Students, take note! This is what you need to know about meiosis for your AQA Biology exams and nothing more. Learn this and you should be set up well! Cell division consists of the division of the nucleus followed by the division of the cell itself (cytokinesis). Mitosis produces two daughter nuclei with the same number of chromosomes as the parent cell, however meiosis produces four daughter nuclei. Each has half the number of chromosomes as the parent cell.
Meiosis is necessary because sexual reproduction is when two gametes fuse. Gametes must have half the number of chromosomes, otherwise the number would double every generation. During meiosis, the chromosome pairs separate, causing the gamete to have a haploid number of chromosomes (23). When they fuse in fertilisation, the diploid number (46, or 23 pairs) is restored.
Meiosis is divided in to two parts:
First division (meiosis 1) is where the homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of the chromatids can be exchanged in the process of crossing-over.
Second division (meiosis 2) is where the chromatids move apart. Four cells formed, each containing 23 chromatids (in humans).
This genetic recombination by crossing over (in meiosis 1) causes genetic variation, helping organisms to adapt to the environment. Another cause of genetic variation which is related to meiosis is the independent segregation of homologous chromosomes (during meiosis 1) where each chromosome lines up alongside its homologous partner. When they arrange themselves in to the line, it’s done randomly. One of each pair passes to each daughter cell. Since the pairs are lined up at random, the combination of chromosomes entering the daughter cell is random.
Now would be a good time for me to remind you of some useful key terms that you’ll be reading about a lot in the topic:
Gene – A section of DNA that codes for a protein;
Locus – The position of a gene on a chromosome/DNA molecule;
Allele – A different form of a particular gene.
Things such as “genetic recombination by crossing over” and the “independent segregation of homologous chromosomes” (both mentioned above) are also important to remember – just like everything else in this article!
Variety from these new genetic combinations happens as follows: Each member of the homologous pair of chromosomes has exactly the same genes, so determines the same characteristics. The alleles may differ however. This random assortment of chromosomes leads to this genetic variation.
Here’s an example:
Imagine one pair of the chromosomes carries an allele for brown eyes and another for blue on one chromosome and one allele for blood group A and another for group B. At the end of meiosis 1, two separate cells have formed. At the end of meiosis 2, the chromosomes have segregated in to chromatids. Four gametes.
One arrangement produces one of 2 types of gamete (brown eyes with blood group B; and blue eyes with blood group A) while the other arrangement produces 2 other types (blue eyes with blood group B; and brown eyes with blood group A).
Gametes will therefore be genetically different resulting from the different combinations of the maternal and paternal chromosomes they contain. Random fusion of these gametes produces the variation. Since the gametes usually come from different parents, even more variety is produced.
The above seems quite complicated but, if you go through it carefully (and make sure you understand each stage) you should be able to get to grips with it.
Finally – let’s look at crossing over, which I’ve now mentioned a couple of times, in a bit more detail. It is known as ‘crossing-over’ since the chromatids cross over one another many times. Broken-off portions of chromatid recombine with another chromatid – this is called ‘recombination’.
The process occurs as follows: Chromatids of each pair of chromosomes (chromosomes with their homologous partners) become twisted round each other; twisting causes tensions and parts of the chromatids break off; the broken portions then rejoin with the chromatids of its homologous partner and, therefore, new genetic combinations are produced.
Note that normally it’s the equivalent portions of homologous chromosomes that are exchanged.
There you have it. Your one-stop guide to meiosis!
If you feel you don’t quite understand this topic after reading this, you might want to try looking at the diagrams available on many biology revision sites, such as “biology mad”.