MITOSIS / MEIOSIS

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MITOSIS / MEIOSIS

• Lab 7

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OBJETIVES

• Name the stages of the cell cycle and describe
  their characteristics.
• Name the phases of mitosis and meiosis and
  describe their characteristics.
• Identify the phases of mitosis and meiosis from
  diagrams, pictures or micrographs.
• Compare the processes and end products of mitotic
  and meiotic cell division.
• Describe the significance of mitotic and meiotic
  cell divisions.

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• Be able to define the following terms in writing
• a- chromosome l- spindle
• b- chromatid m- equatorial
  plane
• c- centromere n- cytokinesis
• d- chromatin o- furrow
• e- nucleolus p- cell plate
• f- centriole q- daughter
  cell
• g- poles
• h- diploid
• i- crossing-over
• j- haploid
• k- tetrad

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Cell Cycle period between two sequential
divisions

• Interphase
• Growth (G1),
• Synthesis (S),
• Growth (G2)
• Mitotic phase
• Mitosis and cytokinesis

Figure 3.30
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Mitosis

• Growth in our bodies, and in those of other
  many-celled organisms, is basically a process of
  increasing the number of cells. This involves two
  processes the distribution of copies of the
  genetic information from the parent cell to the
  two daughter cells and the cytoplasmic division.
  In this way, a body grows or wounds are repaired.
  Each of the new cells will include all the
  genetic information possessed by all of the other
  living cells of the body.

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• Mitosis is an orderly series of events that flow
  without interruption. We have artificially
  divided this smooth flow into stages prophase,
  metaphase, anaphase, and telophase. It is a
  convenience to be able to classify parts of the
  process in this way, but it can be misleading if
  we forget that there is really no pause or
  interruption in the events.
• When we look at a plant or animal cell that has
  been killed and stained while in the process if
  mitosis, we are looking at the stopped action.

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Early and Late Prophase

• Asters are seen as chromatin condenses into
  chromosomes
• Nucleoli disappear
• Centriole pairs separate and the mitotic spindle
  is formed

Fragments of nuclear envelope
Polar microtubules
Pair of centrioles
Early mitotic spindle
Centromere
Aster
Kinetochore
Kinetochore microtubule
Spindle pole
Chromosome, consisting of two sister chromatids
Late prophase
Early prophase
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Metaphase

• Chromosomes cluster at the middle of the cell
  with their centromeres aligned at the exact
  center, or equator, of the cell
• This arrangement of chromosomes along a plane
  midway between the poles is called the metaphase
  plate

Metaphase plate
Spindle
Metaphase
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Anaphase

• Centromeres of the chromosomes split
• Motor proteins in kinetochores pull chromosomes
  toward poles

Daughter chromosomes
Anaphase
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Telophase and Cytokinesis

• New sets of chromosomes extend into chromatin
• New nuclear membrane is formed from the rough ER
• Nucleoli reappear
• Generally cytokinesis completes cell division

Nucleolus forming
Contractile ring at cleavage furrow
Nuclear envelope forming
Telophase and cytokinesis
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Cytokinesis

• Cleavage furrow formed in late anaphase by
  contractile ring
• Cytoplasm is pinched into two parts after mitosis
  ends

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Meiosis

• If a species is to retain the original number
  of chromosomes and produce offspring with
  chromosomes from both male and female parent,
  then the parents must have a mechanism to produce
  cells with half the number of chromosomes. The
  production of these cell with the haploid number
  (one-half of the parental number of chromosomes)
  results from the special cell division called
  Meiosis.
• Meiosis consists of two cell divisions.

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Meiosis

• Many organisms reproduce sexually. Sexual
  reproduction means the formation of new
  individual by a combination of two sex cells
  (gametes). Gametes are product of meiosis and
  they are haploid
• Meiosis is the type of the cell division that
  reduces the number of chromosomes in the daughter
  cells by halve.
• Meiosis is the process that includes two
  consecutive cell divisions. First (Meiosis I) or
  reduction. Second division (Meiosis II) or
  equation division.

