CASE FILES: BIOCHEMISTRY
If a double-stranded DNA molecule undergoes two rounds of replica-
tion in an in vitro system that contains all of the necessary enzymes and
nucleoside triphosphates that have been labeled with 32P, which of the
following best describes the distribution of radioactivity in the four
resulting DNA molecules?
A. Exactly one of the molecules contains no radioactivity.
B. Exactly one of the molecules contains radioactivity in only one
C. Two of the molecules contain radioactivity in both strands.
D. Three of the molecules contain radioactivity in both strands.
E. All four molecules contain radioactivity in only one strand.
A 48-year-old man has had a lengthy history of skin cancer. In the past
6 years he has had over 30 neoplasms removed from sun-exposed areas
and has been diagnosed with xeroderma pigmentosum. Which of the
following best describes the enzymatic defect in patients with xero-
A. DNA polymerase a
B. DNA polymerase y
C. DNA ligase
D. Excision repair enzymes
E. RNA polymerase III
A nsw ers
B. Class Vb viruses, since they have a minus single-stranded RNA
genome, cannot use their genomic RNA directly to encode viral pro-
teins. It is used to synthesize a (+)-stranded viral mRNA that is then
used to encode the viral proteins. The influenza virus does this by the
following mechanism: a virus-specific polymerase first cleaves an
oligonucleotide from a host cell mRNA. It uses this as a primer that
is elongated by the polymerase to synthesize the viral (+)-mRNA
using the (-)-viral RNA genome as the template.
C. After two rounds of replication using 32P-labeled nucleoside triphos-
phates (NTPs), all four DNA molecules will be radioactive; two will be
radioactive in both strands, two will be radioactive in only one strand.
D. Xeroderma pigmentosum is a genetic disease in which the ability
to remove pyrimidine dimers caused by exposure to UV light is
impaired. The mechanism used to remove these pyrimidine dimers
(also used to repair DNA that has formed adducts with carcinogenic
compounds) is excision repair. The enzymes used in this repair mech-
anism cleave the affected strand on either side of damaged nucleotides.
The oligonucleotide containing the damaged nucleotides is removed
and the gap is filled in by DNA polymerase and DNA ligase.