GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Viruses

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

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Example Question #1 : Help With Viral Genomics

Which of the following is true about viral genomes?

Possible Answers:

Viral genomes are incapable of undergoing recombination events

Viral genomes can be either RNA or DNA

Viral genomes typically consist of 20,000+ genes

Viral genomes are always circular

Correct answer:

Viral genomes can be either RNA or DNA

Explanation:

The human genome contains 20,000-25,000 genes, so, as one might expect, viral genomes are typically much smaller than this. Viral genomes can either be circular or linear, and both formats are susceptible to recombination events.

Different classes of viruses have different nucleic acid genomes. Double-stranded DNA, single-stranded DNA, double-stranded RNA, and single-stranded RNA can all be found in certain virions.

Example Question #1 : Help With Viral Replication

An inhibitor that prevents reverse transcription would be most useful in combatting which of the following viruses?

I. Retroviruses

II. DNA viruses

III. RNA viruses

Possible Answers:

III only

I and III

I, II, and III

I only

Correct answer:

I only

Explanation:

Of the three choices, only retroviruses use reverse transcription to make a DNA copy of their RNA genomes.

DNA viruses do not need to do this because their genome already consists of DNA. RNA viruses use specialized enzymes called RNA replicases to make RNA copies of their RNA genomes, and do not go through a DNA intermediate like retroviruses. 

Example Question #1 : Molecular Biology And Genetics

Which type of virus uses reverse transcriptase in order to convert its genome into a viable template for reproduction?

Possible Answers:

Single-stranded DNA viruses

Single-stranded RNA viruses

Double-stranded RNA viruses

Retroviruses

Correct answer:

Retroviruses

Explanation:

Reverse transcriptase is used to convert RNA genomes into DNA genomes. The genome is then inserted into the host's chromosomes, where it can be transcribed numerous times to create viral proteins. Without this process, the RNA genome would have a very short-lived existence in the cell, resulting in very few viral copies.

This technique is used by retroviruses, such as HIV.

Example Question #3 : Help With Viral Replication

Which of the following is the correct sequence for viral replication?

Possible Answers:

Replication, viral assembly, transcription, protein synthesis, penetration, release

Transcription, protein synthesis, replication, viral assembly, release, penetration

Release, replication, transcription, protein synthesis, viral assembly, penetration

Viral assembly, release, penetration, transcription, protein synthesis, replication

Penetration, replication, transcription, protein synthesis, viral assembly, release

Correct answer:

Penetration, replication, transcription, protein synthesis, viral assembly, release

Explanation:

The first step in viral replication is penetration. The virus may be engulfed by the host cell or fuse with the host's membrane. The viral genetic material is released into the cytoplasm. During replication the viral genetic material is copied many times. The viral genetic material is used as a blueprint to make messenger RNA (mRNA) during the transcription stage. The mRNA is used to synthesize viral proteins. The viral genetic material and enzymes are surrounded by their protein coat during viral assembly. Finally viruses are released from the host cell by budding from the cell membrane or bursting the cell. 

Example Question #1 : Help With Viral Replication

Why do viral proteins interact with each other extensively as well as interact with a wide array of host proteins? 

Possible Answers:

To modulate host cellular activities

Viruses encode a small number of proteins compared to the host

All of these

Viral proteins exhibit numerous, small, low-specificity binding sites

Viral proteins have many splice variants

Correct answer:

All of these

Explanation:

All of the answers are correct. Given that viral genomes are small, the proteins that they encode must be able to execute many functions. Due to high mutation rate, many splice variants, and multiple small protein binding domains, viral proteins interact extensively with each other and host proteins. This allows viral proteins to modulate host cellular activities such as inflammation, proliferation, anti-viral response, and DNA replication/transcription/translation so that viral proteins are able to replicate and are not killed by the infected cells. 

Example Question #3 : Viruses

What enzyme would a non retroviral RNA virus use to replicate it's genome? 

