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CHAPTER 30 Genetics & Gene Therapy
Viruses with large genomes (e.g., vaccinia virus) are excel-
lent candidates for this purpose. To construct the recombi-
1. In the lab, a virologist was studying the properties of HIV. She
nant virus, any vaccinia virus gene that is not essential for
infected the same cell with both HIV and rabies virus. (HIV can
viral replication is deleted, and the gene from the other
infect only human CD4-positive cells, whereas rabies virus can infect
virus that encodes the antigen that elicits neutralizing anti-
both human cells and dog cells.) Some of the progeny virions were
body is introduced. For example, the gene for the surface
able to infect dog cells, within which she found HIV-specific RNA.
antigen of hepatitis B virus has been introduced into vac-
Which one of the following is the term used to describe these results?
cinia virus and is expressed in infected cells. Recombinant
(A) Complementation
vaccines are not yet clinically available, but vaccines of this 2. You have isolated two mutants of poliovirus, one mutated at gene
(B) Phenotypic mixing
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mebooksfree.com mebooksfree.com mebooksfree.com (A) If complementation between the mutant gene products occurs, mebooksfree.com
type promise to greatly improve the efficiency of our
(C) Reassortment
immunization programs.
(D) Recombination
X and the other mutated at gene Y. If you infect cells with each one
PEARLS
alone, no virus is produced. If you infect a single cell with both
mutants, which one of the following statements is most accurate?
• Mutations in the viral genome can produce antigenic vari-
both X and Y progeny viruses will be made.
ants and drug-resistant variants. Mutations can also produce
(B) If phenotypic mixing occurs, then both X and Y progeny
attenuated (weakened) variants that cannot cause disease
viruses will be made.
but retain their antigenicity and are useful in vaccines.
(C) If the genome is transcribed into DNA, then both X and Y
• Temperature-sensitive mutants can replicate at a low (per-
viruses will be made.
missive) temperature but not at a high (restrictive) tem-
(D) Because reassortment of the genome segments occurs at high
perature. Temperature-sensitive mutants of influenza virus ANSWERS
frequency, both X and Y progeny viruses will be made.
are used in one of the vaccines against this disease.
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• Reassortment (exchange) of segments of the genome
RNA of influenza virus is important in the pathogenesis of
the worldwide epidemics caused by this virus.
2. (A)
• Complementation occurs when one virus produces a pro-
tein that can be used by another virus. A medically impor-
PRACTICE QUESTIONS: USMLE &
tant example is hepatitis D virus, which uses the surface
antigen of hepatitis B virus as its outer coat protein.
COURSE EXAMINATIONS
• Phenotypic mixing occurs when two different viruses
infect the same cell and progeny viruses contain proteins of
in the Basic Virology section of Part XIII: USMLE (National
both parental viruses. This can endow the progeny viruses
Board) Practice Questions starting on page 720. Also see Part
with the ability to infect cells of species that ordinarily
XIV: USMLE (National Board) Practice Examination starting
parental virus could not. Questions on the topics discussed in this chapter can be found
on page 751.
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