NOTES ON MEIOSIS
Meiosis is a special form of cell
division that produces
GAMETES OR SEX CELLS.
These cells differ from other cells
because they have HALF the
number of chromosomes.
Because they have half the number
of chromosomes, these cells are
called HAPLOID cells, which are
different from SOMATIC OR
BODY CELLS that have the full
number of chromosomes and are
called DIPLOID.
PHASES OF MEIOSIS
Meiosis 1:
During Prophase 1, the DNA
condenses into
CHROMOSOMES,
the nuclear membrane BREAKS
DOWN, and the SPINDLE FIBERS
form. This is the same as mitosis,
but there is an additional step in
meiosis called crossing-over.
Crossing over is when
HOMOLOGOUS chromosomes
line up next to each other and
EXCHANGE DNA.
Homologous chromosomes are a
pair of chromosomes (one from
mom and one from dad) containing
the SAME TYPES of genes.
Crossing-over occurs during
SYNAPSIS, which is when
homologous chromosomes pair up
during PROPHASE 1.
During METAPHASE 1 the
chromosomes line up in the center of the
cell. Notice how they line up IN PAIRS.
During Anaphase 1 one
CHROMOSOME from each
homologous pair moves to each
side of the cell.
TELOPHASE 1 only occurs in
some cells.
During CYTOKINESIS the cell
divides.
Meiosis 2:
There is no PROPHASE 2. During
Metaphase 2 the chromosomes LINE
UP in the center of the cell. This
time, they line up SINGLE FILE.
During Anaphase 2, the
chromosomes break apart and one
CHROMATID moves to each side
of the cell, just like in mitosis.
During Telophase 2, a new
NUCLEAR membrane forms and
the DNA UNCONDENSES.
The cell DIVIDES in cytokinesis.
The final cells are called
HAPLOID because they contain
half the number of chromosomes.
Meiosis Animation:

http://www.lewport.com/10712041113402793/lib/1071204
1113402793/animations/meiosis.html
Meiosis in Males
Meiosis occurs in the TESTES in
cells called SPERMATOCYTES.
A man will begin going through
meiosis once he reaches about 14
years old and will continue for the
rest of his LIFE.
Each time a man goes through
meiosis, he produces FOUR
sperm.
It is the male who determines the
SEX of the baby.
Y
X
X
X
Meiosis in Females
Meiosis occurs in the OVARIES
in cells called the OOCYTES.
Each time a woman goes through
meiosis, she produces one EGG
and three POLAR BODIES.
A female baby is BORN with all
of the eggs she will ever produce.
They have completed MEIOSIS
1 before birth.
Once a girl reaches about 12
years old, there is a RACE once
a month to complete meiosis 2.
The first ovum to complete it
OVULATES, the rest die.
Once a woman reaches
MENOPAUSE she no longer has
enough eggs left in her ovaries to
continue going through meiosis.
Eggs in Human Ovaries Over Lifetime
1 = birth
3500000
2 = 13
Number of Eggs
3000000
3 = 25
2500000
2000000
Series1
1500000
Series2
1000000
500000
5 = 35
6 = 47
0
-500000
4 = 30
1
2
3
4
5
6
Age
7
8
9
10
11
7 = 50