18.2Somatic Cell Division (mitosis) and Meiosis

In somatic cell division (mitosis), a 2n parent cell doubles its DNA in accordance with the cell cycle and distributes it to two daughter cells (Figure 18-2A). On the other hand, in meiosis, 1n cells are created from a 2n cell prior to fertilization. During meiosis, the parent 2n cell first doubles its DNA and then undergoes two successive divisions to become four 1n cells (Figure 18-2B). The major difference between mitosis and meiosis is that in meiosis, DNA distribution occurs after the first cell division without DNA replication*3.

*3 In Chapters 2 and 13 mitosis was discussed as mitotic division, whereas in this chapter, we discuss the two different types of mitotic division, one in which the genome is maintained as a diploid (2n form), and the other where the chromosomes split to become haploids (1n form). The former is referred to as mitosis in the narrow sense of the word and the latter is referred to as meiosis. Meiosis is also a form of mitotic division.

Let’s look at the mitotic and meiotic processes in detail (Figure 18-3). A 2n cell has a pair of paternal and maternal chromosomes known as homologous chromosomes. Each chromosome is doubled by DNA replication to produce two sister chromatids.

Figure 18-3 Meiosis and somatic cell division (mitosis) processes

During mitosis, each homologous chromosome moves independently, and the two sister chromatids of a single chromosome are divided into two cells during the fission process. In meiosis, homologous chromosomes form pairs (synapsis). This pairing also occurs between sex chromosomes (in humans, X and Y) and genetic crossing-over occurs between paternal and maternal homologous chromosomes. Figure 18-4 illustrates formation of multiple crossover points between sister chromatids. Chromosome transfer takes place at these crossover points, resulting in a change in gene combinations in a process known as genetic recombination. The crossover process does not interfere with chromatid segregation; on the contrary, they have been shown to be indispensable to the progress of segregation. The genetic recombination 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 a chiasma, and the paired chromosomes are lined up between the spindles, the chromosome segregation apparatus. Then, proteins that connect the homologous chromosomes are degraded and the homologous chromosomes are segregated and distributed to two cells by the mitotic spindle (first division). The second division follows, in which the sister chromatids that constitute the homologous chromosomes are segregated and each is distributed to one cell.

Microtubules that constitute the mitotic spindle (see Selection 1 of Chapter 13, Column Figure 13-2) bind to chromosomes at kinetochores*4, pushing and pulling the chromosomes (see Selection 3 of Chapter 13, Column Figure 13-3). The location where the kinetochores attach to chromatins differs between mitosis and meiosis. In mitosis (in which paired chromatids are carried in opposite directions), the kinetochores are positioned facing opposite directions, whereas in meiosis I (in which paired chromatids are carried in the same direction), kinetochores are positioned facing the same direction (Figure 18-5).

*4 Kinetochores and centromeres: Centromeres are the constricted region observed in chromosomes during the M phase, and are known to have specific repetitive DNA sequences. Kinetochore complex is formed around the centromere DNA with proteins binding to this region and plays important roles such as interaction with microtubules during division.

Figure 18-4 Genetic crossing-over

Chromosomal crossing-over does not occur between sister chromatids (1 and 2), but each chromatid (1 or 2) can cross with either sister chromatid of the other homologous chromosome (3 or 4).

Figure 18-5 Direction of kinetochores

In Meiosis I, the kinetochores of the two chromatids face the same direction (left), whereas they face opposite directions in mitosis (right).


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 (hetero), whereas females have two X chromosomes (homo). The SRY (sex-determining region of the Y chromosome) plays an important role in the formation of male organs. However in birds, those 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 fruit flies, in which sex is determined by the ratio of sex chromosomes to autosomes. Some plant species, such as the white campion, use sex chromosomes to determine their sex. On the other hand, the sex of many organisms is altered by environmental factors. The sex of some reptile species is determined by their thermal environment during embryogenesis. For instance, 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; Japanese black porgies change from male to female as they age, whereas wrasses 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 effectively create progeny in the natural environment.

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