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Biology 1308Chapter 5

Cell Division

Introduction◈ Cell division allows a zygote to grow into an organism with trillions of cells◈ The purposes of cell division are:reproduction, growth, & repair◈ The continuity of life from 1 cell to another is known as the cell cycle.

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Cell Division◈ Somatic cells are the body cells that continue to undergo cell division into adulthood◈ Diploid (2N) cells w/ a pair of chromosomes

-Human somatic cells have 46 chromosome

-1 set from each parent◈ Gametes are the reproductive cells(egg & sperm) ◈ Haploid (1N) cells w/half of chromosome #

-Egg & sperm cells have 23 chromosomes 4

Chromosomes◈ Chromatin, thin fibers of DNA & histone proteins, replicate & condense to form coiled chromosomes.◈ chromosomes only appear during cell division◈ Each duplicated chromosome consists of 2strands; sister chromatids◈ The area where strands connect is the Centromere.◈ Each cell has about 2 meters (~6ft)of DNA

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Cell Cycle Stages◈ The Cell Cycle includes:

-Interphase - divided into 3 stages The cell spends

90% of the time in interphase◇ G1 ◇ S ◇ G2

-Mitotic Stage◇Mitosis - division of the nucleus◇ Cytokinesis - division of the cytoplasm7

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The Stages of Interphase◈ G1: organelles replicate & accumulates material for

DNA synthesis◆G0 occurs in some cells, (nerve & muscle) which

do not complete the cell cycle◈ S: DNA replicates◆After DNA replication, each chromosome consists

of two sister chromatids◈ G2: the cell synthesizes proteins for cell division◆Final stage before mitosis begins9

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Mitosis◈ Mitosis, the division of the nucleus. -- Distributed to two daughter nuclei- Asexual reproduction◈ Cytokinesis- the division of the cytoplasm◆Results in 2 daughter cells that are

identical to the mother cell. 10

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Control of the Cell Cycle ◈ The cell cycle is controlled by both types of signal.◆Internal signals ensure the stages

follow in the normal sequence.◆They also check that each stage is properly completed prior to the next stage.◆External signals tell the cell whether or not to divide. (growth factors)

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◈ 3 checkpoints control the cell cycle.◆G1 – stops if DNA is damaged; before S ◆G2 - stops if DNA did not finish replicating; after S◆M – stops if chromosomes are not going to be properly distributed◈ Checkpoints are critical for preventing cancer

development.◈ A damaged cell should not complete mitosis

Control of the Cell Cycle

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2 genes control the cell cycle.◈Proto-oncogenes -encode proteins that promote the cell cycle & prevent apoptosis (cell death)◆Mutate to become oncogenes (cancer

causing genes)◈Tumor suppressor genes -encode proteins that stop the cell cycle & promote apoptosis

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Control of the Cell Cycle & Cancer

◈ Cancer is uncontrolled cell growth◈ Carcinogenesis is the development of cancer◈ Carcinogens are environmental agents that may cause cancer◈ Proto-oncogenes genes may mutate to become oncogenes -cancer causing agents◈ When tumor suppressor genes mutate, they can no longer inhibit the cell cycle

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Control of the Cell Cycle & Cancer

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Mitosis - Animal Cell◈ Division of the nucleus that forms 2 daughter cells◈ Clones of parent cell◈ 4 stages are◆ Prophase◆Metaphase◆ Anaphase◆ Telophase

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Prophase◈ Chromatin condenses to form chromosomes ◈ Nuclear membrane breaks down ◈ Nucleolus disappears◈ Centrioles migrate to opposite poles ◈ Spindle fibers appear between centrioles◈ Centromeres attach to spindle fibers

- Chromosomes have 2 sister chromatids17

Metaphase◈ Spindle fibers and asters visible◈ Chromosomes line up at the metaphase plate or equator

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Anaphase◈ Centromeres divide, & sister chromatids are moved to opposite poles by spindle fibers.◆ Kinetochore spindle fibers shorten, pulling

daughters◆ Polar spindle fibers push the poles apart◆ Each pole receives same # of chromosomes

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Telophase◈ Nuclear membrane reforms ◈ Spindle fibers disappears ◈ Nucleolus reappears◈ Chromosomes uncoil & return to chromatin◈ Nucleolus appears in each daughter nucleus◈ Cytokinesis begins

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Cytokinesis◈ Division of the cytoplasm◈ May begin in late anaphase but does not reach completion until the beginning of interphase◈ Animal cells form a cleavage furrow where a contractile ring pinches the cells in half◈ Rigid cell wall of plant cells prevent them from pinching in half, instead they form a cell plate between the 2 cells

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Differences between Plant & Animal Cell ÷

Plant Cell◈ No centrioles◈ Cell plate◈ No cleavage furrow

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Animal Cell◈ Centrioles◈ No cell plate◈ Cleavage furrow

Animal Cell Mitosis

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Meiosis

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◈ Type of cell division that occurs in gametes◈ Sexual reproduction◈ Requires 2 cell divisions◈ Purpose of meiosis is to reduces the chromosome number in half◈ Allows for variations within the population◈ Provides offspring with unique characteristics ◈ Begins with 1 diploid parent cell◈ Ends with 4 haploid cells ◈ Form homologous chromosomes

Meiosis I◈ Homologues line up side by side at the equator (known as synapsis)◈ When homologous pairs separate, each daughter cell receives 1 member of the pair.◈ The cells are now haploid.

