Problem 4 - AP Psychology Premium Prep 2021

Question

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.

Step-by-Step Solution

Answer

The short answer to the question is (D) - the same microtubule in the spindle attaches to both sister chromatids during meiosis II. This is the mechanism that leads to nondisjunction of chromosome 21 and results in Down syndrome.

Step 1: Determine the phase in which nondisjunction occurs

Trisomy 21 results from the failure of chromosome 21 to separate properly during cell division. This failure is called nondisjunction. Trisomy occurs mainly during meiosis, which is the process that generates gametes (sperm and eggs) in humans.

Step 2: Analyze each possible option in detail

Let's go through each option and analyze how it may or may not correspond to the mechanism of Trisomy 21: (A) DNA replication during S phase is not responsible for the failure of chromosomes or chromatids to separate; it is the process that duplicates the DNA in preparation for cell division. So, this option is not correct. (B) Mitosis is the process of dividing somatic (non-reproductive) cells, and nondisjunction leading to trisomy occurs in meiosis. Also, this option talks about the separation of non-sister chromatids, but the nondisjunction of homologous chromosomes occurs during meiosis I and sister chromatids in meiosis II. So, this option is not correct. (C) This option refers to the mitotic spindle attaching to chiasmata instead of kinetochores. However, chiasmata are involved in crossover events during prophase I of meiosis I and not in chromosome separation. Also, the kinetochores are the key structures that attach to the spindle fibers and help in proper chromosome segregation during cell division. So, this option is incorrect. (D) This option says that the same microtubule in the spindle attaches to both sister chromatids during meiosis II. If this happens, sister chromatids will not be properly separated, leading to an incorrect number of chromosomes being distributed to the resulting cells. This can result in an extra copy of chromosome 21, which would lead to trisomy 21. So, this option is correct.

Step 3: Conclusion

Based on the analysis, the correct answer is (D) – the same microtubule in the spindle attaches to both sister chromatids during meiosis II. This is the mechanism that leads to nondisjunction of chromosome 21 and results in Down syndrome.