Meiosis

Meiosis is a type of cell division that occurs in sexually reproducing organisms. It produces four genetically different daughter cells with half the number of chromosomes compared to that of the parent cell. 

Meiosis consists of two divisions:

Meiosis I - this separates the homologous pairs

Meiosis II - this separates the chromatids. 

There are several steps that occur in Meiosis I:

1. Prophase I

2. Metaphase I

3. Anaphase I

4. Telophase I

5. Cytokinesis

During prophase I, the maternal and paternal chromosomes come together in homologous pairs: they are bivalent.

The chromosomes condense to form distinguishable pairs of chromatids attached at a centromere. The centrioles move to the opposite poles of the cell where they synthesise the microtubules of the spindle. Towards the end of prophase, the nuclear envelope also disintegrates and the nucleolus disappears.

Prophase I differs from prophase of mitosis as the chromatids wrap around each other, then partially repel each other, but remained joined at points called chiasmata. At a chiasma, a segment of DNA from one chromatid can be exchanged with the equivalent DNA segment on the other chromatid of the chromosome. Therefore, this provides genetic variation as different combinations of alleles are formed. This crossing over can occur at several places over the length of a chromatid, creating a huge number of different genetic combinations. 

During metaphase I, the spindle fibres extend from either pole and attach to the centromeres of the pairs of chromosomes. The homologous chromosomes then line on the equator, with either chromosome facing either pole. This is called the independent assortment of chromosomes, which produces new genetic combinations, as a mixture of maternal and paternal chromosomes enter the daughter cells. 

During this stage the spindle fibres contract, separating the homologous pairs and pulling the chromosomes to the opposite poles of the cell. Due to the independent assortment at metaphase I, the daughter cells receive a mixture of paternal and maternal chromosomes. 

In telophase I the spindle fibres break down and the chromosomes decondense to form haploid nuclei.  In most species the nuclear envelope does not reform, as meiosis II follows directly after meiosis I. 

In cytokinesis, the membrane furrows and the cytoplasm divides to form two new daughter cells. The sister chromatids do not separate. 

Meiosis II occurs in several stages:

1. Prophase II

2. Metaphase II

3. Anaphase II

4. Telophase II

5. Cytokinesis

During prophase II, the centrioles separate and organise new spindle at right angles to the old spindle. 

During metaphase II, the spindle fibres extend and attach to the centromeres of the pairs of chromatids. The chromatids then align on the equator, with either chromatid facing either pole (independent assortment). 

The spindle fibres contract dividing the centromere, pulling the chromatids to the opposite poles of the cell. 

At telophase II, the chromatids uncoil and lengthen to form chromosomes. The spindle fibres break down, the nuclear envelope reforms and the nucleoli reappear. 

In cytokinesis, the membrane furrows and the cytoplasm divides to form four daughter cells. 

• Meiosis keeps the chromosome number constant from one generation the next.
• Meiosis generates genetic variation in one of two ways:

1. Metaphase I: due to the crossing over of chromatids, so that daughter cells have different combinations of maternal and paternal chromosomes.

2. Metaphase II: daughter cells have different combinations of chromatids.