Chapter 7

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.

A researcher isolates DNA from different types of cells and determines the amount of DNA for each type of cell. The samples may contain cells in various stages of the cell cycle. In order of increasing DNA content, which of the following would have the least amount of DNA to the greatest amount of DNA? (A) Sperm, neuron, liver (B) Ovum, muscle, taste bud (C) Liver, heart, intestine (D) Skin, neuron, ovum

Which of the following mutations would be least likely to have any discernable phenotype on the individual? (A) Translocation of the last 1,000 base pairs of chromosome 1 onto chromosome 2 (B) Nondisjunction of chromosome 7 to produce trisomy7 (C) Nondisjunction of chromosome 8 to produce monosomy-8 (D) Deletion of 400 base pairs, resulting in the loss of an enhancer