MEIOSIS

In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity. 

Interphase is the same in both mitosis and meiosis, but in meiosis, it is followed by two cell divisions. These two division processes are referred to as Meiosis I and Meiosis II, and result in a total of four daughter cells, each with a 1n chromosome number.

In prophase I, notice the difference in how the homologous chromosomes behave. They come together and match up (synapsis) in pairs (tetrads or bivalents). In human females, this stage happens prior to birth when the ovaries are forming, and then stops. A baby girl is born with all the precursor egg cells she will ever have in a sort-of "suspended animation" until puberty (hence abdominal x-rays are dangerous for any young to middle-aged human female, not just pregnant women, and hence there is a greater likelihood that a 40-yr-old mother will have a baby with Down Syndrome – due to incorrect meiosis — than a 20-yr-old mother).

In metaphase I, the bivalents line up, not individual chromosomes, so there’s a 50:50 chance of which chromosome of each pair faces which pole of the cell. Human “eggs” go about this far through meiosis before they are shed from the ovaries at ovulation.

In anaphase I, the homologous chromosomes separate, and one of each pair travels to each of the two poles of the cell, thereby reducing the chromosome number from 2n to 1n. Note that the sister chromatids stay together.

Two daughter cells are formed during telophase I. These usually go immediately into the second cell division (meiosis II) to separate the sister chromatids.

Meiosis II is pretty much like mitosis, in that the sister chromatids are separated. This results in four daughter cells, each with an 1n chromosome number. In human females, meiosis II in the precursor egg cells never happens until/if a sperm first enters the egg to fertilize it. Fertilization triggers Meiosis II, and then the sperm nucleus unites with the resulting egg nucleus. Thus, the unfertilized “eggs” that a woman sheds each month are not true eggs. Also in human females, division of the cytoplasm is not even. This provides a way of keeping as much cytoplasm as possible with the future egg/zygote. Rather than equal-sized gametes, one big egg and three smaller polar bodies with minimal cytoplasm are formed.

Interestingly, because the homologous pairs line up during Metaphase I, there is a 50:50 chance of which one of each pair will go to each of the poles of the cell (like flipping a coin, where you can get either heads or tails). Therefore, in humans with 23 pairs of chromosomes, a gamete (egg or sperm) could have 2²³ or 8,388,604 possible combinations of chromosomes from that parent. Any couple could have 2²³ × 2²³ or 70,368,744,177,644 (70 trillion) different possible children, based just on the number of chromosomes, not considering the actual genes on those chromosomes. Thus, the chance of two siblings being exactly identical would be 1 in 70 trillion. In addition, something called crossingover, in which the two homologous chromosomes of a pair exchange equal segments during synapsis in Meiosis I, can add further variation to an individual’s genetic make-up.