Meiosis is a very specialized process of cell division that produces gametes (eggs and sperm), and is quite distinct from the mitotic cycle of normally dividing cells. In humans, it is estimated that at least 10% of conceptions have defects that probably occurred during meiosis of either paternal or maternal gametes. Most of these will result in miscarriage. Understanding the process of meiosis is therefore fundamental to understanding human health and development. This page uses simple schematics to compare mitosis to meiosis in a generalized cell.
First, consider mitosis (typical cell division) in a normal diploid cell, with two copies of each chromosome. These are indicated by the red and blue lines in the figure, and referred to as homologues. The chromosomes are duplicated during DNA replication, and the duplicates, called sister chromatids, remain attached to one another until chromosome segregation. Each daughter cell then receives one of the sisters from each homologue pair. Importantly, this means that the daughter cells are exact copies of the mother cell, with two copies of each chromosome, so they can go through the same process again. Thus, we call this process the cell cycle. This is the usual process of division by which the cells of our bodies renew themselves. (For further information, you can view lecture notes with a detailed summary of cell cycle control on this site.)
The purpose of meiosis is to reduce the normal diploid cells (2 copies of each chromosome / cell) to haploid cells, called gametes (1 copy of each chromosome per cell). In humans, these special haploid cells resulting from meiosis are eggs (female) or sperm (male). In yeast cells, they are spores.
To carry out this specialized process, the cells duplicate their DNA but follow this by two rounds of division, instead of one. The first division separates the duplicated homologues from each other. This essentially reduces the number of chromosomes in each cell. Thus, we call this the reductional, or Meiosis I division. The second division operates similarly to the mitotic division, and separates the sister chromatids from one another; we call this the equational, or Meiosis II division. The offspring from meiosis have half the number of chromosomes as their parent cell, because they receive just one copy of each chromosome, rather than two. (Compare the offspring in the meiosis diagram to those in mitosis).
An important additional feature of meiosis is the process of recombination, or genetic exchange between the homologous chromosomes. This is indicated by the swapping of red and blue on the homologous chromosomes in the diagram. Although homologous chromosomes contain the same genes, they may have different versions, or alleles, of each gene. Recombination allows the different versions to be shuffled into new combinations. And, the physcial mechanism of recombination is often required for the chromosomes to segregate properly in the Meiosis I division. (A more detailed diagram can be found elsewhere).
Finally, to make a diploid again, the haploid gametes produced by each parent will fuse to form a zygote (during fertilization). Thus, the offspring receives one copy of each chromosome from each parent. Depending upon the recombination events that occurred to produce each gamete, genetic information may be rearranged. This explains why siblings get different combinations of genes from their parents, which is why they look related, but not identical.
All these processes--replication, recombination, and chromosome segregation--are linked, and require successful completion of each preceding stage. Any defects in the careful choreography of meiosis can therefore have profound effects on the health of the gametes and the resulting zygotes. See this page for a discussion of meiotic errors.
© S. L. Forsburg .
