What constitutes a tetrad in meiosis?

A tetrad in meiosis refers specifically to the structure that forms during the first prophase of meiosis, known as prophase I. During this stage, homologous chromosomes pair up and become closely associated, forming what is termed a bivalent or tetrad. Each tetrad consists of four chromatids, which are two homologous chromosomes each containing two sister chromatids. This is where the term "tetrad" derives from, meaning four.

The key feature of a tetrad is that it represents a pair of homologous chromosomes, each composed of two sister chromatids, which are ultimately linked by association at their centromeres. This pairing facilitates genetic recombination through crossing over, which increases genetic diversity in the resulting gametes.

The other options fail to accurately describe a tetrad. For example, while two sister chromatids connected at one centromere are themselves important structures, they do not represent the pairing of homologous chromosomes necessary to form a tetrad. Similarly, the mention of four separate chromosomes does not capture the critical aspect of pairing that defines a tetrad, and chromosomes aligned at the metaphase plate describes a later stage of meiosis, rather than the tetrad formation seen in prophase I.