MEIOSIS 4.2

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MEIOSIS4.2
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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).
• Male spermatogenesis
• Female oogenesis
• Meiosis is similar to mitosis with some
  chromosomal differences.

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Spermatogenesis
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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 - Synapsis

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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.
• Humans have 23 pairs of homologous chromosomes.
• a. 22 pairs of autosomes
• b. 01 pair of sex chromosomes

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Homologous Chromosomes

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Sex Chromosomes
XX chromosome - female
XY chromosome - male
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Karyotype

• A picture of the chromosomes from a human cell
  arranged in pairs by size
• First 22 pairs are called autosomes
• Last pair are the sex chromosomes
• XX female or XY male

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Chromosome Mutations Nondisjunction
During meiosis tetrads may not segregate or in
meiosis II, sister chromatids may stick together
Nondisjunction.
The above karyotype is of a person who has
nondisjunction of the 21st chromosome or Down
syndrome. (note the extra chromosome)
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Detecting disorders in fetus

• Aminocentesis removal of small amount of fluid
  from the sac that surrounds the fetus
• Fetal cells and proteins from fluid are analyzed

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Detecting disorders in fetus

• Chorionic villi sampling removal of small amount
  of tissue that grows between the mothers uterus
  and the placenta
• Tissue contains fetal DNA and can produce fetal
  karyotype

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MEIOSIS10.1
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Interphase I

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

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Interphase I

• Nucleus and nucleolus visible.

chromatin
nuclear membrane
cell membrane
nucleolus
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Prophase I

• Longest and most complex phase (90).
• Chromosomes condense.
• Synapsis occurs homologous chromosomes come
  together to form a tetrad.
• Tetrad is two chromosomes or four chromatids
  (sister and nonsister chromatids).

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Crossing Over

• Crossing over (variation) may occur between
  nonsister chromatids at the chiasmata.
• Crossing over segments of nonsister chromatids
  break and reattach to the other chromatid.
• Chiasmata (chiasma) are the sites of crossing
  over.

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Crossing Over - variation

variation
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Crossing over
Chiasmata sites of crossing over, occur in
synapsis. Exchange of genetic material between
non-sister chromatids. Crossing over produces
recombinant chromosomes.
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Recombined chromosomes
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Prophase I

spindle fiber
centrioles
aster fibers
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Metaphase I

• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS
• 1. Orientation of homologous pair to poles is
  random.
• 2. Variation
• 3. Formula 2n
• Example 2n 4
• then n 2
• thus 22 4 combinations

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Independent assortment
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Independent assortment

• Number of combinations 2n

e.g. 2 chromosomes in haploid 2n 4 n 2 2n
22 4 possible combinations
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Metaphase I


OR
metaphase plate
metaphase plate
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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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Anaphase I

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

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Anaphase 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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Telophase I
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Meiosis II

• No interphase II
• (or very short - no more DNA replication)
• Remember Meiosis II is similar to mitosis

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Prophase II

• same as prophase in mitosis

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Metaphase II

• same as metaphase in mitosis

metaphase plate
metaphase plate
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Anaphase II

• same as anaphase in mitosis
• sister chromatids separate

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Telophase II

• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember four haploid daughter cells
  produced.
• gametes sperm or egg

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Telophase II
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Meiosis
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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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Random fertilization

• At least 8 million combinations from Mom, and
  another 8 million from Dad
• gt64 trillion combinations for a diploid zygote!!!

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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