Question

what is a key difference between meiosis and mitosis? dna divides twice in mitosis and once in meiosis. crossing over only occurs in meiosis. synthesis of dna occurs. all of these are correct.

Explanation:

• For the first option: In mitosis, DNA divides once, and in meiosis, DNA divides twice. So this option is incorrect.
• For the second option: Crossing - over (exchange of genetic material between homologous chromosomes) is a key event that only occurs during meiosis (prophase I). This is a correct difference.
• For the third option: DNA synthesis (replication) occurs before both mitosis and meiosis (during the S - phase of the cell cycle). So this is not a difference.
• For the fourth option: Since the first and third options are incorrect, this option is also incorrect.

Answer:

B. Crossing over only occurs in meiosis.