Final answer:
A tetrad is formed during prophase I of meiosis when homologous chromosomes pair up, form a synaptonemal complex, and undergo crossing over at chiasmata.
Step-by-step explanation:
A tetrad is formed during prophase I of meiosis. This occurs when homologous chromosomes pair up and align closely together, facilitated by the formation of a synaptonemal complex, which is a protein structure that holds the homologous chromosomes together. As they pair, the chromosomes condense and crossing over occurs, where non-sister chromatids exchange genetic material at points called chiasmata. The structure now consists of four chromatids (a tetrad) because each homologous chromosome is made up of two sister chromatids, and this tetrad structure is crucial for the proper segregation of chromosomes during the first meiotic division.During prophase I of meiosis, chromosomes condense, become visible, and the homologous chromosomes move closer together.
The synaptonemal complex forms between the homologous chromosomes, creating a lattice of proteins that spreads along the entire length of the chromosomes. This complex supports the crossing over of chromosomal segments between non-sister homologous chromatids, leading to genetic variation. At the end of prophase I, the homologous chromosomes are held together at the chiasmata and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are visible.