GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Viral Lifecycles

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

varsity tutors app store varsity tutors android store

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

1 Diagnostic Test 201 Practice Tests Question of the Day Flashcards Learn by Concept

Example Questions

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:

Invasive

Latent

Lysogenic

Lytic

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 #2 : Help With Viral Lifecycles

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

Possible Answers:

The virus uses host ribosomes to create viral proteins

Viral DNA is integrated into the chromosome

Viral nucleic acid is injected into the cell

New viruses are assembled inside of the cell

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:

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

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

Virulent viruses reproduce only by the lysogenic cycle

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 #3 : Help With Viral Lifecycles

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

Possible Answers:

lysogenic . . . lytic

lysogenic . . . lysogenic

lytic . . . lytic

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.

Example Question #4 : Help With Viral Lifecycles

Which of the following feature is common between a virus undergoing lytic cycle and a virus undergoing lysogenic cycle?

Possible Answers:

The mode of entry into a cell

More than one of these

The length of incubation

The mode of exit out of a cell

Correct answer:

The mode of entry into a cell

Explanation:

A typical virus life cycle includes the attachment of virus to host cell, entry, replication or latency (depending on the type of life cycle), and exit. If the virus immediately replicates and lyses the cell, it undergoes a lytic cycle. If the virus incorporates itself into the host cell’s genome and becomes latent, then it undergoes lysogenic cycle. Note that a virus in lysogenic cycle never exits the cell; to exit the cell, the virus must become reactivated by environmental cues. Once activated, the virus enters the lytic cycle and causes the eventual lysis of the cell. Therefore, the only common theme between lytic and lysogenic cycles is the mode of entry.

Example Question #6 : Help With Viral Lifecycles

Which of the following are true regarding a virus undergoing the lysogenic cycle?

I. The viral nucleic acid is the only remnant found inside the infected cell

II. The infected individual will have prolonged fevers and other symptoms

III. Cell rupture does not occur

Possible Answers:

III only

I and III

I and II

II only

Correct answer:

I and III

Explanation:

Lysogenic cycle of a virus involves the incorporation of the viral genome into the host cell’s genome. Upon incorporation, the virus becomes a provirus and remains latent until activated. The only remnant of the virus found in provirus is its genome, or its nucleic acid content. Note that viruses can have DNA or RNA. If it has RNA, virus uses an enzyme called reverse transcriptase to convert the RNA to DNA. The DNA can now incorporate itself into the host cell’s genome (host cell genome is made up of DNA only).

One of the key characteristics of a provirus is that it does not cause any symptoms such as fever, headaches, etc. It can only cause infection and symptoms after reactivation, which can occur due to environmental cues.

Cell rupture, or cell lysis, is caused by viruses in lytic cycle. In a lytic cycle, the virus replicates and exits the host cell, causes an infection, and elicits an immune response.

Example Question #7 : Help With Viral Lifecycles

A researcher is analyzing the effects of a virus that is known to be deadly. He administers the virus to an immunosuppressed rat and observes it for two whole weeks. The results reveal that the rat failed to develop any symptoms. What can you conclude about this virus?

Possible Answers:

The virus replicates but the T cells eliminate the pathogen

It undergoes the lysogenic cycle once it enters the host

It undergoes the lytic cycle once it enters the host

More than one of these

Correct answer:

It undergoes the lysogenic cycle once it enters the host

Explanation:

The question states that the virus is known to cause a deadly disease. It also states that the rat did not develop any symptoms for the entire observatory period (2 weeks). One possible explanation for this result is that the virus enters the cell and becomes latent. It incorporates itself into the host cell's genome and causes disease once re-activated. This reactivation can occur years after the primary infection; therefore, a patient infected with this virus can have no symptoms for years. Recall that a virus that enters the cell and becomes latent is undergoing the lysogenic cycle; therefore, this virus must be undergoing this life cycle.

The question states that the rat is immunosuppressed. This means that the rat does not have the ability to activate the immune system (B cells, T cells, macrophages, etc.) to fight infections. Also, remember that even a completely immuno-competent rat will not produce an immune response to this virus. This is because the immune system does not recognize a latent virus in the lysogenic cycle and cannot, therefore, produce an immune response.

Example Question #8 : Help With Viral Lifecycles

When performing gene transfer via pantropic retroviruses, which of the following is true about the cells they infect? 

Possible Answers:

The cells must have an outer lipopolysaccharide layer 

The cells must be actively replicating 

The cells must be of human origin 

None of these

The cells must be in a stationary nonreplicating phase

Correct answer:

The cells must be actively replicating 

Explanation:

The correct answer is that cells must be actively replicating. Retroviral gene integration into host genomes can only occur when the host is actively replicating. Pantropic retroviruses are able to infect many different cell types from many species. Additionally, the presence of a lipopolysaccharide layer indicates that the target cell is a bacterium. Bacteriophages, not retroviruses, infect bacteria. 

Example Question #5 : Help With Viral Lifecycles

What is the defining characteristic of temperate phage?

Possible Answers:

They have DNA genomes

They can enter either the lytic or lysogenic cycle

They can enter the lysogenic cycle

They have an unusual capsid structure

They can enter the lytic cycle

Correct answer:

They can enter either the lytic or lysogenic cycle

Explanation:

Temperate phages a so named because after infecting a bacterial cell, they can enter either the lysogenic or lytic cycle. Other phages cannot enter the lysogenic cycle and will always enter the lytic one. Since they have the choice to either hide out in the cell unnoticed or immediately replicate and lyse the cell lysogenic phages are refereed to as temperate.

Example Question #6 : Help With Viral Lifecycles

In the viral lytic cycle, the step of adsorption is when the virus particle interacts with receptors on the outer surface of a given cell. Which step describes the process in which there is the formation of new (progeny) virions?

Possible Answers:

Translation

Penetration

Replication

Release

Assembly

Correct answer:

Assembly

Explanation:

During assembly, newly synthesized viral proteins and the replicated viral genomes associate with one another to form new virions. The build up of new virions causes rupture and release. Replication describes the production of viral mRNAs which are translated using host machinery. Penetration describes the entry of the original virus into the cell. 

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

1 Diagnostic Test 201 Practice Tests Question of the Day Flashcards Learn by Concept
Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: