In which of the following phases does crossing over occur?

• A. Metaphase I
• B. Prophase I
• C. Anaphase II
• D. Prophase II

Answer: (B)

Verified step by step guidance

1. Understand the context of meiosis, which is a type of cell division that reduces the chromosome number by half, creating four haploid cells. It consists of two stages: Meiosis I and Meiosis II.
2. Recognize that crossing over is a process where homologous chromosomes exchange genetic material. This increases genetic diversity in gametes.
3. Identify that crossing over occurs during Prophase I of Meiosis I. During this phase, homologous chromosomes pair up in a process called synapsis, forming structures known as tetrads.
4. Note that during Prophase I, the exchange of genetic material occurs at points called chiasmata, where the chromatids of homologous chromosomes physically exchange segments.
5. Differentiate Prophase I from other phases such as Metaphase I, Anaphase II, and Prophase II, where crossing over does not occur. Metaphase I involves alignment of tetrads at the cell equator, Anaphase II involves separation of sister chromatids, and Prophase II is a preparatory phase for the second meiotic division.