Tetrads, also known as bivalents, form during the prophase 1 stage of meiosis when duplicated chromosomes, each composed of two identical chromatids, pair up and complete the process of crossing over in which one chromosome exchanges corresponding segments of genetic material with the other chromosome in the tetrad. This exchange of physical parts and genetic information creates a tight grouping of the two chromosomes, each consisting of two sister chromatids.
Pearson Education's BioCoach describes the prophase 1 stage in the process of meiosis, or sexual reproduction, in eukaryotic cells. Although there are many other stages in meiosis, prophase 1 is one of the most important steps and takes up the most time, typically consuming up to 90 percent of the total cell division processing time. Tetrads are incredibly important in developing genetic diversity and in the spreading of genetic information through recombination of the sister chromatids.
At the beginning of prophase 1, the chromosomes in the cell have already been duplicated. First, these chromosomes condense and coil, becoming thicker. This makes them visible under a microscope. These thicker chromosomes then split off into pairs and begin to cross over. Crossing over is the process by which the ends of one chromosome can switch with another.