Meiosis I

The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called Meiosis to produce 4 genetically different, haploid sex cells. Meiosis can only occur in diploid, meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated and so chromosomes are duplicated to produce two sister chromatids joined at the centromere. However, in contrast to segregation in Mitosis and Meiosis II, the homologous chromosomes (each containing two sister chromatids) are separated so both sister chromatids of each chromosome goes into each daughter cell, they are not split by the centromere. DNA replication only occurs before the first division of Meiosis I, Meiosis II continues after the first division with no DNA replication. Meiosis goes through 4 stages similar to mitosis which are called Prophase I, Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in Meiosis II.

Prophase I

This stage is commonly split into 5 sub-stages to describe the appearance of chromosomes in each stage of crossing over in order to exchange DNA. This is key to produce genetic variability.

Lepotene: The duplicated chromosomes start become visible as they condense into the sister chromatids structure.

Zygotene: The homologous pairs start to line up so a synaptonemal complex, a protein structure, forms between them called a synapsis. The structure the paired chromosomes forms is called a tetrad.

Pachytene: In this stage, the chromatids from each chromosome on the inside of the tetrad, that are forming the synapsis, start to cross over

Diplotene: The synaptonemal complex in the synapsis, between the chromatid, starts to break down so as the separate chromosomes pull away, the point where the chromatids cross over is more visible. This point of connection is called a chiasma.

Diakinesis: The homologous chromosomes start to separate as much as possible with the chiasmas still in tact. The chromatids are at their maximum point of condensation. At the end of Prophase I, the nuclear envelope fragments and the spindle starts to form.

Metaphase I

Like in Mitosis, the microtubule components of the spindle fibre going across the cell to and from the centrioles at each pole, attach to the kinetochore, a protein structure in the centromere, of the tetrads. Therefore, the tetrads align themselves along the metaphase plate (the 'equator' of the cell).

Anaphase I

The 2 pairs of sister chromatids ,making up each half of the tetrad, split and move to opposite poles of the cell. the tetrad, therefore, splits and genetic information has been fully exchanged.

Telophase I

A set of chromosomes (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the spindle breaks down. The cell divides and the cells enter The Second Meiotic Division.

These 2 daughter cells are genetically different to each other and to the parent containing diploid set of chromosomes and recombinant DNA.