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: port.com/ /lib/ /animations/meiosis.html port.com/ /lib/ /animations/meiosis.html port.com/ /lib/ /animations/meiosis.html port.com/ /lib/ /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. X X Y 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. 1 = birth 2 = 13 3 = 25 4 = 30 5 = 35 6 = 47 7 = 50