Possible Answers:

A DNA dependent RNA polymerase encoded by the virus

An RNA dependent RNA polymerase encoded by the virus

A DNA dependent RNA polymerase encoded by the virus

An RNA dependent RNA polymerase encoded by the host

An RNA dependent DNA polymerase encoded by the virus

Correct answer:

An RNA dependent RNA polymerase encoded by the virus

Explanation:

In order the replicate its genome, a non retroviral RNA virus must use a polymerase that can use an RNA template to polymerize a complimentary RNA molecule; an RNA dependent RNA polymerase. Since no such enzyme exits in host cells, the virus must encode this enzyme itself.

Example Question #1 : Help With Viral Lifecycles

___________ is a term used to describe a viral lifecycle in which the virus incorporates its DNA into the host's genomic DNA. 

Possible Answers:

Lytic

Invasive

Latent

Lysogenic

Correct answer:

Lysogenic

Explanation:

Lytic viral lifecycles involve using the host's machinery to mass produce further copies of the virus.

Lysogenic lifecycles can become lytic under the right circumstances. The key difference is the time period and method of infection. In the lysogenic cycle, viral DNA is incorporated into the host genome and remains dormant, rather than becoming active immediately to generate more virions. After a period of time the viral DNA will be synthesized in large quantities, transitioning into the lytic cycle, and eventually lysing the cell.

Latent is a term used to refer to the period when a virus lays dormant within a host. Invasive is not a term commonly used to refer to a viral lifestyle. 

Example Question #1 : Viruses

Which event is not part of the viral lytic life cycle?

Possible Answers:

The virus uses host ribosomes to create viral proteins

New viruses are assembled inside of the cell

Viral nucleic acid is injected into the cell

Viral DNA is integrated into the chromosome

Correct answer:

Viral DNA is integrated into the chromosome

Explanation:

The lytic life cycle is the most basic life cycle for a virus. The virus injects its nucleic acid into the host, synthesizes the parts needed for new viruses using the host cell's ribosomes, assembles the viruses, and then breaks out of the host cell (lysis).

At no point in the lytic life cycle will a virus integrates its genome into the host's chromosomes. This technique is employed by viruses with a lysogenic life cycle.

Example Question #3 : Help With Viral Lifecycles

Which of the following statements about viral life cycles are true?

Possible Answers:

Virulent viruses reproduce only by the lysogenic cycle

Temperate viruses are those that can infect a cell that has already been infected by another virus

Once a phage injects its nucleic acid into the host cell, the lytic cycle consists of two stages

Viruses that enter the lytic cycle exist as prophages

It takes a bacteriophage roughly three hours from binding and infecting a host cell to lysing it

Correct answer:

Once a phage injects its nucleic acid into the host cell, the lytic cycle consists of two stages

Explanation:

Virulent viruses are those that reproduce only by the lytic cycle. It takes a bacteriophage only ~30 minutes to complete a lytic cycle. Prophages are latent forms of lysogenic bacteriophages that are incorporated into the host genome. Prophages can remain inactive within the genome through may cell divisions. Temperate viruses are those that may display lysogeny. Once a phage injects its nucleic acid into the host cell, the lytic cycle consists of two stages. In the early stage, transcription of viral genes that code for shutting down host transcription. In the late stage, viral genes are transcribed which code for capsid proteins and proteins that lyse the host cell to release the new viruses. 

Example Question #4 : Help With Viral Lifecycles

A virus that causes cell lysis undergoes the __________ cycle and a virus that induces inflammation undergoes __________ cycle.

Possible Answers:

lytic . . . lytic

lysogenic . . . lytic

lysogenic . . . lysogenic

lytic . . . lysogenic

Correct answer:

lytic . . . lytic

Explanation:

There are two types of viral life cycles. First, a virus enters the host cell, replicates, lyses the cell, and infects another host cell. This life cycle is called the lytic cycle and results in immediate infection, symptoms, and immune response (such as inflammation). Second, a virus can enter the host cell, incorporate itself into the host cell’s genome, and stay latent for years. This life cycle is called the lysogenic cycle and it can take years for symptoms to appear. Symptoms occur when the inactive virus is activated due to some environmental cue. The virus gets activated, replicates, lyses the cell, and causes infection.

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