Chapter 9 Cell Division-Proliferation and Reproduction

Cell Division

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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

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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 elongates 2. Cell wall and plasma membrane begin to divide 3. Cross-wall forms completely around divided DNA 4. Cells separate, there are now two daughter cells that are identical

Binary Fission

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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.

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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.

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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.

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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.

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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.

– G 1 – Stage S Stage (Synthesis) – G 2 Stage

G

1

Stage

• In the G 1 stage of interphase, the cell will gather nutrients and other resources from its environment.

– G 1 is the “first growth” phase during the cell’s primary growth phase – If a cell will remain in the G 1 phase for an extended period of time, it may be also called the G 0 stage. This is because it is not moving forward in the cell cycle.

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S & G

2

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 G 2 Stage.

• Final preparations are made before going into mitosis.

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

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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 of cytoplasm) typically follows telophase.

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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.

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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.

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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.

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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.

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Anaphase

Anaphase, the third stage of mitosis, the sister chromatids move toward opposite ends of the cell.

The kineochore is a protein attached to each chromatid at the centromere.

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Telophase

In telophase, the cell finishes mitosis • Spindle fibers disassembles.

• Nuclear membrane re-forms • Chromosomes uncoil • Nucleolus reforms

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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 .

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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

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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.

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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.

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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.

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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

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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.

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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

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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.

Terminology

During 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

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Sexual Reproduction

Meiosis transforms diploid cells (2N) into haploid daughter cells (N) – Meiosis occurs only in diploid cells, not in haploid cells.

Fertilization, the fusion of two haploid cells, causes the transition from haploid (N) to diploid (2N).

– Haploid cells that fuse are called gametes – 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, spermatozoa begins at puberty and continues daily until at least age 70.

•In females, oogenesis 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)

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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.

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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.

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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.

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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

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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.

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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 • 2 N , where N = # of haploid chromosomes • If N = 1, then there are 2 1 = 2 unique gametes.

• If N = 2, then there are 2 2 = 4 unique gametes.

• If N = 3, then there are 2 3 = 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 10 6 unique sperm will combine with one of the 8.3 x 10 6 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.