Meiosis | Cell division | Biology (article)
Whereas mitosis occurs in somatic tissue and results in tow identical daughter cells, meiosis occurs in gametocytes (germ cells) and results in up to four nonidentical sex cells (gametes). Meiosis shares some likenesses with mitosis. In both processes, for example, the genetic material must be duplicated, chromatin is condensed to form chromosomes and microtubules emanating from centrioles are involved in dividing genetic material. However, the MCAT tends to ask regarding the differences between these two processes.
In contrast to mitosis, which consists of one round of replication and division, meiosis consists of one round of replication pursued by two rounds of division, as shown in Figure 2.5. Meiosis I consequences in homologous chromosomes being isolated, generating haploid daughter cells; this is studied/known as a reductional division. Meiosis II similar to mitosis, in that it results in the segregation of sister chromatids and is known as equational division.
Note that crossing over occurs amidst homologous chromosomes and not between sister chromatids of the same chromosome⎯the latter are identical, so crossing over would not produce any change. Those chromatids included are left with a changed but structurally complete set of genes. Such genetic recombination can unlink joined genes, thereby increasing the variety of genetic combinations that can be produced via gametogenesis.
Linkage refers to the trend for genes to be hereditary together; genes that are found further from each other physically are fewer likely to be inherited together, and more likely to undergo crossing over relative to each other. Thus, as opposed to asexual reproduction, which produces identical offspring, sexual reproduction provides the advantage of great genetic diversity, which is believed to increase the mastery of a family to evolve and adapt to a changing environment.
- 2n → 2n
- Occurs in all separating cells
- Homologous chromosomes do not pair
- No crossing over
- 2n → n
- Occurs in sex cells only
- Homologous chromosomes align on the opposite side of the metaphase plate
- Crossing over can occur