(4) Process of Mieosis

The process of meiosis

including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells 

Meiosis can be divided into two main stages: 

I and II

Both Meiosis I and II are subdivided into: 

- Prophase

- Metaphase

- Anaphase

- Telophase

(a) The nuclear membrane is intact and the chromosomes inside cannot be seen. At this stage the chromosomes are not greatly coiled or condensed which allows genes to be expressed. Each DNA molecule is about 1.8m long but still wound sufficiently such that it can be contained inside a 10um nucleus.

(b) In the G1 stage of the Interphase each chromosome is a single DNA molecule (+histones). Here we can see (although in-reality you cannot since the nucleus is intact) that there are four chromosomes and the diploid number of the cell is 2n=4. Red and blue are a homologous pair as are green and purple.

(c) In S1 of the interphase the DNA molecules replicate. Each copy (sister chromatids) are held together at the centromere (black dot). The cell is now preparing for the meiotic division in which:
Chromosome number will be halved and the Homologous chromosomes will be separated

(d):Early prophase, the nuclear membrane is breaking down.

The spindle of microtubules is forming from opposite ends of the cell.

Centrioles organise the spindle construction at the poles of the cell.

(e) The pairs of sister chromatids attach to the spindle microtubules at the centromere.

The DNA is condensing by super coiling, this will reach it peak in the metaphase.

(f) The pairs of chromatids will move up and down the between the poles but gradually move towards the equatorial plate (centre) of the cell.

The nucleus has now disappeared and the chromosomes are dense enough to be seen with a…