Meiosis Notes

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Meiosis Notes
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Meiosis

• Meiosis occurs in sexual reproduction when a
  diploid germ cell produces four haploid daughter
  cells that can mature to become gametes (sperm or
  egg).

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Meiosis

• Goal reduce genetic material by half
• Why? n (mom) n (dad) 2n (offspring)

Just right!
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Fertilization
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Chromosomes
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Meiosis cell division in two parts
Sister chromatids separate
Chromatids separate
(equational division)
2n
n
n
Result one copy of each chromosome in a gamete.
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Meiosis I the reduction division
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Prophase I

• Chomatids pair and condense.
• Crossing over occurs
• Spindle forms.
• Nuclear envelope
• fragments.

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Metaphase I

• Chromatid pairs align along the equator of the
  cell.

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Anaphase I

• Chromosomes separate and move to opposite poles.
• Sister chromatids remain attached at their
  centromeres.

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Telophase I

• Nuclear envelopes reassemble.
• Spindle disappears.
• Cytokinesis divides cell into two.

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Meiosis II
Gene X
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Meiosis II the equational division
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Prophase II

• Nuclear envelope fragments.
• Spindle forms.

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Metaphase II

• Chromosomes align along equator of cell.

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Anaphase II

• Sister chromatids separate and move to opposite
  poles.

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Telophase II

• Nuclear envelope
• assembles.
• Chromosomes
• unravel.
• Spindle disappears.
• Cytokinesis
• divides cell into
• two.

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Results of meiosis

• Four haploid cells
• One copy of each
• chromosome

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Meiosis allows the creation of unique
individuals through sexual reproduction.
Meiosis keeps the number of chromosomes the same
for each generation.
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Mitosis Meiosis
Number of divisions 1 2
Number of daughter cells 2 4
Genetically identical? Yes No
Chromosome Same as parent Half of parent
Where Somatic cells Sex cells
When Throughout life At sexual maturity
Role Growth and repair Sexual reproduction
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MITOSIS MEIOSIS
Occurs in somatic (body) cells Occurs only in gonads (sex organs ovary/testes)
Produces cells for repair, maintenance, growth, asexual reproduction Only produces gametes (sex cells egg/sperm)
Results in identical diploid (2n) daughter cells Reduction division results in haploid (n) cells
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Mistakes in Meiosis

• Nondisjunction The failure of homologous
  chromosomes to separate properly during meiosis

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• Trisomy When a gamete with extra chromosomes is
  fertilized by a normal gamete, the zygote will
  have an extra chromosome
• Ex. In human, if a gamete with an extra
  chromosome number 21 is fertilized by a normal
  gamete, the resulting zygote has 47 chromosomes
  instead of 46. This will develop into a baby
  with Down syndrome.

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• Monosomy When a gamete that is missing a
  chromosome fuses with a normal gamete during
  fertilization, the resulting zygote lacks a
  chromosome.
• Most often this is lethal. One non-lethal
  example is Turner syndrome, in which human
  females have only a single X chromosome instead
  of two.

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• Polyploids organisms with more than the usual
  number of chromosomes sets
• Often results in death of the zygote. However,
  polyploids often occur in plants