Meiosis

1
Meiosis

• The process of creating sex cells

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What is Meiosis?

• Meiosis involves two successive nuclear
  divisions that produce four haploid cells. The
  first division (meiosis I) is the reduction
  division the second division (meiosis II)
  separates the chromatids.

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Advantages of Meiosis

• Mitotic cell division produces new cells
  genetically identical to the parent cell.
• Meiosis increases genetic variation in the
  population.

4
Interphase

• Interphase is an important stage preceding
  meiosis. Without this stage meiosis would not
  occur.
• During this stage, each individual chromatid
  replicates, similar to mitosis..
• At this stage, the chromosomes are long and
  stringy and are not visible.
• Remember All somatic cells are diploid in
  number (2n), therefore for each chromatid there
  also exists its homolog, which also replicates
  during interphase.

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The graphical representation here shows that each
chromotid pair is connected by a centromere (the
gold structure). Each pair of the chromosome are
referred to as a sister chromatids

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Prophase I

• Prophase I is one of the most important stages of
  meiosis.
• The chromotid threads begin to twist and
  condense, creating chromosomal structures which
  are visible to the microscope.
• Each chromosome then actively seeks out its
  homologous chromosome, as shown in the graphical
  representation.
• After the homologous chromosomes pair, the
  structure is referred to as a tetrad (four
  chromatids). The point at which two non-sister
  chromatids intertwine is known as a chiasma.
• Sometimes a process known as crossing over occurs
  at this point. This is where two non-sister
  chromatids exchange genetic material. This
  exchange does not become evident, however, until
  the two homologous pairs separate.

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Prophase I
Cell Photographs of Prophase I
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Metaphase I

• At metaphase, each chromosome has reached its
  maximum density.
• The homologous pairs and their sister chromatids
  also prepare for separation.
• They interact with spindle fibers which form from
  either side of the nuclear envelope of the cell.
• The spindle fibers are produced from a structure
  called a centriole.
• There is a centriole at opposite ends of the
  cell, which is referred to as poles.
• During metaphase, the chromosomes are lined by
  the spindle fibers at what is known as the
  metaphase plate.
• As in the cell picture, it looks like a line
  through the center of the cell.

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Metaphase I
10
Anaphase I

• Anaphase I pulls apart the tetrad, separating
  each homologous chromosome.
• In the graphical representation in the next
  slide, the affects of crossing over are evident.
  It is by random chance that a certain chromosome
  is pulled to a certain pole.

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Anaphase I
12
Telophase I

• Telophase I varies from species to species.
• Sometimes Telophase I is skipped and meiosis
  starts its second division immediately.
• In general, however, two nuclear envelopes begin
  to surround the separate chromosomes and
  cytokinesis (splitting of the cytoplasm into two
  separate entities) will sometimes occur. Then a
  phase called interkinesis will follow, which
  essentially is a resting period from Telophase I
  to Prophase II.
• This differs from mitosis because DNA replication
  does not occur.

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Telophase I
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Prophase II

• During Prophase II, each dyad (1/2 a tetrad) is
  composed of a pair of sister chromatids and they
  are connected by a centromere.
• The centrioles (replicated during Telophase I)
  which produce the spindle fibers also start to
  move toward the poles of the cell.

15
Prophase II
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Metaphase II

• Metaphase II is similar to Metaphase I in that
  the dyads are lined up at a metaphase plate by
  the spindle fibers.

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Anaphase II

• Anaphase II separates the dyads into individual
  chromatids.
• Each sister chromatid ends up on one side of the
  cell.

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Telophase II

• At the end of Telophase II, the nuclear envelopes
  forms around each set of DNA and the cytoplasm
  divides once again.
• As a result, four haploid cells have formed from
  one diploid cell.
• The chromosomal content of a haploid cell in
  one-half the chromosomal content of a diploid
  cell ( n as opposed to 2n )

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Cell Photograph of Telophase II