Chapter 9 Cell Division-Proliferation and Reproduction

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Chapter 9Cell Division-Proliferation and
Reproduction
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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

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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
  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

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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.

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Chromosomes

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

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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.
• G1 Stage
• S Stage (Synthesis)
• G2 Stage

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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 cells
  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.

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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.

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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.

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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.

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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)

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Terminology

• Homologous chromosomes have the same appearance
  and genes.
• Diploid cells will have many sets of homologous
  chromosomes

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Terminology

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

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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.

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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.

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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.

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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

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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.

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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

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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.

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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

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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.

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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.

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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.

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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

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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.

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Genetic Diversity Independent Assortment

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

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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

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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.

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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.

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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.

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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.