MCAT Biology : Viruses

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #2 : Understanding Viral Life Cycles And Replication

A mad scientist is working with a rare retrovirus under the hood, when he accidentally spills the viral sample over three live tissue cultures. Assuming the cultures were composed of stem cells, epithelial cells, and muscle cells, in which sample would the mad scientist expect to find the lowest viral titer?

Possible Answers:

Epithelial cells

There would be no significant difference in viral titer between the three cell types

Muscle cells

Stem cells

Correct answer:

Muscle cells

Explanation:

This question requires knowledge of two concepts: first, that retroviruses integrate their own genetic material into the host and are propagated during normal cell division; second, that some cell types do not continue to divide once they have reached maturity (muscle and nerve).

Example Question #2 : Viruses

Which of the following is the best description of a bacteriophage?

Possible Answers:

A fungus

An obligate intracellular parasite

A non-living organism

A prokaryote

A living organism

Correct answer:

An obligate intracellular parasite

Explanation:

A bacteriophage is a virus that infects bacteria. They are not considered living (becuase they cannot replicate on their own) organisms, nor are they techincally considered non-living organisms. They are called obligate intracellular organisms, because they are parasites (kill the cell) that require a host in order to replicate. 

Example Question #4 : Divisions Of Life And Viruses

Which of the following is not a component of HIV?

Possible Answers:

Protein

Reverse transcriptase

Ribosomes

Proteases

RNA

Correct answer:

Ribosomes

Explanation:

HIV is a retrovirus, meaning that it contains RNA as its genetic material, and thus reverse transcriptase to code for DNA from the RNA template. It also has a protein coat, and must use proteases in order to degrade the host cell membrane to inject its RNA. It does not, however, have ribosomes. It will eventually use the host ribosomes to translate its own genetic material.

Example Question #3 : Understanding Viral Life Cycles And Replication

Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear. 

Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:

PrPC  + PrPRes  → PrPRes + PrPRes

The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.

This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:

Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:

Rabbit 1 and 2: injected with normal saline on days 1 and 2

The above trials serve as controls.

Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2

The above trials use unmodified brain matter.

Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2

The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.

Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2

The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.

Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2

The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.

Some scientists argue that there must be a virus or bacterium that cause infectious diseases, and claim that there is likely a heretofore undiscovered microbe causing neurodegeneration that most scientists claim are caused by PrPRes. What is a key way for these scientists to distinguish viruses from bacteria?

Possible Answers:

Bacteria have mitochondria, viruses do not.

Bacteria have true membrane bound organelles, viruses do not.

Bacteria have nuclear pores, viruses do not.

Bacteria have ribosomes, viruses do not.

Viruses have ribosomes, bacteria do not.

Correct answer:

Bacteria have ribosomes, viruses do not.

Explanation:

Bacteria have ribosomes to facilitate protein synthesis, but lack other membrane bound organelles such as nuclei, Golgi, or mitochondrion. Viruses lack all of these, including ribosomes.

Example Question #1 : Microbiology

Which of the following is not a described type of virus?

I.  A virus containing double strand DNA

II.  A virus containing single strand DNA

III.  A virus containing single strand RNA and single strand DNA

IV.  A virus containing single strand RNA

V.  A virus containing double strand RNA

Possible Answers:

III

IV

II

V

I

Correct answer:

III

Explanation:

The question addresses the Baltimore classification system for viral genetic information.  Viruses contain some form of DNA or RNA, but never both.

Example Question #6 : Divisions Of Life And Viruses

What molecule would you not expect to find in a retrovirus?

Possible Answers:

Adenine

Thymine

Uracil

Guanine

Cytosine

Correct answer:

Thymine

Explanation:

Retroviruses carry RNA and when they infect a host cell, they use their own reverse transciptase to made DNA from that RNA; therefore, within the virus, there are only the molecules that make up RNA. Uracil is used in RNA place of thymine, which can be found in DNA. Cytosine, guanine, and adenine can be found in both DNA and RNA.

Example Question #7 : Divisions Of Life And Viruses

Sexually transmitted diseases are a common problem among young people in the United States. One of the more common diseases is caused by the bacterium Neisseria gonorrhoeae, which leads to inflammation and purulent discharge in the male and female reproductive tracts.

