Trisomy 21, which results in Down syndrome, results from nondisjunction of chromosome 21 in humans. Nondisjunction occurs when two homologous chromosomes, or two sister chromatids, do not separate. Which of the following describes the mechanism of this defect?
(A) During DNA replication in S phase of the cell cycle, the two new strands do not separate.
(B) During mitosis, at the metaphase plate, non-sister chromatids do not separate.
(C) The mitotic spindle attaches to chiasmata rather than kinetochores.
(D) The same microtubule in the spindle attaches to both sister chromatids during meiosis II.