Sexual reproduction requires meiosis, a process in which a parent cell divides to produce cells with half the genetic material of the parent. A diploid parent cell, for example, divides to make four haploid cells. In sexual reproduction, haploid gametes from two individuals then combine to produce a diploid zygote. An offspring resulting from sexual reproduction is genetically different from both parents.
In the accompanying animation, we examine the events of meiosis, using a model cell with two pairs of chromosomes. One chromosome of each pair is maternally derived, while the other is paternally derived; each is distinguished by different colors. The colors allow us to track two mechanisms of producing unique daughter cells: independent assortment of chromosomes and the phenomenon of crossing over.
Before meiosis begins, in the preceding interphase, DNA replication takes place. At this point, the chromosomes are not condensed and are not visible under a light microscope. However, we depict them in a condensed manner so that you can see that the cell replicates each chromosome to form two chromatids.
Meiosis consists of two cell divisions—meiosis I and II. As the diploid cell enters the first phase of meiosis I, called prophase I, its chromatin condenses into discrete chromosomes. The cell has two centrosomes, which form the mitotic spindle as the centrosomes migrate to opposite poles of the cell.
This diploid cell contains homologous chromosomes, which pair up later in prophase I. One chromosome in each pair is maternally derived (indicated by blue) and one is paternally derived (indicated by red). The pairing is called synapsis. At this time, as well, the nuclear envelope breaks down.
Late in prophase I, the homologous chromosomes exchange genetic material, as indicated by crossing-over points called chiasmata. Recombinant chromatids consist of both maternally and paternally derived DNA. In this phase, microtubules attach to the chromosomes, directing them to the equatorial plate.
At metaphase I, homologous chromosomes have lined up on the equatorial plate in a pair-wise fashion, with one homolog on either side of the plate. Note that the chromosomes assort independently: for example, the maternal chromosomes align randomly, not necessarily on the same side of the plate.
During anaphase I, chromosomes from each pair move to opposite poles of the cell. The centromeres of the chromosomes do not divide, so each chromosome still consists of two sister chromatids, which now may not be genetically identical due to crossing over.
In some species, telophase I occurs, a stage in which chromosomes decondense, nuclear membranes re-form, and the cytoplasm divides in a process called cytokinesis. A short interphase period, called interkinesis, may then follow telophase I. Note that DNA replication does not take place during interkinesis.
During the second half of meiosis, in prophase II, the chromatin again condenses into discrete chromosomes. There are now only a haploid number of chromosomes per cell. Each chromosome consists of two chromatids joined together by a centromere.
At metaphase II, the chromosomes have lined up on the equatorial plate. Microtubules from opposite poles attach to each sister chromatid of a chromosome.
During anaphase II, the centromeres divide, and both chromatids become independent chromosomes and move to opposite poles of the cell.
During telophase II, chromosomes again decondense and nuclear membranes re-form. Depending on the species, cytokinesis may occur.
Meiosis and sexual reproduction produce genetic diversity.
When a diploid cell divides by meiosis to produce gametes, each of the four daughter cells is genetically unique. The uniqueness arises in part from the independent assortment of chromosomes in meiosis. Through independent assortment, each daughter cell randomly receives either a maternally derived homolog or the paternally derived homolog from each chromosome pair. In addition, the process of crossing over produces chromosomes that have unique combinations of paternally and maternally derived regions.
This shuffling of the genetic material produces genetically unique gametes, each of which can then fuse with another unique gamete during fertilization to produce a unique zygote of the next generation.