Meiosis

• First nuclear division– 46 X’s split into 2 nuclei with 23 X’s (one of

each kind of chromosome)

• Second nuclear division– 23 X’s split into 2 nuclei with 23 single copy

chromosomes

• Cytokinesis follows both divisions, but interphase only follows second division.

Meiosis (Prophase I)

• Process: synapsis

• Two pairs of homologous = tetrad

• Trading chromosome pieces is crossing over.

• The X formed by “legs” of the chromsomes overlapping is called a chiasma

Where

• Mitosis: Any cell not in G0 phase, except in the gonads (ovaries and testes)

• Meiosis: In the gonads, the germ cells give rise to the gametes (egg and sperm)

When

• Mitosis: throughout life in some cells, but cell division stops in nerve cells and muscle cells early in life (2-3 years old).

• Meiosis:– Females: Begins before two years old, then

starts again during puberty with one egg made each month. Menopause is when eggs are no longer formed.

– Males: Happens throughout life from puberty to death.

Result

• Mitosis: 2 Diploid cells with 46 single copy chromosomes.

• Meiosis: 4 haploid cells with 23 single copy chromosomes (one of each kind)– Males: All four are viable cells (sperm)– Females: Of the four cells created, only one

will become an egg and the other three becoming polar bodies with little cytoplasm, yet having the full 23 chromosomes.