Cell division 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 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 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  meiosis animation meiosis animation  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.

Gametogenesis  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.