Meiosis Study Guide

1) The purpose of meiosis is to produce haploid gametes for reproduction. Haploid cells have only one copy of each chromosome (instead of two- which would be diploid). Gametes are specialized cells meant for reproduction (sperm and ova (egg) in humans).

2) Meiosis can be divided up into distinct phases similar to mitosis but with some important differences. The phases of meiosis are divided up into Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II and Telophase II.

The Process of Meiosis

3) Meiosis starts with one diploid (two chromosomes) cell and produces four haploid gametes. This is different than mitosis which also starts with one diploid cell but produces two diploid cells.

Mitosis vs Meiosis

4) In Prophase I of Meiosis, crossing over occurs between homologous chromosomes. For example, the two copies of chromosome 5 pair up and exchange parts of their chromosomes. This happens at the same time with the rest of the 23 sets of chromosomes in humans. Crossing over increases genetic diversity.

5) When homologous chromosomes pair (synapsis) for crossing over (in Prophase I of Meiosis I), a synaptonemal complex made of protein forms between them.

6) Since each homologous chromosome exists as pairs of sister chromatids during synapsis, there are a total of 4 chromosomes linked together resulting in a tetrad.

7) In Anaphase I of Meiosis, homologous chromosomes are pulled apart (disjoin) and move to opposite polls. They are pulled apart by the spindle fibers which are connected to the centromeres of each chromosome.

8) In Anaphase II of Meiosis, the sister chromatids are pulled apart (disjoin) and move to opposite polls.

9) So to repeat: Anaphase I:homologous chromosomes disjoin; Anaphase II: sister chromatids disjoin.

10) The X and Y chromosomes, while different behave as homologous chromosomes during meiosis. They link up at the pseudoautosomal regions (PAR). X and Y are called the sex chromosomes to distinguish them from the other 22 autosomal chromosomes.

23 Human Chromosomes

11) Sex is determined by the X and Y chromosomes in humans. Females are XX. Males are XY.

12) Since males produce gametes with either an X or a Y chromosome they are the heterogametic sex. Females always produce eggs that have an X chromosome so they are called the homogametic sex.

13) The X chromosome is much larger than the Y chromosome and has many more genes. Roughly 2000 compared to 75 genes. The Y chromosome does contain "important" genes and is not a wasteland as previously thought.

14) Besides genetic information encoded on nuclear chromosomes, humans also have a small amount of genetic content stored in the mitochondria. Mitochondrial DNA is inherited from the mother.