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 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 to each daughter cell. They, therefore, do not split by the centromere. DNA replication only occurs before the first division of Meiosis I, Meiosis II continues after the first division without another DNA replication and chromosomes remain condensed. 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 phased is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of chromosomes in each stage of crossing over. Crossing over is the physical exchange of DNA between homologous chromosomes and results in genetic recombination. This is key to producing 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 laterally so a synaptonemal complex, a protein structure, forms between them called a synapsis. The structure the paired chromosomes forms is called a tetrad or bivalent. 

Pachytene: In this stage, the chromatids from each chromosome on the inside of the tetrad, that are forming the complete 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 and where crossing over just took place is called a chiasma.

In some organisms, prophase I can take years to complete. For example, in female homo sapiens, the development of egg cells start during embryo development, but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the chromosomes de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.

The number bivalents is equal to the number of chromosomes in a haploid cell of that organism. More than one chiasma indicates more than one crossing over has occurred.

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). The tetrads are randomly orientated on the metaphase plate so either the paternal or maternal homologue can go to the opposite pole. They are also aligned so a homologous chromatid pair face the opposite end.

Chiasmas help position and stabilise the tetrads on the metaphase plate. If crossing over did not occur, then a chiasma has not manifested. This is essential to the lining of tetrads on the metaphase plate and so, therefore, they might not be able to separate properly.

Anaphase I

The microtubules of the spindle start to contract to pull apart the tetrads.The 2 pairs of sister chromatids ,making up each half of the tetrad, split and move to opposite poles of the cell. As the homologous chromatid pairs separate completely, 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. The chromosomes do not de-condense.

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