Unit Three. The Continuity of Life
While the general features of meiosis that you have just reviewed are similar in all eukaryotes, the detailed mechanism of meiosis varies somewhat in different organisms. This is particularly true of chromosomal separation mechanisms, which differ substantially in protists and fungi from the process in plants and animals that we describe here. Despite such differences in detail, however, two consistent features are seen in the meiotic processes of every eukaryote: synapsis and reduction division. Indeed, these two unique features are the key differences that distinguish meiosis from mitosis, which you studied in chapter 8.
The first of these two features happens early during the first nuclear division. Following chromosome replication, homologous chromosomes or homologuespair all along their lengths, with sister chromatids being held together, as mentioned earlier, by cohesin proteins. While homologues are thus physically joined, genetic exchange occurs at one or more points between them. The process of forming these complexes of homologous chromosomes is called synapsis, and the exchange process between paired homologues is called crossing over. Figure 9.8a shows how the homologous chromosomes are held together close enough that they are able to physically exchange segments of their DNA. Chromosomes are then drawn together along the equatorial plane of the dividing cell; subsequently, homologues are pulled apart by microtubules toward opposite poles of the cell. When this process is complete, the cluster of chromosomes at each pole contains one of the two homologues of each chromosome. Each pole is haploid, containing half the number of chromosomes present in the original diploid cell. Sister chromatids do not separate from each other in the first nuclear division, so each homologue is still composed of two chromatids joined at the centromere, and still considered one chromosome.
The second unique feature of meiosis is that the chromosome homologues do not replicate between the two nuclear divisions, so that chromosome assortment in the second division separates sister chromatids of each chromosome into different daughter cells.
In most respects, the second meiotic division is identical to a normal mitotic division. However, because of the crossing over that occurred during the first division, the sister chromatids in meiosis II are not identical to each other. Also, there are only half the number of chromosomes in each cell at the beginning of meiosis II because only one of the homologues is present. Figure 9.8b shows how reduction division occurs. The diploid cell contains four chromosomes (two homologous pairs). After meiosis I, the cells contain just two chromosomes (remember to count the number of centromeres, because sister chromatids are not considered separate chromosomes). During meiosis II, the sister chromatids separate, but each gamete still only contains two chromosomes, half as many of the germ-line cell.
Figure 9.8. Unique features of meiosis.
(a) Synapsis draws homologous chromosomes together, all along their lengths, creating a situation (indicated by the circle) where two homologues can physically exchange portions of arms, a process called crossing over. (b) Reduction division, omitting a chromosome duplication before meiosis II, produces haploid gametes, thus ensuring that the chromosome number remains the same as that of the parents, following fertilization.
Because mitosis and meiosis use similar terminology, it is easy to confuse the two processes. Figure 9.9 compares the two processes side-by-side. Both processes start with a diploid cell, but during meiosis, you can see that crossing over occurs and that during the first division in meiosis, the homologous pairs line up along the metaphase plate, while in mitosis, centromeres line up along the metaphase plate. These two differences result in haploid cells in meiosis and diploid cells in mitosis.
Figure 9.9. A comparison of meiosis and mitosis.
Meiosis differs from mitosis in several key ways, highlighted by the orange boxes. Meiosis involves two nuclear divisions with no DNA replication between them. It thus produces four daughter cells, each with half the original number of chromosomes. Also, crossing over occurs in prophase I of meiosis. Mitosis involves a single nuclear division after DNA replication. Thus, it produces two daughter cells, each containing the original number of chromosomes, which are genetically identical to those in the parent cell.
Key Learning Outcome 9.4. In meiosis, homologous chromosomes become intimately associated and do not replicate between the two nuclear divisions.