2. Goals for today:
1. Knows details of the stages of meiosis I, and
meiosis II.
2. Understands the cells that result and their
characteristics.
3. Is aware of similarities and differences
between meiosis and mitosis.
5. Interphase: Preparing for meiosis
Before meiosis, the cell duplicates its diploid chromosome during a special stage of
interphase.
c d
c c d d
a a b b
a b
6. Prophase I
The nucleus dissolves, releasing the
duplicated, homologous pairs into the cell.
Some chromosomes undergo crossover during
prophase I.
7. Crossover
Crossover is the process by which homologous pairs exchange info.
Without crossover With crossover
Crossover introduces variation into an organism’s gene pool.
8. Metaphase I
The homologous pairs line up together and
each attaches to a spindle fiber.
In this step, the homologous pairs are split up.
Note: In mitosis, the duplicated
chromosomes are split in two; in meiosis I,
the chromosomes remain intact.
9. Anaphase I
The homologous pairs are pulled to opposite
ends of the cells during anaphase I.
The cell begins to elongate and cleave.
Cleave (v): to split or divide by or as if by a
cutting blow, especially along a natural line of
division.
10. Telophase I
The cytoplasm continues to separate and
cleave in two.
A nucleus re-forms around the hereditary
material, as the cell prepares to split during
cytokenesis I.
11. Cytokenesis I
Cytokenesis I is not officially part of Meiosis
but completes the division of the cell.
During Cytokenesis I, the two cells separate
completely.
12. Meiosis I
Prophase I Metaphase I Anaphase I Telophase I Cytokenesis I
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13. Duplicated, diploid to unduplicated, haploid
duplicated
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haploid
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II
14. Meiosis II
Meiosis II is identical to mitosis; it has the same steps and these steps look the same.
The only difference:
Meiosis II splits 2 haploid cells with duplicated chromosomes into 4 haploid cells with
unduplicated chromosomes.
AND
Mitosis splits 1 diploid cell with duplicated chromosomes into 2 diploid cells with duplicated
chromosome.
16. Prophase II
Before prophase II, the chromosomes do not need
to duplicate. The duplication that took place before
meiosis I was never undone.
The nucleus dissolves, releasing the haploid,
duplicate chromosomes.
17. Metaphase II
The duplicated chromosomes line up in the middle
and a spindle fiber from each centriole attaches to
the chromosome.
Metaphase II is the same as metaphase during
mitosis.
18. Anaphase II
The duplicated chromosomes are now split
into halves.
If there were no crossovers, both halves are
identical. They are each half of a duplicated
chromosome.
19. Telophase II
Now 4 nuclei form, these nuclei will become the
4 new gametes during cytokenesis II.
20. Cytokenesis II
Cytokenesis II results in 4 gametes.
Like all gametes, these haploid cells contain
unduplicated chromosomes .
As a result of crossover and the split of
homologous pairs, all the gametes contain
slightly different genes.
22. Meiosis: Step by step
Interphase
Meiosis I
Prophase I II
MEIOSIS Metaphase I Anaphase I Telophase I Cytokenesis I
Meiosis II
or
Prophase II Metaphase II Anaphase II Telophase II Cytokenesis II