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• During the first division of meiosis the
  chromosomes line up at the equator of the cell in
  pairs. Each pair separates and one member of the
  pair moves to one pole the other member of the
  pair moves to the other pole. When the two nuclei
  reorganize and cytokinesis occurs, the resultant
  two cells have one-half as many chromosomes as
  the testis cell had. Reduction from the diploid
  number of chromosomes to the haploid number of
  chromosomes has been accomplished.

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Meiosis I or Reduction division

• Prophase I
• Chromosomes descondensed even farther.
  Homologous chromosomes pair and form very close
  contacts. As a result of synapses, the exchange
  of DNA (crossing over) between sister chromatids
  may occur.
• Pairs of homologous chromosomes are called
  bivalents (2 chromosomes and 4 chromatides).

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Meiosis I (cont)

• Prometaphase I
• The nuclear m. dissapears.One kinetochore
  forms per chromosomes some rather than one per
  chromatid, and the chromosomes attached to
  spindle fibers begin to move.
• Metaphase I
• Bivalents, each composed of two chromosomes (four
  chromatids) align at the metaphase plate. (50/50
  chance for the daughter cells to get either
  mother or fathers homologues)

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Meiosis I (cont)

• Anaphase I Homologous chromosomes separate and
  move to the opposite poles. There is haploid set
  of chromosomes at each pole but each chromosome
  has two chromatids. Each daughter receive the
  reduce number of chromosomes. For this reason,
  the first division of meiosis is called the
  reduction division . In this stage occurs a
  misbalance between chrom. and DNA , to restore
  the balance , the second meiosis is needed

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Meiosis I (cont)

• Telophase I Nuclear envelopes may or may not
  form around the chromosomes. Chromosomes stay
  condensed
• Interkinesis This stage is analagous of
  cytokinesis of mitosis. Two complete daughter
  cell form. Chromosomes stay condensed

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• Each of the two cells produced as a result of
  meiosis I go through a second division (Meiosis
  II).
• This second division results in the formation of
  four cells. During this division, chromosomes
  line up on the equators of the cells. The
  centromeres divide and one of the chromatids
  moves to one pole the other chromatid moves to
  the other pole.

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Meiosis II or Equation division

• Prophase II
• Centrosomes begin to move to oppossite poles of
  the cell and microtubules cross the cell to form
  the mitotic spindle.

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Meiosis II (cont)

• Prometaphase
• Microtubules attach at the kinetochores, and the
  chromosomes begin moving
• Metaphase II
• Spindle fibers align the chromosomes along the
  equator of the cell in the metaphase plate. All
  chromosomes are laying in one plane, their sister
  chromatids leaning toward the opposite poles.
  This organization helps to ensure that each new
  nucleus will receive one copy of each pre-S-phase
  chromosomes

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Meiosis II (cont)

• Anaphase II The paired sister chromatids
  separate at the centromeres and move to opposite
  poles of the cell. Now each chromosome has only
  one DNA molecule.
• ( chromosomes DNA molecules)
• For this reason, the second meiosis division is
  called equation division.
• Telophase II Chromatids arrive to the opposite
  poles of the cell, and new nucleus reconstructed
  around them

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Cytokinesis

• In animal cells, cytokinesis occur when cell
  membrane contracts pinching the cell into two
  daughter cells, each with one nucleus.
• In plant cells, the rigid cell wall requires the
  partitioning membrane to be synthesized between
  the two daughter cells.
• One diploid cell is entered meiosis. Four haploid
  daughter cells emerged as the result of the type
  of cell division.

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• The four cells that result from these meiotic
  divisions may mature into sperm cells. Each cell
  has the haploid number of chromosomes and each
  chromosome is composed of a single chromatid.
  These four cells are not necessarily alike in
  the genetic information that they contain. Each
  cell contains similar packages of information,
  but the information in each package could be
  quite different.