IB Biology - Process of Mieosis

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

In IB Biology curriculum, meiosis can be divided into two main stages: I and II

Both Meiosis I and II are subdivided into:

• Prophase
• Metaphase
• Anaphase
• Telophase

In IB Biology, there are nine key ideas in this stage:

1. 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.
2. 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.
3. 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
4. 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.
5. 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.
6. 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 light microscope.
   • Note that the red and blue homologous pair are 'crossing over' , see metaphase for details.
7. The metaphase is marked by all pairs of sister chromatids aligned on the equator. 
   • The chromosomes are at their most condensed and therefore most visible at the metaphase.
8. Cross-over. Notice that the chromosome of one homologous chromosome is exchanging with the chromosome of the parallel non-sister chromosome.
   • Cross-over is the exchange of genetic material between non-sister chromatids during Prophase I but is most readily seen during the metaphase.
   • The point at which the chromosomes exchange genetic information is called the chiasma.
   • This may occur many times along a chromosome and not just once as shown in the diagram.
9. Early anaphase with the homologous pairs are aligned together on the equatorial plate of the cell. The spindle microtubules contract and pull the homologous pairs (alleles) apart.
   • The homologous pairs separate one to either pole.
   • This is the case with all homologous pairs.
10. Late anaphase the pairs of chromatids are moving to the poles.
    • Notice that there has been an exchange of genetic material on the 'arm' of the red and blue homologous pair.
    • New combinations of genes are not found on the same chromosome.
11. Illustrates how to identify anaphase by the 'arrow' shape make by the pair of sister chromatids points towards the poles.
12. Chromosomes are now in two sets at opposite ends of the cell. Each set contains one from each of the homologous pairs.
    • In some species a nuclear membrane may form, in others there is a progression straight into Prophase II.
    • The cell membrane 'pinches' towards the centre in a 'cleavage furrow' the membrane will fuse at a central point and the cell will have divided in half.
    • This marks the end of meiosis one (reduction division) in which the homologous pairs have been separated.

End of Meiosis I (remember to read Meiosis II)

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Drafted by Gina (Biology)