12.2Somatic Cell Division and Meiosis
In somatic cell division, a 2n parent cell doubles its DNA in accordance with the cell cycle and distributes it to two daughter cells (Fig. 12-2A). In meiosis, on the other hand, a 2n cell doubles its DNA and undergoes two successive divisions to become four 1n cells (Fig. 12-2B). These two cell division types are different in that, with meiosis, DNA distribution occurs after the first cell division without DNA replication. However, this is not the only difference between the two types.
Fig. 12-2. Somatic cell division and meiosis
A) The somatic cell division cycle. A cell doubles its chromosomes in the S phase (the DNA synthesis phase) and enters the M phase (the karyokinesis phase, which is followed by cytokinesis) via the G2 phase. In the M phase, the doubled chromosomes are evenly distributed to two daughter cells, which then go through the G1 phase and enter the S phase, thus repeating the cell cycle.
B) The meiosis cycle. A meiosis-induced cell performs one DNA replication (premeiotic DNA synthesis) and then successively goes through the first and second meiotic divisions, thereby creating 1n cells. In the case of an ovum, however, four equal ova are not necessarily created.
Let’s look at the meiotic process in detail (Fig. 12-3). A 2n cell has a pair of paternal and maternal chromosomes known as homologous chromosomes. Each chromosome is doubled by DNA replication and become two sister chromatids. During the somatic cell division process, each homologous chromosome moves independently, and two sister chromatids of the single chromosome are divided into two cells during the fission process. In meiosis, on the other hand, homologous chromosomes form pairs (synapsis). This pairing also occurs between sex chromosomes (in humans, X and Y), and genetic crossover takes place between paternal and maternal homologous chromosomes. Figure 12-4 shows formation of multiple crossover points between homologous chromosomes and chromatids; chromosome transfer takes place at crossover points, resulting in a change in gene combinations in a process known as genetic recombination. This process occurs randomly between homologous chromosomes, and involves the creation of a variety of chromosomes in which paternal and maternal regions are mixed. The point at which paternal and maternal chromosomes cross and attach is called chiasma, and these paired chromosomes are lined up in the center of a pair of mitotic spindles - the chromosome segregation apparatus. Then, following the degradation of proteins that connect the homologous chromosomes, they are segregated and distributed by the mitotic spindle to two cells (representing the first division). The second division then occurs, in which the sister chromatids that constitute the homologous chromosomes are segregated, and each is distributed to one cell.
The microtubules that constitute the mitotic spindle bind to chromosomes with their kinetochores*2, pushing and pulling them (see Chapter 6). The directions of kinetochores are different in somatic cell division and in meiosis. In somatic cell division (in which paired chromatids are carried in opposite directions), kinetochores are positioned facing opposite directions, and in meiosis I (in which paired chromatids are carried in the same direction), kinetochores are positioned facing the same way (Fig. 12-5).
Fig. 12-4. Genetic Crossover
Each chromatid (1 or 2) can cross with either sister chromatid (3 or 4).
Kinetochores: Kinetochores are regions found in chromosomes. They contain highly repetitive DNA sequences, and are bound to by many proteins. During cell division, microtubules are attached to these regions for chromosome segregation (kinetochore). Kinetochores are equivalent to the primary constriction sites of chromosomes in higher eukaryote.
Sex Determination and Reversal
The sex of mammals, including humans, is determined by the combination of X and Y chromosomes. Males have one X and one Y chromosome, while females have two X chromosomes. The SRY (i.e., the sex-determining region of Y) of the Y chromosome plays an important role in forming male organs. On the other hand, birds with heterozygous sex chromosomes become females, and those with homozygous sex chromosomes become males. In such cases, Z and W are used to express the sex chromosomes; females have ZW chromosomes, and males have ZZ chromosomes. In addition, there are many organisms, including fruitflies (Drosophila melanogaster), in which the sex is determined by the ratio of sex chromosomes to autosomes*3. Some plant species, such as the evening campion, use sex chromosomes to determine their sex.
On the other hand, the sex of many organisms is changed by environmental factors. The sex of some reptile species is determined by their thermal environment. By way of example, turtles tend to become male in low-temperature conditions and female in high-temperature conditions. Conversely, alligators tend to become female under low temperatures and male under high temperatures. Sexual reversal is also found in fish; black porgies change from male to female as they age, while giltheads change from female to male. Sex determination mechanisms are therefore diverse, and are believed to have evolved as a survival strategy that enables organisms to create progeny effectively in the natural environment.
Sex chromosomes and autosomes: Chromosomes that differ by sex and have genes that are involved in sex determination are called sex chromosomes. Other chromosomes are collectively called autosomes.