During which stage of meiosis do homologous chromosomes form a bivalent?

During prophase 1 of meiosis, homologous chromosomes undergo a critical process called synapsis, where they pair up closely to form structures known as bivalents or tetrads. This pairing is essential for the subsequent exchange of genetic material through a process called crossing over, which enhances genetic diversity.

The bivalent consists of four chromatids—two from each homologous chromosome—and it allows for proper alignment and segregation of chromosomes during the later stages of meiosis. As the cell progresses through prophase 1, further events like crossing over can occur, leading to increased genetic variation in the gametes ultimately produced.

In contrast, other stages such as metaphase 1 involve the alignment of these bivalents at the metaphase plate but do not involve their formation. Prometaphase 1 is characterized by the breakdown of the nuclear envelope and further preparation for chromosome alignment, while anaphase 1 is when the homologous chromosomes are pulled apart to opposite poles of the cell. Thus, the formation of bivalents is uniquely tied to prophase 1 of meiosis.