Meiosis

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Meiosis

• Gametes are special cells used in sexual
  reproduction. It is important that these cells
  contain 1/2 the normal number of chromosomes than
  the regular body cells (autosomes).
• When the sperm and egg unite the resulting
  zygote will contain the normal number of
  chromosomes for that organism

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• Mitosis- formation of body cells. At the end of
  mitosis each of the 2 daughter cells has 46
  chromosomes
• These are your body cells, cells that make up
  tissues, organs, etc

• Meiosis-formation of sex cells called gametes. At
  the end of meiosis each of the 4 daughter cells
  has 23 chromosomes.
• They contain half the number of chromosomes

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Cell division cell type of
daughter of cells
chromosomes Mitosis somatic(body)
2 46(diploid) Meiosis
gametes(sex) 4
23(haploid)
Meiosis reduces the number of chromosomes to half
the number in body cells. This is called the
haploid number. Reductive division is necessary
so that when reproduction occurs, the zygote
contains the necessary 46.
Egg(23 chrom) sperm(23 chrom) zygote(46 chrom)
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Meiosis
There are 2 divisions of meiosis- Meiosis I and
Meiosis II. It must be this way in order to end
up with the sex cells only having 23 chromosomes.
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Meiosis I
During the first meiotic division, chromosomes
are replicated.
1. Prophase I- chromatin makes a copy of itself
and begins to coil up
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2. Metaphase I- homologous pairs of chromosomes
line up together at the middle of the cell. This
forms a tetrad(meaning 4).
Homologous chromosomes are paired chromosomes.
Each contains genes for the same trait.
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3. Anaphase I- homologous chromosomes that form
each tetrad separate and move to opposite ends.
The centromeres do not split.
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4. Telophase I- spindle disappears and the cell
divides.
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Meiosis II
During the second meiotic division, nothing is
replicated.
1. Prophase II- centriole doubles, spindle forms
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2. Metaphase II- chromosomes line up at the middle
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3. Anaphase II- individual chromatids move to
opposite ends
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4. Telophase II- when the 2 daughter cells divide
into 4 new daughter cells, each new daughter cell
has 23 chromosomes. This is half the number of
the original parent cell.
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Meiosis

• Meiosis occurs in the testes of the male and is
  called spermatogenesis
• And in the ovaries of the female it is called
  oogenesis.