Section Test:Microbiology of Viruses

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Topics Covered

This Section Test deals with the concepts of virology commonly found in the microbiology topics of the MCAT. Viral structure, including the physical structure, size, genomic content, and other physical characteristics are covered. The viral life-cycle is also covered. See The Microbiology of Viruses for the article relating to this section test.

Section Test

1. Which of the following would never be found in a virus?

polymerase
→ While many viruses do not require a polymerase to replicate their genetic material because they use their host's, some require special polymerases to perform unusual tasks, such as reverse transcription (RNA to DNA) or RNA to RNA replication.
ribosomes
→ All viruses require the host protein synthesis machinery to produce its proteins
RNA
glycoproteins
→ Viral proteins synthesized by the host can often also be glycosolated using host enzymes.

2. What is a lysogenic cell?

a cell that contains viral DNA integrated into its genome
a cell resistant to viral infection
a cell filled with virus progeny about to lyse
a cell which contains digested viral particles from a previous infection
→ the lysogenic stage of a bacteriophage occurs when instead of causing virus replication and cell lysis, the viral genome is integrated into the hosts and lies dormant until later activation

3. Prions are like viruses in that?

they both carry genetic material (DNA or RNA)
they both require host cells for replication
they both have a capsid and/or often an envelop surrounding their contents
they both utilize their host's ribosomes to replicate protein
→ While the last choice sounds plausible, prions do NOT use ribosomes to replicate. They are simply a malformed protein capable of using their own structure as a template to convert other host proteins of the same family into their infectious and damaging state. They can only replicate as long as the host is producing the prion like protein to begin with it.

4. Which event does not occur in the life cycle of a retrovirus?

integration of the viral genome into the host's genome
conversion of RNA into DNA
binding of viral receptors to host surface proteins
translation of mRNAs encoding viral proteins
→ retroviruses include all viruses which utilize reverse transcriptase to produce DNA from RNA. Integration however, is unique only to the HIV retrovirus.

5. When an enveloped virus enters the human circulatory system, the most likely component of the virus to be targeted for destruction by antibodies would be?

the viral envelope
the viral receptor proteins
the viral capsid
the viral RNA (or DNA)
→ the only component of a virus accessible by antibodies is the surface of the virus. Furthermore, the plasma membrane envelop which surrounds the virus is not unique to the virus as it is derived from a host cells plasma membrane, therefor it is unlikely to be targeted by antibodies. The surface receptors are generally large and protrude outwards and therefore the most likely target for an antibody.

6. Which of the following human cell types would a virus be least likely to replicate in effectively?

lung epithelial cell
→ The common cold and influenza both infect lung epithelial cells efficiently.
B-cell
→ While viruses that infect B-cells are more rare, it is not impossible, mono for instance infects B-cells.
erythrocyte
→ Erythrocytes, or red blood cells, do NOT have a nucleus and lack most of the machinery necessary for protein synthesis, it is therefore the cell type least likely to be able to sustain ANY virus.
hepatocyte
→ Hepatocytes are an extremely common cell type or viral infection since they are exposed to many viral particles while they filter the circulating blood supply. Viral hepatitis the effect of such an infection.

7. What type of immunity is most responsible for fighting viral infections?

Innate Immunity
Humoral Immunity
Cell-mediated Immunity
→ Cell-mediated, or adaptive immunity, is the immune response that targets T-cells to virally infected cells and antibodies to viral components.
Homeostatic Immunity
→ This is NOT a real component of the immune system.

8. Which is not true regarding capsids?
          i) it is composed of repeating subunits
          ii) it is able to self-assemble into virions
          iii) helical structure has a fixed number of subunits and is spherical in shape

i only.
i and ii.
ii, and iii.
i, ii, and iii are all correct.
→ Capsids are composed of the same protein subunit repeated over and over until they form a much larger structure. The assembly of these structures generally occurs on their own due to the stable structure that forms as a consequence. There exist many forms of the final shape, but the helical form (tobacco mosaic virus) is typically a helical structure of a variable number of subunits which becomes a long cylinder, NOT a sphere.

9. In AIDS patients, the virus homes in on the _______________ T cells.

CD1+
CD2+
CD3+
CD4+
→ While it is possible for HIV to enter many cell types, the intended host cell target of HIV is CD4+ T cells. It does this by utilizing its surface receptor gp120 to find CD4 on the surface of host cells.

10. Copying the HIV virus' nucleic acid depends on

replicase
reverse transcriptase
→ HIV is a retrovirus, which has the enzyme reverse transcriptase. This enzyme is used to convert its RNA to DNA
transcriptase
nucleases

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