Meiosis

Meiosis is a specialized type of cell division necessary for sexual reproduction in eukaryotes because the cells produced by meiosis are ova and sperm. It consists of 2 successive cell divisions, meiosis I and meiosis II. Unlike in mitosis, in meiosis the chromosomes undergo a recombination that shuffles the genes from each parent, producing a different genetic combination in each gamete. The outcome of meiosis is 4 genetically unique haploid cells, whereas the outcome of mitosis is 2 genetically identical diploid cells.

Interphase consists of the G1 and S phases (there is no G2 phase in meiosis) and is followed by meiosis I and then meiosis II. Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, as in the mitotic cell cycle. In the G1 phase, each of the chromosomes consists of a single (very long) molecule of DNA. At this stage in humans, the cells contain 46 chromosomes, the same number as in somatic cells. During S phase, the chromosomes duplicate, so that each of the 46 chromosomes becomes a complex of 2 identical sister chromatids.

During meiosis I, homologous chromosomes (a matched pair, 1 derived from each parent) separate into 2 cells. The entire haploid content of each chromosome is contained in each of the resulting daughter cells; the first meiotic division thus reduces the ploidy of the original cell by half.

During meiosis II, each chromosome’s sister strands (the chromatids) are decoupled, and the individual chromatids are segregated into haploid daughter cells. The 2 cells resulting from meiosis I divide during meiosis II, creating 4 haploid daughter cells.

Chromosomal crossing over is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes. It occurs during prophase of the first meiotic division (prophase I), usually when matching regions on matching chromosomes break and then reconnect to the other chromosome. Although the same genes appear in the same order, the alleles are different. It is theoretically possible to have any combination of parental alleles in an offspring. This theory of independent assortment of alleles is fundamental to genetic inheritance. However, the chances of recombination are greater the farther apart 2 genes are from each other. The genetic distance is described in centimorgans (cM; named for Thomas Hunt Morgan, who described crossing over), and a distance of 1 cM between genes represents a 1% chance of their crossing over in 1 meiosis.

Genetic linkage describes the tendency of genes to be inherited together as a result of their proximity on the same chromosome. Linkage disequilibrium occurs when combinations of alleles are present in a population more or less frequently than would be expected based on their distances apart from each other. This concept is applied in searches for a gene that may cause a particular disease.

Although crossovers typically occur between homologous regions of matching chromosomes, a mismatch or unbalanced recombination may occur. This rare event can be a local duplication or deletion of genes on 1 chromosome, a translocation of part of 1 chromosome onto a different one, or an inversion of a part of the chromosome.

Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.