1.3.2 The reduction of chromosome number: meiosis
If you look at the chromosomes shown in Figure 8 you will see that they have been lined up in pairs. The members of each pair are of similar shape and size, and unlike the members of other pairs. At a molecular level these distinctions are maintained: the order of the bases in the DNA is very similar in both members of a pair, but is quite different from that found in other pairs. By ‘very similar’ we mean that the order of the particular genes on each chromosome of the pair is the same, but the exact order of the bases within those genes may be slightly different.
To produce pictures like those in Figure 8, chromosomes from a dividing cell are photographed down a microscope. Each individual chromosome's photograph is cut from the resulting print, and the chromosomes are lined up and matched into pairs based on size and the position of the constriction which is found somewhere along each chromosome's length. (This constriction is called the centromere, and has structural properties which enable it to attach to the mitotic spindle during cell division) The resulting matched pairs of chromosomes are photographed again, to give a karyotype, as shown. Note that such pictures are obtained only for dividing cells, in which the chromosomes are replicated – each one consisting of two strands, which are held together at the centromere. In fact, each strand is itself a chromosome, but while still joined to its partner like this, is called a chromatid.
Q If the order of bases in two copies of a particular gene is slightly different, what does this imply about the protein that the gene codes for?
A The amino acids may also be slightly different, if the base change caused a change in the code (.
It is these subtle differences in the amino acid sequences of our proteins that make us individuals. For example, we all have genes for hair colour, but we do not all have the same colour hair! These different forms of the same gene are called alleles.
The fact that the members of a pair of chromosomes carry the same genes, albeit slightly different forms of them, means that each body cell is actually carrying two sets of genetic information. One set is derived from the mother, the other from the father. Each gamete, however, carries only one set of chromosomes; and each chromosome is a mixed combination of alleles from the chromosome pair that the mother inherited from her father and mother. The same is true of each sperm cell from the father. (The cell division process by which this gene mixing comes about is called meiosis, and is discussed below.) Thus, when fertilization occurs the fertilized egg will contain the two sets of chromosomes characteristic of the body cells. The individual chromosomes, however, will be different (i.e. have different combinations of alleles) from those carried by either the mother or the father – this is why we are not identical to our parents or to our siblings (unless we have an identical twin, who would be derived from the same conceptus). What happens is that during meiosis a process occurs which allows alleles to swap between members of the pair. This recombination of alleles is of vital importance to us, both as individuals (as we have seen, this process is what makes us individuals), and as a species, because it gives rise to the genetic variations that allows our species to be fexible, make compromises and survive.
Because each pair of chromosomes is qualitatively different from all the other pairs, a mechanism is needed to make sure that each egg and sperm cell has a complete set of chromosomes, and not just any 23. This is achieved by the process of meiosis, which is shown diagrammatically in Figure 9. The key points to remember about meiosis are: (a) that it halves the number of chromosomes per cell, and (b) that it gives rise to new gene combinations.
As with mitosis, by the time they become visible each meiotic chromosome has replicated, i.e. it has been accurately copied along its entire length (Figure 9a). The pairs of chromosomes move so that they come to lie side by side (Figure 9b), and the pairs migrate towards the equator of the cell (Figure 9d). During this process, the arms of the chromosomes make contact with each other (Figure 9c), and members of a pair can cross over and exchange material by two chromatids breaking at the corresponding point, then rejoining with the ‘wrong’ chromatid.
Because all the chromosomes have replicated and become chromatid pairs, there are actually sets of four chromatids lined up together at stage (c), allowing a large number of possible exchanges. Because of the way the pairs of chromosomes were aligned at the equator of the cell, one member of each pair (i.e. one set of two chromatids) moves to each pole (Figure 9e). Note that it is not the case that all the chromosomes originating from this individual's mother goes towards one pole, while all those originating from its father goes to the other: the pairs are randomly assorted, so that at each pole there is a mixture of maternally-derived and paternally-derived chromosomes, although there will be a complete set of 23 at each pole (Figure 9f). The cell membrane now pinches in, producing two new cells.
Although each new cell now holds 23 chromosomes, there is still twice the amount of genetic material as is needed for a gamete.
Q Why is this so?
A Because each chromosome has replicated, giving two copies of each.
The two copies (i.e. the pair of chromatids) are held together at the centromere, the constriction lying part of the way along each chromosome that is the point of attachment to the meiotic spindle (not shown in Figure 9).
This stage is illustrated in Figure 9g. So meiosis needs to proceed further, to reduce the genetic material to one copy per gamete. The 23 chromosomes in each new cell assemble at the equator once more (Figure 9h), and one chromatid is pulled towards each pole via the centromere (Figure 9i). Once again, the cell divides (Figure 9j), yielding from the original one a total of four cells, each of which contains one copy of each of 23 different chromosomes. These four cells are now ready to be gametes.
Q What about mitosis? What is the most important difference between meiosis and mitosis?
A In mitosis, there is one round of DNA replication and one cell division. In meiosis there is also one round of DNA replication, but there are two cell divisions. This ensures that each of the four resulting cells contains only one copy of the genetic information.
Also, whereas in mitosis the chromosomes in the two progeny cells are identical with those of the parent cell, in meiosis the gametes’ chromosomes are subtly different because of random assortment of the chromosomes and the exchange of material between similar strands that has taken place during the First cell division.
But although the cells now have the right number of chromosomes to be gametes, they need to undergo various steps of maturation before they can take part in fertilization. We shall now look at these steps in men and women.