploid cells (2N) into haploid

daughter cells (N)– Meiosis occurs only in diploid cells, not in haploid cells.

• FertilizationFertilization, the fusion of two haploid cells, causes the transition from haploid (N) to diploid (2N).– Haploid cells that fuse are called gametesgametes– This first diploid cell is called a zygote.This first diploid cell is called a zygote.

• Depending on the organism, mitosis (asexual reproduction) may occur in haploid cells only, diploid cells only, or in both haploid and diploid cells.

Alternation of Generations-Human•In humans, the only haploid stages are gametes.•In males, spermatozoaspermatozoa begins at puberty and continues daily until at least age 70.•In females, oogenesisoogenesis begins as a fetus.•This egg completes meiosis after fertilization by a sperm.

Alternation of Generations-Plant•In flowering plants and conifers, the dominant phase of the life cycle is also diploid.•Haploid cells are produced in anthers and in ovaries.•These haploid cells undergo a few cycles of mitosis to produce multicellular haploid stages.•Fusion of sperm (N) with egg (N) produces a zygote (2N)

Meiosis• In meiosis, chromosome copy once, divide twice.

– Like mitosis, meiosis is preceded by interphase.• This is followed by two rounds of separating

chromosomes to end up with four haploid cells.

Meiosis• Meiosis

• Meiosis I• Prophase I• Metaphase I• Anaphase I• Telophase I

• Meiosis II• Prophase II• Metaphase II• Anaphase II• Telophase II

Interphase

• During interphase, the chromosomes are copied and the copies (sister chromatids) remain attached at the centromere.

• The machinery necessary to move chromosomes (centrioles, etc.) is copied too.

Meiosis I•Prophase 1 begins as the chromosomes condense for packing.•Spindle fibers begin to form.•Homologous chromosomes pair up, these are known as tetrads.•Homologous chromosomes typically exchange some sections of DNA in a process called crossing over

Meiosis I•In metaphase I, all the tetrads are lined up midway between the two poles. •Anaphase I begins when the homologous chromatids in the tetrads release each other.

•Spindle fibers pulls the homologous chromatids to opposite poles of the cell.•The homologous chromosomes are separated from each other = segregated.

Meiosis I

•In telophase I, the chromosomes begin to uncoil, the new nuclei form, and the cytoplasm splits.•The end of meiosis I produces two daughter cells, each with only half the unique genetic information of the parent

Meiosis II

•The second cycle of division in meiosis is just like mitosis, except the chromosomes are not copied during interphase.•Spindle fibers attach to the centromeres of sister chromatids during prophase II.

Meiosis II

•By metaphase II, all the sister chromatids are lined up midway between the two poles.•Anaphase II begins when the centromeres holding the sister chromatids split and the sisters become daughters.

Meiosis II

•In telophase II, the new nuclei begin to form, chromosomes unwind, and the cell begins to divide.•During spermatogenesis, cytokinesis is equal.•During oogenesis, cytokinesis is unequal.

Meiosis•Meiosis is the key to sexual reproduction, which generates populations of genetically diverse individuals.•Among these individuals will be those whose characteristics allow them to thrive in a changing environment.

Genetic Diversity: Mutations• Five processes are responsible for producing offspring

which differ genetically from their parents.• 1) Mutations: from point mutations to chromosomal

mutations• Cystic fibrosis is a common lethal genetic disorder in the

U.S.• It is caused by a mutation to a gene which produces

proteins that regulate mucus production.

Genetic Diversity: Segregation• As a result of meiosis, each haploid cell will inherit

only one of the two alleles present in one parent.• Because homologous chromosomes end up in

different cells during Meiosis I, the two parental alleles are segregated (separated).

• Gametes will have either the allele for type A blood or type O blood

Genetic Diversity: Crossing Over•During prophase of Meiosis I, homologous chromosomes exchange sections of DNA - crossing over.•This moves some alleles from one homologous chromosome to another•This creates new combinations of alleles.•Without crossing over, the purple homologous chromosome has alleles for type O blood, attached earlobes, and normal hemoglobin.

•After crossing over, one purple chromatid has alleles for type O blood, attached earlobes, and sickle-cell hemoglobin

Genetic Diversity: Crossing Over• Without crossing over, meiosis produces two

unique haploid cells.• After a single cross-over, there are now four

unique haploid cells.

Genetic Diversity: Crossing Over• Crossing over can happen at multiple places between

homologous chromosomes during Prophase I.• This additional crossing over events creates more

unique combinations of alleles.• During meiosis in humans, 2-3 crossing over events

occur between each pair of homologous chromosomes.

Genetic Diversity: Independent Assortment

• We know that each haploid cell will end up with one of two homologous chromosomes from segregation.

Genetic Diversity: Independent Assortment

•But the arrangement of one pair of homologous chromosomes has no impact on the arrangement of non-homologous chromosomes are inherited independently•Occurs in Metaphase.•Independent Assortment

Genetic Diversity: Independent Assortment• In general, the number of possible

combination of non-homologous chromosomes due to independent assortment • 2N, where N = # of haploid chromosomes• If N = 1, then there are 21 = 2 unique gametes.• If N = 2, then there are 22 = 4 unique gametes.• If N = 3, then there are 23 = 8 unique gametes.• What about humans?

• If we add in mutations and crossing over, each gamete is truly unique.

Genetic Diversity: Random Fertilization• During fertilization, one unique sperm combined

with one unique egg to create a zygote, the initial diploid stage.

• In humans, that means one of the 8.3 x 106 unique sperm will combine with one of the 8.3 x 106 unique eggs.

• Each zygote is the product of these unique combinations.

• Leads to 70 trillion unique compositions of diploid cells.

Nondisjunction•In meiosis, the number of chromosomes in diploid cells is reduced to haploid. However, there are cases, where homologous chromosomes do not segregate properly.•Nondisjunction occurs when homologous chromosomes do not separate during cell division.

Nondisjunction•If one of these abnormal gametes unites with a normal gamete, the offspring will have an abnormal number of chromosomes.•In monosomy, a cell has just one pair of homologous chromosomes.•In trisomy, a chromosome is present in three copies. •Down syndrome is a result

of trisomy-21.