Answer: Extra copy of chromosome 21 (nondisjunction).
- A Deletion of part of chromosome 21 during meiosis
- B Extra copy of chromosome 21 (nondisjunction)
- C Translocation of chromosome 14 onto another chromosome
- D Loss of one sex chromosome during gamete formation
Correct answer: B. Extra copy of chromosome 21 (nondisjunction)
Explanation: Down syndrome: extra copy of chromosome 21 (2n+1 = 47). Most often caused by nondisjunction during maternal meiosis I.
The cell cycle: G1 (growth and preparation), S (DNA replication, chromosome number stays 2n but DNA content doubles), G2 (final checks before division), and M (mitosis itself) - together, G1+S+G2 make up interphase.
Concept context
Mitosis and meiosis: how cells divide for growth, repair, and reproduction.