Cell division Meiosis Meiosis In meiosis a single

Cell division Meiosis

Meiosis In meiosis, a single diploid cell gives rise to 4 haploid cells. Occurs in two distinct stages, Meiosis I and Meiosis II. PMAT still applies but now it is doubled. PMAT

Meiosis I Prophase I DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form tetrads, a step unique to meiosis. During synapse, the time when the homologues are paired up, crossing over may occur between non-sister chromatids of homologous chromosomes.

Prophase I (cont. ) The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move. A kinetochore is a protein which attaches the spindle fiber to the centromere.

Metaphase I Tetrads, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50 -50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

Anaphase I Chromosomes, each with two chromatids, move to separate poles. An important difference between anaphase in mitosis and anaphase I in meiosis is that the centromeres remain intact in anaphase I.

Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis II

Cytokinesis I Analogous to mitosis where two complete daughter cells form. The cells now enter into interphase II. There is no replication of DNA during interphase II.

Meiosis II proceeds exactly the same way as mitosis. �The steps involved are called prophase II, metaphase II, anaphase II and telophase II. Just like mitosis, meiosis II is followed by cytokinesis The end result of meiosis II is four genetically different haploid cells. meiosis animation

Gametogenesis is the production of gametes, and is different in males and females. In males the process is called spermatogenesis, in females the process is called oogenesis.

Spermatogenesis The process begins with spermatogonium, which divides by mitosis to form a primary spermatocyte. The spermatocyte undergoes meiosis to ultimately create 4 sperm cells.

Oogenesis An oogonium divides by mitosis to form a primary oocyte. The products of the first meiotic division are not equal in size. The secondary oocyte forms an egg cell and one polar body during meiosis II. The first polar body becomes 2 polar bodies during meiosis II. The end result is one egg cell and 3 polar bodies. The polar bodies (which are basically chromosomes wrapped in a cell membrane are reabsorbed.