Revision as of 17:07, 23 November 2012

Meiosis is a type of cell division. It can only occur in diploid cells, resulting in four unidentical haploid daughter cells.This contrasts to mitosis which can occur in both haploid and diploid cells, producing only two identical daughter cells. Therefore meiosis results in variation where as mitosis produces exact copies of the parent cell. There are two stages of Meiosis, Meiosis I and II. This leads to the nucleus dividing twice but chromosome replication only occuring once. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase. 

Meiosis I ^[1]

Meiosis 1 is separated into 4 stages. These are Prophase I, Metaphase I, Anaphase I and Telophase I.

Prophase I

During prophase I chromosomes pair, condense and crossing over occurs between non-sister chromotids. It is separated into 5 different stages. Similar to mitosis, centrioles move to opposite poles and spindle fibres start to form.

Leptotene

Leptotene is the first stage of Prophase I. During the Leptotene stage chromosomes coil and condense. This is when the chromosomes first become visible. The two chromatids are joined together at the centromere.

Zygotene

During Zygotene the homologous chromosomes pair up and undergo synapsis where the synaptonemal complex between the homologous chromosomes starts to form.

Pachytene

Synapsis is now complete and the a bivalent (pair of homologous chromosomes) is formed. Chiasmata (singular: chiasma) form between non-sister chromotids of homolohous chromosomes. This is the point where crossing over occurs and DNA exchange occurs.

Diplotene

The snaptonemal complex breaks down, allowing the chromosomes to separate. The chiasmata are now visible and are point at which the chromosomes are still held together.

Diakensis

During diakenesis the nuclear envelope starts to breakdown. The bivalents are now ready for metaphase.

Metaphase I

Bivalents or tetrads (four chromatids) align on on the metaphase plate (the equator of the cell) and spindle fibres attach to the kinetichores; protein structures located at the centromeres. In this stage, the nuclear envelope has been fully disintegrated.

Anaphase I

Disjunction occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles.

Telophase I

Nuclear envelope may form around the dyads and cytokinesis (cell division) occurs.

Interphase

Interphase does not have to occur between Telophase I and Prophase II, but it can occur. Unlike traditional interphase, there is no DNA replication but growth and biosynthetic activities can still occur.

Meiosis II
 

Prophase II

Unlike Prophase I no chiasmata form and no crossing over occurs. If a nuclear envelope has formed during Telophase I, it is broken down. Centroiles move to opposing poles and spindle fibres start to form.

Metaphase II

The dyads align on the metaphase plate and spindle fibres attach to the kinetechores.

Anaphase II

The dyads are pulled apart by spindle fibres and the myads arrive at opposite poles.

Telophase II

The nuclear envelope reforms around the myads. Cytokenesis occurs and the cells divide. This leaves us with 4 unidentical daughter cells, also known as gametes.

References

1. ↑ Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers