Meiosis has three unique features :
The mechanism of cell division varies in important details in different organism. This is particularly true of chromosomal separation mechanism, which differ substantially in protists and fungi from the process in plants and animals that we will describe here. Meiosis in a diploid organism consists of two rounds of division, mitosis of one. Although meiosis and mitosis have much in common, meiosis has three unique features: synapsis, homologous recombination, and reduction division.
Synapsis :
The first unique feature of meiosis happens early during the first nuclear division. Following chromosome replication, homologous chromosomes, or homologous pair all along their length. The process of forming these complexes of homologous chromosomes is called synapsis.
Homologous Recombination :
The second unique feature of meiosis is that genetic exchange occurs between the homologous chromosomes while they are thus physically joined. The exchange process that occurs between paired chromosomes is called crossing over. Chromosomes are then drawn together along the equatorial plane of the dividing cell; subsequently, homologues are pulled by microtubules toward opposite poles of the cell. When this process is complete, the cluster of chromosomes at each pole contains one of the two homologues of each chromosome. Each pole is haploid, containing half the number of chromosomes present in the original diploid cell. Sister chromatids do not separate from each other in the first nuclear division, so each homologue is still composed of two chromatids.
Reduction Division :
The third unique feature of meiosis is that the chromosomes do not replicate between the two nuclear divisions, so that at the end of meiosis, each cell contains only half the original complement of chromosomes. In most respects, the second meiotic division is identical to a normal mitotic division. However, because of the crossing over that occurred during the first division, the sister chromatids in meiosis 2 are not identical to each other.
Meiosis is a continuous process, but it is most easily studied when we divide it into arbitrary stages. The stages of meiosis are traditionally called meiosis 1 and meiosis 2. Like mitosis, catch stage is subdivided further into prophase, metaphase, anaphase and telophase. In meiosis, however, prophase 2 is more complex than in mitosis. In meiosis, homologous chromosomes become intimately associated and do not replicate between the two nuclear divisions.
The Red Queen Hypothesis. One evolutionary advantage of sex may be that it allow populations to “store” recessive alleles that are currently bad but have promise for reuse at some time in the future. Because populations are constrained by a changing physical and biological environment, selection is constantly acting against such alleles, but in sexual species can never get rid of those sheltered in heterozygotes. The evolution of most sexual species, most of the time, thus manages to keep pace with ever-changing physical and biological constraints. This “treadmill evolution” is sometimes called the “Red Queen hypothesis”, after the Queen of Hearts in Lewis Carroll’s Through the Looking Glass, who tells Alice, “Now, here, you see, it takes all the running you can do, to keep in the same place.”
Miller’s Ratchet. The geneticist Herman Miller pointed out in 1965 that asexual populations incorporate a kind of mutational ratchet mechanism – once harmful mutations arise, asexual populations have no way of eliminating them, and they accumulate over time, like turning a ratchet. Sexual populations, on the other hand, can employ recombination to generate individuals carrying fewer mutations, which selection can then favor. Sex may just be a way to keep the mutational load down.
The Evolutionary Consequences of Sex
While our knowledge of how sex evolved is sketchy, it is abundantly clear that sexual reproduction has an enormous impact on how species evolve today, because of its ability to rapidly generate new genetic combinations. Independent assortment, crossing over, and random fertilization each help generate genetic diversity.
Whatever the forces that led to sexual reproduction, its evolutionary consequences have been profound. No genetic process generates diversity more quickly; and, as you will see in later chapters, genetic diversity is the raw material of evolution, the fuel that drives it and determines its potential directions. In many cases, the pace of evolution appears to increase as the level of genetic diversity increases. Programs for selecting larger stature in domesticated animals such as cattle and sheep, for example, proceed rapidly at first, but then slow as the existing genetic combinations are exhausted; further progress must then await the generation of new gene combinations. Racehorse breeding provides a graphic example: thoroughbred racehorses are all descendants of a small initial number of individuals, and selection for speed has accomplished all it can with this limited amount of genetic variability – the winning times in major races ceased to improve decades ago.
Paradoxically, the evolutionary process is thus both revolutionary process is thus both revolutionary and conservative. It is revolutionary in that the pace of evolutionary change is quickened by genetic recombination, much of which results from sexual reproduction. It is conservative in that evolutionary change is not always favored by selection, which may instead preserve existing combinations of genes. These conservative pressures appear to be greatest in some asexually reproducing organism that do not move around freely and that live in especially demanding habitats. In vertebrates, on the other hand, the evolutionary premium appears to have been on versatility, and sexual reproduction is the predominant mode of reproduction by an overwhelming margin.
The close association between homologous chromosomes that occurs during meiosis may have evolved as mechanism to repair chromosomal damage, although several alternative mechanism have also been proposed.
The evolutionary origin of sex is a puzzle.
Why Sex?
Not all reproduction is sexual. In asexual reproduction, an individual inherits all of its chromosomes from a single parent and is, therefore, genetically identical to its parent. Bacterial cells reproduce asexually, undergoing binary fission to produce two daughter cells containing the same genetic information. Most protists reproduce asexually except under conditions of stress; then they switch to sexual reproduction. Among plants, asexual reproduction is common, and many other multicellular organism are also capable of reproducing asexually. In animals, asexual reproduction often involves the budding off of a localized mass of cell, which grows by mitosis to form a new individual.