Meiosis

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Meiosis
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Meiosis

• meiosis is the process by which one diploid
  eukaryotic cell divides to generate four haploid
  cells often called gametes.
• meiosis is essential for sexual reproduction and
  therefore occurs in all eukaryotes (including
  single-celled organisms) that reproduce sexually.
• meiosis does not occur in archaea or bacteria,
  which reproduce via asexual processes such as
  mitosis or binary fission.
• a few eukaryotes, notably the Bdelloid rotifers,
  have lost the ability to carry out meiosis and
  have acquired the ability to reproduce by
  parthenogenesis.

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Meiosis

• meiosis is a "one-way" process, it cannot be
  said to engage in a cell cycle as mitosis does.
• exchange of genetic material between maternally
  and paternally derived chromosomes.
• the preparatory steps (G1, S and G2
  Interphase) that lead up to meiosis are identical
  in pattern and name to the interphase of the
  mitotic cell cycle.
• Interphase is immediately followed by meiosis I
  and meiosis II.

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Meiosis I

• meiosis I consists of segregating the homologous
  chromosomes from each other, then dividing the
  diploid cell into two haploid cells each
  containing one of the segregates.
• meiosis I consists of prophase I, metaphase I,
  anaphase I and telophase I.
• prophase I is a complicated phase which itself
  is subdivided into five sections namely
  Leptotene, Zygotene, Pachytene, Diplotene and
  Diakinesis.

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Leptotene/Leptonema
individual chromosomes begin to condense into
long strands within the nucleus. However the two
sister chromatids are still so tightly bound that
they are indistinguishable from one another.
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Zygotene/Zygonema

• homologous chromosomes are attracted and pair
  (synapsis).
• synaptonemal complex structure starts to form
  between paired homologous chromosomes.

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Zygotene/Zygonema
synaptonemal complex structure
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Pachytene/Pachynema

• pairing is now completed, and the chromosomes
  contract further.
• homologous chromosomes are closely associated
  (now called a bivalent).

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Pachytene/Pachynema

• genetic crossing over occurs with the physical
  exchange of DNA between maternal and paternal
  chromosomes.
• chiasmata frequency per bivalent is directly
  related to chromosome length. Long chromosomes
  may have several chiasmata, but to ensure proper
  segregation at anaphase I, all bivalent must have
  at least one chiasmata.

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Diplotene/Diplonema

• chromosome contraction continues.
• each chromosome is now clearly visible and acts
  as if it is repulsing its closely paired
  homologue, but they are held together at the
  sites of crossing over (chiasmata).

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Diakinesis

• contraction of the chromosomes is nearly
  maximal.
• the nuclear membrane dissociates.
• the paired chromosomes, held together by
  chiasmata, rotate in various planes so that they
  position themselves in a state of maximum
  repulsion and start to orientate on the metaphase
  plate.

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Diakinesis

• number of chiasmata in locust

i) bivalent, three chiasmata ii) bivalent, two
chiasmata, ring formed iii) bivalent, one
terminal chiasmata vi) bivalent, cross-shaped,
one chiasmata.
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Metaphase I

• the chromosomes lie on the equatorial plate,
  centromeres attached to the spindle fibres.

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Anaphase I

• the bivalents separate and the homologues are
  pulled to opposite poles.

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Telophase I and Interphase

• this is often a very rapid process such that
  cytokinesis may not occur.
• There is no replication of DNA, so each nucleus
  contains half-bivalents, I.e. the haploid
  chromosome number.

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Metaphase II

• the chromosomes align on metaphase plate of
  newly formed spindle.

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Anaphase II

• the centromeres split and one daughter chromatid
  moves to each pole.

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Telophase II

• the interphase nuclei are reformed and
  cytokinesis occurs, forming four haploid daughter
  nuclei.

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What is the significance of meiosis?
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Significance of Meiosis

• maintains chromosome number.
• produces genetic variation.