What is a tetrad?

• A. A single replicated chromosome
• B. A structure formed when corresponding homologous chromosomes pair up during Prophase I
• C. The exchange of genetic material between chromatids
• D. Two sister chromatids

Answer: (B)

A tetrad is the paired set of homologous chromosomes that forms during Prophase I of meiosis. After DNA replication, each chromosome consists of two sister chromatids. During synapsis, homologous chromosomes pair up and align side by side, so the structure contains two homologous chromosomes, each with two sister chromatids—four chromatids in total. This arrangement, a tetrad, is where crossing over can occur between non-sister chromatids, increasing genetic diversity. The other descriptions don’t fit: a single replicated chromosome is just one chromosome with two sister chromatids; crossing over describes the exchange of genetic material within a tetrad but is not the tetrad itself; two sister chromatids refer to one replicated chromosome, not the paired homologous set.