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Meiosis
(Gamete Formation)
Meiosis – cell division that creates gametes (sex
cells, sperm and egg) with half the number of
chromosomes as the parent body cell
2 cell divisions – Meiosis I and Meiosis II
1. Interphase (just like mitosis)
2. Meiosis I
a.
Prophase I
nucleus disappears
homologous chromosomes
pair up to form a tetrad
crossing over occurs
b.
Metaphase I
pairs of homologous
chromosomes line up in the
middle of the cell
random assortment occurs
c.
Anaphase I
pairs of homologous
chromosomes separate and
move to opposite ends of the
cell
d.
Telophase I and Cytokinesis
chromosomes uncoil
nuclei reappear
cytoplasm divides
3. Meiosis II
a.
Prophase II through Telophase II
JUST LIKE MITOSIS!
Meiosis
# of cells
# of
chromosomes/cell
Before Meiosis
After Meiosis
http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
http://biology.about.com/od/meiosis/ss/meiosisstep.htm
http://www3.moe.edu.sg/edsoftware/ir/files/biomeiosis/stages_meiosis1/stages_meiosis1_frameset.html
In humans…
Why
are
you
“you”nique?
- Meiosis accounts for why we’re all
different! (except for identical twins)
- Sources of variation provided by meiosis
(when YOU are making sperm/egg):
1. During Interphase, DNA can change
(mutation) during DNA replication.
2. During Prophase I, crossing over causes
chromosomes to exchange DNA and shuffle
your mom and dad’s genes.
3. During Metaphase I, your mom and dad’s
DNA line up in different ways each time you
make gametes (random assortment).
4. During Metaphase II, occasionally
chromosomes do not separate like they
should (nondisjunction) leading to conditions
such as Down syndrome.
5. During fertilization, one unique sperm meets
one unique egg.
With ALL that variation, you make 8 million
different sperm or egg. This means you your
spouse can make a total of 70 trillion different
children!!