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Meiosis I◈ No replication of DNA between meiosis I & II◈ Centromeres divide & sister chromatids migrate to opposite poles to become individual chromosomes.◈ Each of the 4 daughter cells have the haploid chromosome number (23)◆ Each chromosome is composed of one

chromatid28

Meiosis II and Fertilization◈ Fertilization◆Daughter cells of meiosis mature into gametes◇Sperm and eggs fuse in fertilization◆Fertilization restores the diploid number.◇Fertilized cell is known as a ZYGOTE◇Zygote will develop into a new individual

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◈ Meiosis I is divided into 4 phases◆ Prophase I◆ Metoaphase I◆ Anaphase I◆ Telophase I◈ Meiosis help ensure genetic variation◈ Genetic variation occurs in 2 ways◆ Crossing over◆ Independent Assortment30

Meiosis I

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Prophase I◈ Nuclear membrane breaks down◈ Spindle fibers appear ◈ Nuclear membrane & nucleolus disappears◈ Synapsis occurs; pairing of homologues ◈ Homologues line up side by side & crossing-over occurs; where parts of chromatid from each homologue breaks off & joins to the opposite side. ◈ New chromosomes have genes unique to parents

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Crossing-Over During Prophase I

Metaphase I ◈ Metaphase I◆Homologous pairs line up at metaphase plate

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Anaphase I ◈ Independent assortment occurs when homologues separate from each other◈ Results in cells with different # of maternal & paternal chromosomes◈ In humans, with 23 pairs of chromosomes, the number of possible combinations is 223, or 8,388,608

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Telophase I ◈ May or may not occur at end of meiosis I◈ Nuclear membrane reforms◈ Nucleolus reappear ◈ Cytokinesis may occur, producing 2 daughter cells which are haploid

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Interkinesis◈ Interkinesis◆Period of time between Meiosis I & Meiosis II◆No replication of DNA

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Meiosis II◈ Prophase II◈Metaphase II◈Anaphase II◈Telophase II◈ Cytokinesis

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Prophase II◈ Cells have 1 chromosome from each homologous pair.◈ A spindle appears and the nuclear envelope disassembles.◈ Each duplicated chromatid attaches to the spindle.◈ The nucleolus disappears.

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Metaphase II◈ Sister chromatids line up at the metaphase plate.

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Anaphase II◈Sister chromatids separate and become daughter chromosomes that migrate toward the poles.

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Telophase II◈Spindle fibers disappear◈Nuclear membrane reforms◈ Cytokinesis begins◈After completion of cytokinesis the results will be 4 haploid daughter cells

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Importance of Meiosis◈ Combining of chromosomes from different gametes help ensure offspring are not identical to parents.◈ Genetic variability is the main advantage of sexual reproduction.◈ Long-term, genetic variation increases the survival of a species.

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Mitosis & Meiosis I Comparison Chart

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MEIOSiS I◈ Homologous chromosomes pair up & crossing over in Prophase I◈ Paired homologous chromosomes line up at equator in Metaphase I◈ Homologous chromosome separate & move to opposite poles in Anaphase I◈ 4 unique haploid daughter cells in Telophase II

MITOSIS◈ No pairing or crossing over in Prophase ◈ Chromosomes line up at equator in Metaphase◈ Centromeres split & sister chromatids move to opposite poles in Anaphase◈ 2 identical to parent, diploid daughter cells in Telophase

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Mitosis & Meiosis II Comparison Chart

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MEIOSiS II◈ No pairing of chromosomes in Prophase II◈ Haploid # of chromosomes line up at equator in Metaphase II◈ Sister chromatids separate & move to poles in Anaphase II

MITOSIS◈ No pairing of chromosomes in Prophase ◈ Diploid # of chromosomes at equator in Metaphase◈ Sister chromatids move to opposite poles in Anaphase

Spermatogenesis & Oogenesis ◈ Meiosis in the testes of males is

called spermatogenesis.◆Produces sperm◈ Meiosis in the ovaries of females is

called oogenesis.◆Produces eggs 47

Spermatogenesis◈ Process begins at puberty & continues throughout life◈ Primary spermatocytes (2n) divide in meiosis I to form 2 secondary spermatocytes (1n).◈ Secondary spermatocytes divide in meiosis II to produce four spermatids.◈ Spermatids then mature to form sperm (spermatozoa)

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Oogenesis◈ Begins in the fetus◆ Primary oocytes are arrested in prophase I in ovary◈ Resumes at puberty◆ 1 primary oocyte continues the process of meiosis during each menstrual cycle.◆ Primary oocyte (2n) divides in meiosis I to produce one secondary oocyte (1n) and one polar body (1n).

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Oogenesis◈ Division is unequal as secondary oocyte receives most of the cell contents & half the chromosomes◈ If the secondary oocyte is fertilized, meiosis II will proceed.◆ Another unequal division will occur, with the egg

receiving most of the cytoplasm.◆ A second polar body is also formed.◈ If the secondary oocyte is not fertilized, it disintegrates

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