Cells of pluricellular organisms can be classified in two broad categories: somatic and germinal cells. Somatic cells account for nearly all the body and proliferate by mitosis giving two new somatic cells after every division. Germinal cells are much less abundant. They are found in gonads where they can produce either more germinal cells by mitosis or gametes by meiosis. Gametogenesis is the process by which germinal cells divide by meiosis to give haploid cells that become gametes. Two things occur during gametogenesis: meiosis and cellular differentiation.
Before going further, let's make some concepts clear. Every cell contains two set of chromosomes, one coming from the mother and the other from the father (Figure 1). For example, if the cells of a species contain 4 chromosomes (in humans there are 46), the female provided two chromosomes (1m and 2m) and the male the other two (1p and 2p). 1m and 1p chromosomes have the same genes and they are sorted in the same order along the chromosome. They are known as homologuous chromosomes. However, these genes do not show identical nucleotide sequences. Although they express the same proteins, there are some variations when comparing the nucelotide sequences. Genes located in the same position of two homologue chromosomes coding for the same messenger RNA with slightly different sequences are known as alleles. So, 1p and 1m chromosomes have alleles, two per gen, one coming from the male and the other from the female. The same happens to 2m and 2p chromosomes. In this species, there are two couples of homologous chromosomes, whereas in humans there are 23 pairs of homologous chromosomes. After meiosis, there is just one chromosome of each pair of homologous chromosomes per gamete. Thus, in this species, every gamete contains two chromosomes (1m-2p, or 1p-2m). There are 23 chromosomes in human gametes. Meiosis reduces the number of chromosomes by half, and there is always one chromosome of every pair of homologous chromosomes in each gamete. Germinal and somatic cells, which have the two members of each pair of homologous chromosomes, are known as diploid, whereas gametes, having only one member of each pair, are known as haploid.
Meiosis is a cellular process that reduces by half the number of chromosomes. Every gamete keeps one chromosome of every pair of homologous chromosomes that were present in the germinal cell. Thus, during sexual reproduction, the fusion of two gametes (fertilization) from two individuals, a male and a female, gives a new cell known as zygote with the normal number of pairs of homologous chromosomes, i.e. it is a diploid cell that will give a diploid animal by proliferation and cell differentiation.
Do not confuse meiosis and mitosis (Figures 2 and 3). During mitosis, the DNA is duplicated and then divided between the two daughter cells, so that the two new cells have the same information as the progenitor cell. During meiosis, although the DNA is also duplicated, germinal cell divides twice (meiosis I and meiosis II) giving four haploid cells. In males, these four cells become sperm and in females one of these cells develops into the oocyte.
Meiosis is not just a reduction by half of the number of chromosomes. During meiosis, there is also a molecular mechanism known as recombination (Figure 3). In somatic and germinal cells, the genetic information coming from the male and female are in separated chromosomes. Both, male and female provide one chromosome to every pair of homologous chromosomes. During recombination, there is exchange of segments between homologous chromosomes. It means that some genes of the chromosome provided by the female are now in the chromosome provided by the male, and the other way around. In this way, there are chromosomes with combination of genes (aleles) that did not exist previously. After fertilization, these chromosomes with genes from both progenitors may influence the features of the new organism.
Regarding chromosomes, the meiotic process is similar in both male and female germinal cells giving spermatozoa and oocytes, respectively. Chromosome recombination and reduction of the number of chromosomes happen in all eukaryotic species with sexual reproduction. However, remarkable differences are observed between spermatogenesis (male gamete differentiation) and oogenesis (female gamete differentiation). Gametogenesis is also quite different between species. These differences give clues about how fertilization and posterior embryo development of a particular species are going to be.
Meiosis has several phases which are similar in both males and females of all species with sexual reproduction. Briefly, duplication of DNA during S phase, meiotic prophase (leptotene, zygotene, pachitene, diplotene, diakinesis), meiosis I (first division: metaphase I, anaphase I, interphase without S phase), meiosis II (second division: prophase II, metaphase II, anaphase II). Chromosome recombination takes place during prophase I, and reduction of the number of chromosomes during divisions: meiosis I and II.