The bacterium has a number of systems to evade host defenses. Upon infection, it uses pili to adhere to host epithelium. The bacterium also uses an enzyme, gonococcal sialyltransferase, to transfer a sialyic acid residue to a gonococcal surface lipooligosaccharide (LOS). A depiction of this can be seen in Figure 1. The sialyic acid residue mimics the protective capsule found on other bacterial species.

Once infection is established, Neisseria preferentially infects columnar epithelial cells in the female reproductive tract, and leads to a loss of cilia on these cells. Damage to the reproductive tract can result in pelvic inflammatory disease, which can complicate pregnancies later in the life of the woman.

Untitled

Which of the following is true of Neisseria, but not true of a virus?

Possible Answers:

Neisseria infects only one cell type; viruses have wide variety in their cellular targets

Neisseria is enveloped; no viruses are enveloped

Neisseria does not have flagella; all viruses have flagella

Neisseria has ribosomes; viruses do not have ribosomes

Neisseria contains DNA; all viruses contain RNA

Correct answer:

Neisseria has ribosomes; viruses do not have ribosomes

Explanation:

Neisseria synthesizes proteins as a free living prokaryote. Viruses are generally dependent on host cellular machinery, and synthesize their proteins upon hijacking of a host ribosome.

Example Question #5 : Viruses

What component is common for all viruses?

Possible Answers:

A protective envelope

DNA

RNA

A protein coat

Correct answer:

A protein coat

Explanation:

All viruses contain a protein coat (or capsule) that protects the genetic material (which can be either DNA or RNA). In some instances (such as herpes simplex), the virus can also contain an lipid envelope that serves as a membrane.

Example Question #2 : Microbiology

In the crusade to create a vaccine for Poliomyelitis, Jonas Salk and Albert Sabin created two separate vaccines that proved to be successful in preventing Polio onset.

The Salk vaccine, which is given by standard injection, contained virus particles inactivated by an organic solvent. This method has the advantage of inactivating each of the three Polio strains with no bias.

Albert Sabin's vaccine, given by oral inoculation via sugar water, contained live virus particles that had been genetically attenuated. With this method, each of the three Polio strains acquired separate mutations that made them unable to infect the human host cells. Strain 2 in particular contained one single nucleotide polymorphism in the internal ribosomal entry site (IRES) that prevented successful viral replication. 

Based on the passage above, what is the biggest disadvantage to Albert Sabin's vaccine?

Possible Answers:

The mutation of only one nucleotide in Poliovirus strain 2 represents a much larger chance of sequence reversion, which would allow the virus to regain its virulence.

Genetic manipulation of the Poliovirus leads to a greater chance of creating an immune-resistant strain over time, thus increasing the chances that Polio will regain infectivity with every successive generation of individuals that is inoculated.

The collective mutations in every strain of polio have a large chance of reversion that would restore virulence to the virus.

Live virus cannot sustain itself in a sugar solution, thus, by the time the vaccine is delivered to the patient, the virus will already be dead and unable to confer immunity. 

Correct answer:

The mutation of only one nucleotide in Poliovirus strain 2 represents a much larger chance of sequence reversion, which would allow the virus to regain its virulence.

Explanation:

Because Polio strain 2 only has a single nucleotide mutation, there is a much higher chance of the RNA dependent RNA polymerase (RDRP), which is highly error prone, spontaneously reverting the mutation and restoring virulence. 

Poliovirus is actually very stable in sugar solutions, which eliminates the possibility that virus would be dead upon deliverance. Viruses are also incapable of synthesizing their genome without the aid of the host cell, so the sugar content of the inoculum solution would have no effect on viral infectivity.

Example Question #3 : Microbiology

Which of the following is not found in a virus?

Possible Answers:

DNA

Ribosomes

Proteins

RNA

Correct answer:

Ribosomes

Explanation:

Viruses use the hosts ribosomes to translate their genes, and do not possess their own organelles for replication. Viruses are made of a protein buffy coat with genetic material (DNA or RNA) inside.

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