Meiosis is a special type of cell division that occurs in the gonads or sex organs.
- At the end of meiosis, the number of chromosomes is reduced by half. That is, meiosis of diploid cells produces haploid daughter cells.
- These cells can then function as gametes that can undergo fertilization resulting in the restoration of the diploid number of chromosomes in the zygote.
- Meiosis and fertilization introduce genetic variation
in three ways:
The meiosis
consist of two main stages:
- Meiosis I: This is made up of four sub-stages: Prophase I, Metaphase I, Anaphase I and Telophase I
- Meiosis II: This is made up of four sub-stages: Prophase II, Metaphase II, Anaphase II and Telophase II
MEIOSIS I
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Prophase I
- The chromosomes condense and
become visible.
- The centrioles form and move
toward the poles.
- The nuclear membrane begins
to dissolve.
- The homologs pair up, forming
a tetrad.
- Each tetrad is comprised of
four chromotids - the two homologs, each with their sister chromatid.
- Genetic material from the homologous chromosomes is randomly swapped. This process is known as crossing over.
- Crossing over increases genetic diversity by creating four unique chromatids.
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- Microtubules grow from the
centrioles and attach to the centromeres
- The tetrads line up along the
cell equator.
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Anaphase I
- The centromeres break and homologous
chromosomes separate with the sister chromatids are
still attached.
- Cytokinesis begins.
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- The chromosomes may
decondense.
- Cytokinesis reaches
completion, creating two haploid daughter cells
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MEIOSIS II
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Prophase II
- Centrioles form and move toward
the poles.
- The nuclear membrane
dissolves.
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- Microtubules grow from the
centrioles and attach to the centromeres.
- The sister chromatids line up
along the cell equator.
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Anaphase II
- The centromeres break and sister
chromatids separate.
- Cytokinesis begins.
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- The chromosomes may
decondense.
- Cytokinesis reaches
completion, creating four haploid daughter cells.
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