A dermatologist examines a 10-year-old girl who has numerous freckles on her face, neck, arms, and hands. The parents mention that she is very sensitive to sunlight. The dermatologist identifies two basal cell carcinomas on her face. Based on these observations, which process is most likely 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 patient's sensitivity to sunlight, extensive freckling in sun-exposed areas, and early onset skin cancer suggest that she may have xeroderma pigmentosum (XP). Individuals with XP have a deficiency in one of the XP proteins required for nucleotide excision repair of pyrimidine dimers in DNA damaged by ultraviolet radiation. Double-strand breaks are repaired by nonhomologous end joining (error-prone) or homologous recombination (error-free). Methylation is not involved in strand discrimination in eukaryotic mismatch repair. Uracil is removed from DNA by a specific glycosylase in base excision repair, but this defect does not cause XP.

Telomeres are complexes of DNA and protein that protect the ends of linear chromosomes. They shorten with each division in most normal human somatic cells. However, telomeric length is maintained in stem cells and cancer cells. 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 that is essential for maintaining telomeres. It consists of an RNA that serves as the template for the synthesis of telomeric DNA by the reverse transcriptase of telomerase. The telomeric RNA does not have any catalytic activity. Telomerase acts as an RNA-directed DNA polymerase, synthesizing DNA using its RNA template. The direction of synthesis, like all DNA synthesis, is 5′→3′, and it extends the 3′-end of the already longer 5′→3′ strand.

During the study of a small gene that was sequenced as part of 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 are found in the complete double-stranded molecule?
A. A=40, G=50, C=60, T=44
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
E. A=42, G=55, C=55, T=42

Correct answer = E. The complete double-stranded molecule would have 42 adenine (A) bases, 55 guanine (G) bases, 55 cytosine (C) bases, and 42 thymine (T) bases.
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