Chapter 10: Cell Division

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Chapter 10 Cell Division
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Stages of the Cell Division Cycle

• A single cell divides into two daughter cells
  when cell division takes place.
• Each daughter cell must have a full complement of
  DNA (chromosomes) from the parent cell.
• In addition, each daughter cell must have enough
  organelles and cytosol to maintain normal
  cellular processes.
• Before division occurs, the components necessary
  for the cell to live, such as DNA and organelles,
  are duplicated.

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The cell division cycle has two major stages

• The two stages of cell division are mitosis and
  interphase (Figure 10.2).
• Mitosis ends with the physical division of the
  parent cell into two daughter cells.
• Interphase is the time between mitoses in an
  actively dividing mammalian cell.

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Interphase prepares the cell for division

• Interphase consists of three major stages S, G1,
  and G2 (Figure 10.2).
• S phase is when DNA is replicated.
• G1 phase and G2 phase are times when the cell
  grows.
• During G1 phase, particular proteins must be made
  and activated for S phase to occur.
• G2 prepares the cell for mitosis as another set
  of proteins promotes the cellular events
  necessary for the physical dividing of the cell.
• Not all cells in the body are actively dividing
  rapidly like skin or intestinal cells.

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Mitosis and Cytokinesis From One Cell to Two
Identical Cells

• 1 to 2 meters of DNA is packed in the nucleus of
  a eukaryotic cell, which has a diameter of less
  than 5 micrometers.
• In order to fit the DNA into such a small space,
  it is packaged with proteins. The DNA protein
  complex is called chromatin.
• Chromatin is further looped and packed to form
  chromosomes (Figure 10.3).
• The central event of mitosis is the equal
  distribution of the parent cell DNA to two
  daughter cells (DNA segregation).

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Each species has a distinctive karyotype

• Each species has different numbers and shapes of
  chromosomes, a karyotype (Figure 10.4a).
• Cells in the human body (except eggs and sperm)
  contain 46 chromosomes each.
• There are 22 homologous pairs and a nonhomologous
  pair of chromosomes (sex chromosomes) in human
  cells.
• Each pair of homologous chromosomes contains one
  chromosome inherited from the mother and one from
  the father.
• The sex chromosomes determine the gender of the
  organismin humans, XX is female and XY is male.
• DNA is duplicated in S phase in preparation for
  cell division.
• During S phase, chromosomes are in the form of
  two identical, side-by-side strands called
  chromatids.
• At the beginning of mitosis, the nucleus of a
  human cell contains twice the usual amount of
  DNA, held together at a constriction called the
  centromere (Figure 10.4b).
• The stages of mitosis are prophase, prometaphase,
  metaphase, anaphase, telophase.

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Karyotyping
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Karyotyping
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Chromosomes become visible during prophase

• The five stages of mitosis are depicted in Figure
  10.5.
• Prophase is the first stage of mitosis.
• Chromatin condenses during prophase and becomes
  visible in the nucleus.
• Centrosomes move to opposite poles of the cell.
• The microtubules begin to grow outward from the
  centrosomes.

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Chromosomes are attached to the spindle during
prometaphase

• Prometaphase is marked by the dissolution of the
  nuclear envelope.
• Spindle microtubules attach to chromosomes at
  protein structures in the centromere called
  kinetochores.

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Chromosomes line up in the middle of the cell
during metaphase

• Spindle microtubules move the chromosomes into a
  plane equidistant from each pole of the cell
  during metaphase.

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Chromatids separate during anaphase

• In anaphase, DNA segregation takes place.
• Spindle microtubules pull one copy of each
  chromosome to each spindle pole.

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New nuclei form during telophase

• Telophase begins when a complete set of
  chromosomes arrives at a spindle pole.
• Spindle microtubules break down, chromosomes
  begin to become less condensed, and nuclei start
  to form.

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In plants, vesicles containing cell wall
components accumulate where the metaphase plate
was, and a new cell wall begins to form.
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Cell division occurs during cytokinesis

• In animal cells, physical separation is performed
  by the contraction of a ring of actin filaments
  between the two new nuclei.
• In plant cells, the new cell wall that began to
  form in telophase is completed.
• Cytokinesis is the end of the cell division cycle.

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Meiosis Halving the Chromosome Number

• Human gametes are produced by meiosis.
• Gametes contain half the number of chromosomes
• The fusion of two gametes is known as
  fertilization.
• A zygote is the result of the fusion of a male
  and female gamete (Figure 10.7).
• Gametes contain half the number of chromosomes as
  other cells in the body, so that the resulting
  zygote is diploid.
• Each gamete contributes one chromosome of each
  homologous pair in the zygote.

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Gametes are haploid, while other cells are diploid

• In humans, gametes have 23 chromosomes, the
  haploid (n) number, whereas all other cells have
  46 chromosomes, the diploid (2n) number.
• Meiosis makes haploid cells from diploid cells
  (Figure 10.8).

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Meiosis I is the reduction division

• The first step in producing haploid gametes is
  halving the number of chromosomes.
• Meiosis I has all the same stages as mitosis
  however, the chromosomes pair differently in
  meiosis I (Figure 10.8).
• During prophase I in meiosis, each chromosome
  pairs with its homologue to form a bivalent pair
  that has four chromatids.
• Microtubules from only one pole attach to each
  chromosome of a homologous pair so that each pair
  is pulled apart at anaphase I.
• The resulting daughter cells of meiosis I are
  haploid (contain one chromosome from each
  homologous pair).

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Meiosis II is similar to mitosis

• The two haploid cells formed after meiosis I go
  through a second round of division, called
  meiosis II.
• Unlike in meiosis I, in meiosis II the chromatids
  separate at anaphase II equally, segregating the
  chromosomes into daughter cells (Figure 10.8).
• The two haploid cells produced after meiosis I
  give rise to four haploid cells (gametes).

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• Shockwave Meiosis