Principles of Genetics and Breeding

1.3.9 Meiotic Division

• Meiosis is the process during which the primary gametocytes develop into eggs or sperm. A number of important cellular changes occur during meiosis, but for a geneticist, the important ones are those that affect the genes and chromosomes. Meiosis consists of two cell divisions.
• The first meiotic division is called the “reduction division” and the second is called the “equational division”. Prior to the reduction division, the homologues that form each chromosome pair replicate. The replicated homologues are called “sister chromatids”. The replication of the chromosome is usually uneventful in that they replicate themselves perfectly.
• During the early phase of the reduction division, the replicated homologues that form each chromosome pair come together. Because each homologue is composed of a pair of sister chromatids, each bundle is composed of four chromosomes called a “tetrad”.
• The chromosomes that form a tetrad typically twist around each other and break. When the chromosomes rejoin, pieces can be exchanged between homologues. When this happens, pieces of chromosomes, along with the genes that are located along those pieces, are transferred from one homologue to the other. This process is called "crossing over”.
• Crossing over is one of the most important biological processes, because it greatly increases genetic variability. Crossing over tends to occur at particular locations along each chromosome. Each gene has a given cross over frequency. Genes located near the centromere cross over less frequently than those further away. Genes that are located near each other on a chromosome cross over as a unit and are called “linked”; the closer they are, the more tightly linked they are.
• During reduction division, the chromosome number is reduced from the diploid (paired) state to the haploid (unpaired) state. The homologues that form each chromosome pair separate and are parceled into the two daughter cells (for males, the two secondary spermatocytes; for females, the secondary oocyte and first polar body). When the chromosomes divide, they are not parceled into maternal and paternal sets. The direction in which each replicated homologue (sister chromatids) of each chromosome pair goes is random and independent of that for all other chromosomes. This process is called “independent assortment”. Although each secondary gametocyte contains a haploid set of chromosomes, each chromosomes is duplicated.
• The second meiotic division is called the “equational division”. During the equational division, the replicated homologues (sister chromatids) in each secondary gametocyte separate and are parceled into either two sperm cells or into the egg and second polar body.
• This produces haploid gametes; each gamete contains a single chromosome from each pair, which means that it contains a single copy of each gene.