MEIOSIS

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MEIOSIS
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Meiosis

• The form of cell division by which gametes, with
  half the number of chromosomes, are produced.
• Diploid (2n) ? haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis II).

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Meiosis

• Sex cells divide to produce gametes (sperm or
  egg).
• Gametes have half the of chromosomes.
• Occurs only in gonads (testes or ovaries).

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Three sexual life cycles differing in the timing
of meiosis and fertilization
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Moss with Sporophytes
Gametophytes
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Meiosis
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Spermatogenesis
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Oogenesis
polar bodies
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Interphase

• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consists of two
  identical sister chromatids attached at their
  centromeres.
• Centriole pairs also replicate.

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Interphase

• Nucleus and nucleolus visible.

chromatin
nuclear membrane
cell membrane
nucleolus
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Meiosis I (four phases)

• Cell division that reduces the chromosome number
  by one-half.
• Four Phases
• a. prophase I
• b. metaphase I
• c. anaphase I
• d. telophase I

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Prophase I

• Longest and most complex phase (90).
• Chromosomes condense.
• Synapsis occurs homologous chromosomes come
  together to form a tetrad.

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Homologous Chromosomes

• Pair of chromosomes (maternal and paternal) that
  are similar in shape and size.
• Homologous pairs (tetrads) carry genes
  controlling the same inherited traits.
• Each locus (position of a gene) is in the same
  position on homologues.
• a. 22 pairs of autosomes
• b. 01 pair of sex chromosomes

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Homologous Chromosomes

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Prophase I

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Prophase I - Synapsis

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Crossing Over

• Crossing over (variation) may occur between
  nonsister chromatids.
• Crossing over segments of nonsister chromatids
  break and reattach to the other chromatid.

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Crossing Over - variation

variation
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Metaphase I


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Metaphase I

• Shortest phase
• Tetrads align on the equator
• INDEPENDENT ASSORTMENT OCCURS genes for
  different traits
• sort independently of one another
• into gametes

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Anaphase I
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Anaphase I

• Homologous chromosomes separate and move towards
  the poles.
• Sister chromatids remain attached at their
  centromeres.

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Telophase I
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Telophase I

• Each pole now has haploid set of chromosomes.
• Cytokinesis occurs and two haploid daughter cells
  are formed.

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Meiosis II

• No interphase
• (or very short - no more DNA replication)
• Remember Meiosis II is similar to mitosis but
  the cells are different from each other

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Prophase II

• same as prophase in mitosis

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Metaphase II

• same as metaphase in mitosis

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Anaphase II

• same as anaphase in mitosis
• sister chromatids separate

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Telophase II
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Telophase II

• Same as telophase in mitosis.
• Nuclei form. Cytokinesis occurs.
• Remember four haploid daughter cells produced 3
  polar bodies and an
• egg in females, 4 sperm in males
• gametes sperm or egg

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Fertilization

• The fusion of a sperm and egg to form a zygote -
  a fertilized egg

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Variation

• Important to population as the raw material for
  natural selection.
• Question
• What are the three sexual sources of genetic
  variation?

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Answer

• 1. crossing over (prophase I)
• 2. independent assortment (metaphase I)
• 3. random fertilization
• Remember variation is good !

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Karyotype

• A method of organizing the chromosomes of a cell
  in relation to number, size, and type.

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Question

• A cell containing 20 chromosomes (diploid) at the
  beginning of meiosis would, at its completion,
  produce cells containing how many chromosomes?

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Answer

• 10 chromosomes (haploid)

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Question

• A cell containing 40 chromatids at the beginning
  of meiosis would, at its completion, produce
  cells containing how many chromosomes?

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Answer

• 10 chromosomes

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Question

• In terms of Independent Assortment -how many
  different combinations of sperm could a human
  male produce?

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Answer

• Formula 2n
• Human chromosomes 2n 46
• n 23
• 223 8 million combinations

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1) Three major differences between meiosis and
the mitosis are
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2) Compare the number of chromatids present in
human cells before and after DNA replication
occurs in interphase. 3) Compare and contrast
oogenesis with spermatogenesis.
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Determine the number of chromo- somes in a cell
whose diploid (2n) number is 10 after a.
meiosis I b. meiosis II c. mitosis
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What is the primary difference between sexual
life cycles in animals when compared to
fungi and plants?