Meiosis and meiotic errors

Abstract

AbstractDerived from the Greek word for diminution, meiosis means “to halve”. The basic purpose of meiosis is straightforward: to reduce by half the chromosome complement of a diploid cell during gametogenesis (from 46 to 23 in humans) to give rise to haploid gametes. Subsequent fusion of a haploid spermatozoan and a haploid oocyte at fertilization restores the somatic diploid chromosome number in the resulting offspring.The process of meiosis is conserved through evolution although with marked differences between species and also between sexes. Unlike in mitosis (the division of somatic cells), where daughter cells should have the same genetic component as the parent cell from which they were derived, meiosis serves to effectively “shuffle” the genetic material in two ways, in order to generate an almost infinite genetic variety from the starting chromosomes.The first of the mechanisms giving rise to variation in alleles is by the independent segregation of the chromosome pairs to the resulting cells; in humans this mechanism alone would give rise to 529 different genetic cell types, and is termed “random assortment”. Perhaps surprisingly, meiotic segregation errors are common in human female meiosis with a large proportion of oocytes having abnormal chromosome numbers (aneuploidy). This does not necessarily impair their fertilization capability, but results in chromosomally unbalanced embryos, which either fail to implant, abort spontaneously, cause intrauterine/perinatal death, or cause the development of liveborn children with chromosome disorders, such as Trisomy 21 Down syndrome.The second mechanism occurs when chromosome arms exchange homologous regions of DNA through crossing‐over, the formation of chiasmata, leading to the recombination of parental alleles. Each individual chromosome pair during meiosis can (and usually does) undergo several different recombination events at any point along their length, and the many different ways in which these can then be resolved into the resulting cells create an “endless” level of variety, such that no two oocytes or sperm from one individual are genetically identical. Chiasmata also serve a most important purpose during meiosis; that of holding homologous chromosomes together until anaphase I.In this article, we will describe cytogenetic aspects of the process of meiosis, with special reference to the situation in human males and females. Thus, we will summarize achievements in obtaining information on meiosis by microscopic analysis of human germ cells.