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Unit 2
December 3, 2013
MEIOSIS
Pages 470­483
Meiosis is the division of sex cells called gametes ﴾eggs and
sperm﴿. It involves the halving of genetic material. This means that an
egg ﴾or sperm﴿ would have a monoploid number of chromosomes.
Human body cells have a diploid number of 46 chromosomes, therefore,
each egg or sperm has only 23 chromosomes.
Monoploid: Diploid: The number of PAIRS of chromosomes in a cell.
Represented as n. Also called haploid. Ex. Humans
have 23 pairs of chromosomes in their body cells
﴾n=23﴿
The TOTAL number of chromosomes in a cell. Represented as 2n. Ex. Humans have 46 chromosomes in each body cell
﴾2n=46﴿
Therefore, sex cells are monoploid and somatic ﴾body﴿ cells are diploid.
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Why is it important to have a monoploid number of chromosomes?
When fertilization takes place, the chromosomes from the egg and
sperm join to form a zygote. In humans, a zygote must have 46
chromosomes ﴾2n﴿. Therefore, when two monoploid ﴾n﴿ gametes fuse,
they form a diploid zygote ﴾2n﴿. ie. 23 + 23 = 46
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What are the stages of meiosis?
As with mitosis, cells are in interphase before division. Chromosomes will replicate themselves before meiosis begins. The stages of meiosis are the same as those in mitosis. However, cells must go through two completions of the four stages. These two completions are called Meiosis I and Meiosis II.
prophase I metaphase I anaphase I telophase I Interphase: ­
­
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prophase II
metaphase II
anaphase II
telophase II
Chromosomes duplicate
occurs before prophase I ONLY!!
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Meiosis I
Prophase I: a. The replicated pair of chromatids will line up with
a pair of similar chromatids called its homologous pair. This pairing process is called synapsis ﴾occurs at the centromere﴿. The chromosomes now form a tetrad.
Tetrad
b. Nuclear membrane disappears.
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Metaphase I: a. Each tetrad moves to the equator of the cell.
b. Spindle fibers attach to the centromeres.
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Anaphase I: ­
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One pair of homologous chromosomes is pulled to each pole.
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Telophase I: ­
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The cell membrane pinches in to form two daughter
cells.
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Meiosis II: NO REPLICATION OF CHROMOSOMES TAKES PLACE BEFORE MEIOSIS II!
Events are similar to those in meiosis I except in anaphase II: the spindles pull the sister chromatids apart; one chromatid ­
moves to one pole and the other chromatid moves to the opposite pole.
Prophase 2 Metaphase 2 Anaphase 2 Telophase 2
­ Contain half the original chromosomes
­ mixture of maternal and paternal chromosomes
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Crossing Over
Sometimes, genetic material can be exchanged between homologous
chromosomes during meiosis. This can increase genetic variability in
addition to causing some genetic disorders.
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What is the end result of meiosis?
When meiosis is complete, there are four new monoploid cells. In
males, all four cells survive to become sperm. In females, only the
largest of the cells will survive to become the egg, the remaining three
cells "die".
Why is meiosis important?
It allows for variation among organisms because fertilization will
result in the genetic information of both parents to be combined in the
offspring. This allows organisms to adapt to changing environments.
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How are mitosis and meiosis different?
MITOSIS
MEIOSIS
Division of body (somatic) cells.
Division of sex cells (gametes)
Offspring are identical to the parents and to each other because they have the same genetic material.
Offspring are different from the parents and each other because genetic material is randomly distributed to gametes.
Only one completion of PMAT is necessary.
Two cellular divisions are necessary: meiosis I and meiosis II.
Chromosomes duplicate before each Chromosomes duplicate before meiosis I only.
mitotic division.
Sister Chromatids are separated during anaphase.
Homologous chromosomes are separated during anaphase I. Sister chromatids are separated during anaphase II.
Two diploid daughter cells result.
Four monoploid daughter cells result.
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