MEIOSIS

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MEIOSIS

• Chapter 9

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• Why must organisms reproduce?

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• A. Types of Reproduction
• 1. Asexual Reproduction
• involves only 1 parent
• offspring are genetically identical to the parent
  (clones)
• advantageous when organism is well-adapted to a
  stable environment
• Ex. binary fission, parthenogenesis

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• 2. Sexual Reproduction
• involves 2 parents
• parents produce haploid (1n) gametes which fuse
  to form a diploid (2n) zygote (first cell of
  offspring).
• offspring are genetically diverse
• advantageous in a changing environment
• Haploid gametes are produced by a type of cell
  division called meiosis.

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• B. Meiosis
• Type of cell division that halves the number of
  chromosomes.
• involves 2 nuclear divisions (meiosis I meiosis
  II)
• 1 diploid (2n) cell ? 4 haploid (1n) cells
• occurs in germ cells
• In humans, where would germ cells be located?

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• Germ cell replicates its chromosomes in S phase
  of interphase.
• 1. Meiosis I (reduction division)
• Prophase I
• replicated chromosomes condense
• spindle apparatus forms fibers attach to
  chromosomes
• nucleolus disappears
• nuclear membrane breaks down
• synapsis crossing over occurs

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• Synapsis - homologous chromosomes (homologs) pair
  up.
• Homologs - chromosomes that carry genes for the
  same traits in the same sequence.

A diploid human cell has 23 homologous pairs.
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• Crossing over - non sister chromatids of homologs
  exchange chromosomal material.

Chiasma - visible point of cross over between
homologs.
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Animal cell in prophase I
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• Metaphase I
• paired homologous chromosomes line up along
  equator of spindle

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• Anaphase I
• homologous chromosomes separate move to
  opposite poles

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• Telophase I
• spindle apparatus breaks down
• chromosomes decondense
• nuclear membranes partially reform around two
  nuclei

Cytokinesis usually occurs between meiosis I II
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• 2. Meiosis II (equational division)
• Prophase II
• chromosomes condense
• spindle apparatus forms fibers attach to
  chromosomes
• nuclear membrane breaks down
• Metaphase II
• chromosomes line up single-file along equator of
  spindle

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• Anaphase II
• centromeres part sister chromatids (now called
  chromosomes) separate move toward opposite
  poles
• Telophase II
• spindle apparatus breaks down
• chromosomes de-condense
• nuclear membranes reform
• nucleoli reappear
• Cytokinesis divides two cells into four
  nonidentical cells.

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Meiosis II
Four non-identical haploid cells
Prophase II Metaphase II Anaphase II
Telophase II
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• C. Gametogenesis in Humans
• 1. Spermatogenesis (meiosis in the ?)

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• Structure of human sperm

• Head
• nucleus - 23 chromosomes
• acrosome - contains enzymes that help sperm
  penetrate egg
• Midpiece
• mitochondria
• Tail

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• 2. Oogenesis (meiosis in the ?)

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Comparison of Oogenesis Spermatogenesis