Meiosis is a special type of cell division. Unlike mitosis, the way normal body cells divide, meiosis results in cells that only have half the usual number of chromosomes, one from each pair. For that reason, meiosis is often called reduction division. In the long run, meiosis increases genetic variation, in a way which will be explained later.
Sexual reproduction takes place when a sperm fertilizes an egg. The eggs and sperm are special cells called gametes, or sex cells. Gametes are haploid; they have only half the number of chromosomes as a normal body cell (called a somatic cell). Fertilization restores the chromosomes in sex cells to the diploid number.
The basic number of chromosomes in the body cells of a species is called the somatic number and is labelled 2n. In humans 2n = 46: we have 46 chromosomes. In the sex cells the chromosome number is n (humans: n = 23). So, in normal diploid organisms, chromosomes are present in two copies, one from each parent (23x2=46). The only exception are the sex chromosomes. In mammals, the female has two X chromosomes, and the male one X and one Y chromosome.
All eukaryotes that reproduce sexually use meiosis. This also includes many single-celled organisms. Meiosis does not occur in archaea or bacteria, which reproduce by simple cell division.
Meiosis as part of sexual reproduction
The significance of fertilization
The offspring gets a set of chromosomes from each parent so that, overall, half of its heredity comes from each parent. But the two sets of chromosomes are not identical with the parental chromosomes. This is because they are changed during the reduction division by a process called crossing-over.
The significance of meiosis
This is two-fold:
1. First, to reduce the chromosomes in each egg or sperm to one set only. The two chromosomes in a pair are not identical because at any particular locus on the chromosomes there may be different alleles (forms of the gene) on the two chromosomes. Think about each pair of chromosomes, one set from each parent. From each pair, only one goes into each gamete. So, in the first place, gametes differ because of each pair the gamete may have the father's chromosome or the mother's. This is called 'independent assortment'. 2. To allow crossing-over to take place between each pair of parental chromosomes. Crossing-over changes which alleles sit on a particular chromosome. So, though described as 'mother's' and 'father's' chromosomes, in practice the gamete chromosomes are not all the same as the parents' chromosomes. This is because bits of chromosomes have been exchanged in the process of meiosis.
Although the gametes have only one set of chromosomes, that set is a shuffled mixture of genetic material from both parents. Every single egg or sperm may have a different selection of alleles from the parental chromosomes.
As with shuffling a deck of cards, many different combinations of genes can be produced without a change (mutation) in any individual gene. However, mutations do occur, and they may add alleles which were not in the population before. At any rate, the shuffling increases the variety of the offspring, and the variety gives at least some of the offspring a better chance of surviving in difficult times. The shuffling of alleles which takes place in meiosis may be the reason why sexual reproduction exists at all. "It is not difficult to think of reasons why sexual populations should have long-term advantages over asexual ones. They can evolve more rapidly to meet changing circumstances..."
Special cases
Cyclical parthenogenesis
Several quite large taxa (groups of organisms) use cyclical parthenogenesis. This is when several generations are born by virgin birth, and then a generation occurs with normal sexual reproduction. Examples include aphids, and cladocerans (small crustacea called water fleas). Aphids usually operate as follows: when the weather is good, and their plant hosts are at their best, they use parthenogenesis. At the end of the season, when the weather gets worse, they use sexual reproduction. This system of reproduction is called apogamy.
In parthenogenesis, the eggs contain only the mother's genetic material, and they are not fertilized. The egg cells may be produced either by meiosis or mitosis. When meiosis occurs, crossing-over produces a genetic fingerprint which differs somewhat from the mother's. So, the parthenogenetic greenfly offspring are not identical, and do show some genetic variation: some chromosome segments differ because of meiosis. Mitosis would produce identical offspring.
Amongst these parthenogenetic taxa are a number of species which have entirely abandoned sex.