This set of Virology Multiple Choice Questions & Answers (MCQs) focuses on “Genome Organization of Viruses”.
1. Which of the following is the genetic material of Simian virus 40 (SV40)?
a) Linear double-stranded DNA
b) Linear single-stranded DNA
c) Circular single-stranded DNA
d) Circular double-stranded DNA
View Answer
Explanation: The genome of SV40 is a circular double-stranded DNA which is 5243 bp in length. A single origin of replication is present in the genome. New synthesis of both the strands occurs at the origin of replication in a coordinated manner. RNA primers are used for the initiation of new strand synthesis.
2. What is the role of large T antigen in Simian virus 40 (SV40) replication?
a) Initial recognition of the origin of replication
b) Extension of leading strand
c) Extension of lagging strand
d) Removal of primers
View Answer
Explanation: The role of large T antigen in SV40 replication is initial recognition of the viral replication origin. Large T antigen binds specifically to the origin of replication of the DNA sequence. It then assembles a double hexamer around the DNA. The hexamer has helicase activity and causes the separation of DNA strands.
3. Which of the following genetic materials is packaged in virions of defective parvoviruses?
a) Negative-sense DNA
b) Positive-sense DNA
c) Both positive- and negative-sense DNA
d) Negative-sense RNA
View Answer
Explanation: The parvovirus family has both autonomous and defective viruses. The defective parvoviruses package both positive- and negative-sense DNA strands in separate virions. Either one of them can be infectious. The defective viruses are dependent on coinfection with helper virus for their replication.
4. Which animal virus has the largest RNA genome?
a) Coronavirus
b) Picornavirus
c) Poxvirus
d) Parvovirus
View Answer
Explanation: Coronavirus has the largest single molecule RNA virus genome. The genome is approximately 30,000 nucleotides long. Picornavirus has the smallest animal virus RNA genome. It is approximately 7500 nucleotides long. Poxvirus and parvovirus have DNA as their genetic material.
5. Viruses with RNA genomes can mutate more rapidly than viruses with DNA genomes.
a) True
b) False
View Answer
Explanation: The enzymes of viruses and host cells which are involved in RNA synthesis cannot proofread. This means they are not able to correct the incorrectly inserted bases. But in DNA synthesis correction may occur. Therefore, due to the lack of proofreading activity RNA genomes mutate more rapidly than DNA genomes.
6. What will happen to the number of mutations if the size of the RNA genome of a virus increases?
a) Number of mutations will decrease
b) Number of mutations will remain unchanged
c) Number of mutations will increase
d) Number of mutations will either decrease or increase
View Answer
Explanation: The error rate during RNA virus replication is 10-3 to 10-5 per base per genome replication event. This means that there will be one mutation per 1000 to 100,000 bases per genome replication event. Therefore, as the size of the genome increases the number of mutations will also increase.
7. Which of the following events occurs in the genomes of defective–interfering (DI) viruses?
a) Deletion
b) Insertion
c) Translocation
d) Inversion
View Answer
Explanation: Defective-interfering virus particles are produced as the result of errors in their nucleic acid synthesis. In these mutant viruses the genomes have large deletions. The remaining RNA comprises of about 10% of the infectious genome from which it was replicated. DI viruses cannot reproduce by themselves without the help of the infectious virus particle from which they were derived.
8. Which of the following genetic materials is present in Baltimore class 3 viruses?
a) Double-stranded DNA
b) Single-stranded DNA
c) Single-stranded RNA
d) Double-stranded RNA
View Answer
Explanation: Baltimore class 3 viruses contain multiple segments of double-stranded RNA as genetic material. For example: Reoviruses have RNA genomes which consist of 10 segments. Each of the segments replicates independently of the others. The replication of double-stranded RNA genomes occurs in a conservative manner.
9. Which process is used for the replication of RNA of viroid genome?
a) Gap-fill synthesis
b) Rearrangement
c) Rolling circle mechanism
d) Displacement synthesis
View Answer
Explanation: The replication of viroid genome RNA occurs by rolling circle mechanism which is similar to the replication of circular DNA. The RNA polymerase II begins replication at a particular point on the genome. The replication process generates a linear concatemeric RNA. From the concatemer genome length RNA molecules are excised.
10. Which of the following molecules is required for excision of the linear concatemeric RNA formed during viroid replication?
a) Polymerase
b) Ribozyme
c) Helicase
d) Ligase
View Answer
Explanation: The excision of linear concatemeric RNA depends on the action of an unusual RNA sequence present within the newly synthesized RNA. This molecule is called ribozyme. The ribozymes adopt a complex three-dimensional structure and cleave the RNA autocatalytically at a specific site. This process generates genome length single-stranded RNA molecule. The linear molecules are then converted into circles.
11. Which of the following is the active form of reverse transcriptase enzyme?
a) Homodimer
b) Heterodimer
c) Homotrimer
d) Heterotrimer
View Answer
Explanation: The active form of the reverse transcriptase enzyme is a dimer formed from two polypeptide chains. Therefore, it is a heterodimer. The composition of the heterodimers varies between retroviruses. For example: Reverse transcriptase obtained from avian retroviruses is composed of α subunit (Molecular weight-60,000) and β subunit (Molecular weight-90,000).
12. Which of the following enzymatic activities can be performed by the reverse transcriptase protein?
a) RNA polymerase activity
b) DNase activity
c) RNase H activity
d) Ligase activity
View Answer
Explanation: The reverse transcriptase protein can perform three types of enzymatic activities. They are- reverse transcriptase activity which means synthesis of DNA from an RNA template, DNA polymerase activity which means synthesis of DNA from a DNA template, and RNase H activity which means digestion of the RNA strand from an RNA-DNA hybrid to leave single-stranded DNA.
13. Which enzyme is required for the integration of retroviral DNA into cell DNA?
a) Polymerase
b) Reverse transcriptase
c) Ligase
d) Integrase
View Answer
Explanation: Integration of retroviral DNA into cell DNA requires the integrase enzyme and the presence of short inverted repeat sequences at the terminal end of the viral nucleic acid. The integrase enzyme must be present inside the infecting particle when it enters into the host cell. The enzyme cannot be added at a later stage.
14. Which of the following viruses has the genome that can directly act as mRNA?
a) Positive-strand RNA virus
b) Negative-strand RNA virus
c) Positive-strand DNA virus
d) Negative-strand DNA virus
View Answer
Explanation: Positive-strand RNA viruses have genomes that act as mRNA. Their genomes are translated after entry into the host cell. The first product of translation is an RNA-dependent RNA polymerase. It catalyzes the synthesis of negative-strand RNAs. These are then used to make more positive-strand RNAs.
15. Human hepatitis B virus (HBV) particles have a partially double-stranded circular DNA genome.
a) True
b) False
View Answer
Explanation: The genome of HBV particles has two linear DNA strands which form a circle through base pairing. The negative-strand DNA is covalently linked to the P protein coded by the virus at its 5′ end. It covers the full circumference of the circle and beyond. The positive-strand DNA overlaps the 5’-3’ junction of the negative strand DNA. It is incomplete and acts as a sticky end to circularize the genome. So, HBV particles have a partially double-stranded circular DNA genome.
Sanfoundry Global Education & Learning Series – Virology.
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