Meiosis

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11-4 Meiosis
Making Gametes
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Function

• Form gametes or sex cells
• Types of gametes
• Sperm
• Eggs
• Spores

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Meiosis

• Nuclear division that cuts the number of
  chromosomes in half.
• Necessary to ensure that the zygote does not
  receive double the number of chromosomes after
  fertilization.

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Review Terms

• Zygote fertilized egg.
• Fertilization when sperm and egg unite to make
  one cell. This one cell will go through mitosis
  until it develops into a new individual.

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The cells produced by meiosis are called haploid
(1N), meaning they have half of a set of
chromosomes (1 of each type of chromosome).
These chromosomes add to those donated by the
other parent to form a complete set that is
diploid (2N).
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There are two complete divisions of the
chromosomes in meiosis called

• Meiosis I
• Meiosis II

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In Meiosis I, the homologues are separated into
separate cells.
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Meiosis I
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In Meiosis II, the centromeres split and the
chromatids are separated into separate cells.
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Meiosis II
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The phases in meiosis I and II are similar to
those in mitosis Prophase Metaphase
Anaphase Telophase Each phase is given a
number to indicate whether it is part of Meiosis
I or II.
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The events in each phase of meiosis are also
similar to those in mitosis. There are, however,
some differences.
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In Prophase I, the homologous pairs group
together through a process called synapsis and
form a tetrad (group of 4 chromatids). While
close together, they swap matching genes
resulting in genetic recombination (shuffling the
deck). This is crossing over.
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In Anaphase I, the homologues separate, but the
centromere does not split.
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During Telophase I, the cell does not go back to
Interphase because Meiosis II still has to happen.
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In meiosis II, the steps are the same as in
mitosis, except the cells at the end are haploid
instead of diploid.
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At the end of meiosis, there are four genetically
different haploid gametes.
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Section 3 Meiosis
Chapter 8
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The process that produces sperm is called
spermatogenesis. At the end of spermatogenesis
there are four genetically different sperm.
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The process that produces eggs in a female is
called oogenesis. At the end of this oogenesis
there are 3 polar bodies and 1 ootid (egg).
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The 3 polar bodies would be useful eggs if it
were not for the unequal splitting of the
cytoplasm in each division. The ootid receives
more cytoplasm to help it survive until
fertilization and implantation.
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The polar bodies break down and are absorbed by
the body. The egg is released for
fertilization during the menstrual cycle.
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Sperm and Egg
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The End!