In meiosis, what is the structure formed by two homologous chromosomes called?

During meiosis, the structure formed by two homologous chromosomes, which pair up closely together, is known as a bivalent. This pairing occurs during prophase I of meiosis, where homologous chromosomes align with each other to facilitate genetic recombination or crossing over.

Each homologous chromosome consists of two sister chromatids, and together, the bivalent consists of four chromatids in total. This feature is crucial for the accurate segregation of chromosomes into gametes, ensuring that each gamete receives the correct number of chromosomes and genetic diversity is achieved through the exchange of genetic material between the paired chromosomes.

In context, while terms like "diplotene," "chromatid," and "tetrad" relate to different aspects or stages of meiosis, they do not specifically denote the pairing of two homologous chromosomes. For example, tetrads can often refer to the entire structure comprising the bivalent, but the specific designation of the pair of homologous chromosomes prior to this stage is best referred to as a bivalent.