10.1 Essential ideas
10.1.1 Meiosis: a closer look
- You have already learned in 3.1.3 of the two functions of meiosis:
- To halve the chromosome number – during meiosis diploid cells divide to form haploid cells. The reduction of chromosome number occurs in meiosis I.
- To produce genetic variety – bits of DNA are exchanged between non-sister chromatids in prophase I, through a process called crossing over. Furthermore, there is random assortment of chromosomes during metaphase I.
Behaviour ofduring prophase I
- DNA replication occurs during the S-phase of interphase (before the start of meiosis).
- In the earliest stage of prophase I, the duplicated chromosomes condense, shorten and coil to form a single body composed of two sister chromatids.
- As prophase I progresses, homologous chromosomes migrate towards each other and line up in pairs. This pairing is called synapsis.
Figure 10.1.1a – Bivalent
Homologous chromosomes during synapsis.
- A group of four chromatids, or a tetrad, results from synapsis. This is also known as a bivalent because it is a pair of homologous chromosomes.
- In some cells, a protein complex, called the synaptonemal complex, forms between the homologous pairs.
nd genetic recombination
- Crossing over is the exchange of DNA material between non-sister homologous chromatids during prophase I.
- Crossing over occurs when there are breaks in the DNA structure at identical locations on sister chromatids.
- Non-sister chromatids invade homologous sequences and bind to the region of the break.
- The crossover points are called chiasmata (singular: chiasma).
- Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
Skill: Drawing chiasmata formed by crossing over
Figure 10.1.1d – of
Drawing a chiasma formed by crossing over.
- Use different colours to represent homologous chromosomes.
- Draw chiasma formation as a series of small diagrams.
- The chromosomes should be shown very close together on the page. They should be arranged in a tetrad both before and after formation of the chiasma.
- Make sure only non-sister chromatids cross over.
- Labels may not be necessary. If you are asked for diagrams, label the centromere, bivalent, sister chromatids, homologous chromosomes, and chiasma only once.
DistinguishingI and II
- The main difference between the first and second meiotic divisions is that homologous pairs separate during meiosis I, and sister chromatids separate in meiosis II.
- The genetic variety that is found in daughter cells at the end of meiosis can be traced back to meiosis I:
Random orientation and independent assortment
- One of the principles that Mendel discovered in his pea-breeding experiments was the ‘principle of independent assortment’.
- Independent assortment means that how the alleles of one gene separate does not depend on how the alleles of another gene separate.
- Assuming there is independent assortment, for every number (n) of chromosomal pairs, there should be 2n different chromosome combinations in the daughter cells.
Did you know?
Food for thought
Random assortment of homologues produces 8,388,608 (n = 23, so 223) possible chromosome combinations in humans!