Choose the ONE best answer.
0.1. A 10-year-old girl is brought by her parents to the dermatologist. She has many freckles on her face, neck, arms, and hands, and the parents report that she is unusually sensitive to sunlight. Two basal cell carcinomas are identified on her face. Based on the clinical picture, which of the following processes is most likely to be defective in this patient?
A. Repair of double-strand breaks by error-prone homologous recombination
B. Removal of mismatched bases from the 3′-end of Okazaki fragments by a methyl-directed process
C. Removal of pyrimidine dimers from DNA by nucleotide excision repair
D. Removal of uracil from DNA by base excision repair
Correct answer = C. The sensitivity to sunlight, extensive freckling on parts of the body exposed to the sun, and presence of skin cancer at a young age indicate that the patient most likely suffers from xeroderma pigmentosum (XP). These patients are deficient in any one of several XP proteins required for nucleotide excision repair of pyrimidine dimers in ultraviolet radiation– damaged DNA. Double-strand breaks are repaired by nonhomologous end joining (error prone) or homologous recombination (“error free”). Methylation is not used for strand discrimination in eukaryotic mismatch repair. Uracil is removed from DNA molecules by a specific glycosylase in base excision repair, but a defect in this process does not cause XP.
0.2. Telomeres are complexes of DNA and protein that protect the ends of linear chromosomes. In most normal human somatic cells, telomeres shorten with each division. In stem cells and in cancer cells, however, telomeric length is maintained. In the synthesis of telomeres:
A. telomerase, a ribonucleoprotein, provides both the RNA and the protein needed for synthesis.
B. the RNA of telomerase serves as a primer.
C. the RNA of telomerase is a ribozyme.
D. the protein of telomerase is a DNA-directed DNA polymerase.
E. the shorter 3′→5′ strand gets extended.
F. the direction of synthesis is 3′→5′.
Correct answer = A. Telomerase is a ribonucleoprotein particle required for telomere maintenance. Telomerase contains an RNA that serves as the template, not the primer, for the synthesis of telomeric DNA by the reverse transcriptase of telomerase. Telomeric RNA has no catalytic activity. As a reverse transcriptase, telomerase synthesizes DNA using its RNA template and so is an RNA-directed DNA polymerase. The direction of synthesis, as with all DNA synthesis, is 5′→3′, and it is the 3′-end of the already longer 5′→3′ strand that gets extended.
0.3. While studying the structure of a small gene that was sequenced during the Human Genome Project, an investigator notices that one strand of the DNA molecule contains 20 A, 25 G, 30 C, and 22 T. How many of each base is found in the complete double-stranded molecule?
A. A=40,G =50,C =60,T =44
E. A=42,G =55,C =55,T =42
B. A=44,G =60,C =50,T =40
C. A=45,G =45,C =52,T =52
D. A=50,G =47,C =50,T =47