Which of the following events occurs during Prophase I of meiosis?

• A. Sister chromatids separate and move to opposite poles
• B. Homologous chromosomes pair and crossing over occurs
• C. Chromosomes align at the metaphase plate as individual chromosomes
• D. The nuclear envelope reforms around separated chromosomes

Answer: (B)

Verified step by step guidance

1. Understand the context of the question: Prophase I is a stage in meiosis, a type of cell division that reduces the chromosome number by half and is essential for sexual reproduction. It is distinct from mitosis and has unique events.
2. Recall the key events of Prophase I: Homologous chromosomes (pairs of chromosomes, one from each parent) come together in a process called synapsis. This pairing forms structures called tetrads.
3. Recognize that crossing over occurs during Prophase I: During synapsis, homologous chromosomes exchange genetic material at points called chiasmata. This process increases genetic diversity in gametes.
4. Eliminate incorrect options: Sister chromatids separating and moving to opposite poles occurs during Anaphase II of meiosis, not Prophase I. Chromosomes aligning at the metaphase plate as individual chromosomes happens during Metaphase II. The nuclear envelope reforms around separated chromosomes during Telophase I or Telophase II, depending on the stage.
5. Conclude that the correct answer is: 'Homologous chromosomes pair and crossing over occurs,' as this is the defining event of Prophase I in meiosis.