Download Science 9 – Section 6.1 3 The Process of Meiosis Meiosis I 1

Science 9 – Section 6.1
The Process of Meiosis
Meiosis I
1. Prophase I
 Chromosomes coil, shorten, thicken and become visible.
 Nuclear membrane disappears.
 Spindle forms.
 Homologous chromosomes move towards each other.
2. Metaphase I
 Homologous chromosomes (each joined to its sister chromatids) pair up along the midline (equator)
of the cell.
3. Anaphase I
 Homologous chromosomes move apart, one of each pair going to each end of cell.
 Each cluster of chromosomes at end of cell has one of each type of chromosome – half the number
the parent cell had.
 Paired chromatids remain attached.
4. Telophase I and Cytokinesis
 Paired chromatids uncoil, lengthen, and become thinner and less visible.
 Nuclear membrane forms around each chromosome cluster.
 Spindle disappears.
 Cytokinesis occurs.
 2 daughter cells with only 1 of the homologous chromosomes (sister chromatids still attached)
results.

During meiosis I, parts of non-sister chromatids cross over each other and exchange segments of
DNA. This event is called crossing over. Each chromosome picks up new genetic information from
the other creating chromosomes with different variation of a gene. Since multiple crossovers are
possible, this important event can create an infinite number of genetic possibilities.
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Science 9 – Section 6.1

Another event called independent assortment also creates genetic diversity. During independent
assortment, homologous pairs of chromosomes move apart in anaphase I, sorting themselves into each
daughter cell. There are 2 possibilities for how each of our 23 pairs of chromosome will eventually
sort itself.

Crossing over and independent assortment are the main reasons why people, even from the same
family, look different from each other.
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Science 9 – Section 6.1
Meiosis II
1. Prophase II
 Paired chromatids coil, shorten, thicken and become visible.
 Nuclear membrane disappears.
 Spindle forms.
2. Metaphase II
 Paired sister chromatids line up along the midline (equator) of the cell.
3. Anaphase II
 Each pair of chromatids is pulled to the opposite pole and splits to form two independent
chromosomes.
 New chromosomes move to opposite ends of cell.
4. Telophase II and Cytokinesis
 Chromosomes uncoil, lengthen, and become thinner and less visible.
 Nuclear membrane forms around 4 new nuclei.
 Spindle disappears.
 Cytokinesis occurs.
 4 haploid daughter cells (gametes) results.
Comparing Meiosis to Mitosis
mitosis
meiosis
Parent cell
(chromosome
number)
2n
2n
Sister chromatids...
Number of
daughter cells
separate during anaphase
stay together in meiosis I,
but separate in meiosis II
2
4
Number of
chromosomes in
daughter cells
2n
n
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Science 9 – Section 6.1
Meiosis
Keeps the number of chromosomes from doubling
each generation by producing haploid sperm or
egg cells.
Ensures variation in a species because
chromosomes from each parent are combined.
Has two cell divisions
Makes the diploid sex cell become a haploid egg
or sperm cell.
Mitosis
Is responsible for growth, tissue repair, and some
forms of asexual reproduction.
Ensures that all cells produced are identical
Has one cell division
Makes two exact copies (daughter cells) of a
parent cell.
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