MCAT Biology : Viral Life Cycle

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Viral Life Cycle

Host cells are often killed by a virus as the result of __________.

Possible Answers:

secretion of chemicals from the virus to the membrane of the host cell

toxic enzymes that damage the host cell that are coded for in the viral genome

lysis of the host to release new viruses

integrating into the host cell chromosomes

replication of the viral genetic material within the host

Correct answer:

lysis of the host to release new viruses

Explanation:

Host cells tend to be killed as newly formed viruses try to exit the cell. Lysis is one means a virus can use to leave the cell. This ruptures the cell membrane, which ultimately kills the host cell.

Example Question #2 : Viral Life Cycle

What is a key difference between the lytic and the lysogenic viral life cycles? 

Possible Answers:

When the lysogenic cycle is initiated, the virus cannot go back into the lytic cycle.

When the lytic cycle is initiated, the virus cannot go back into the lysogenic cycle.

Viral DNA/RNA is incorporated into the host in the lytic cycle; it is not in the lysogenic cycle.

Cells are lysed in the lysogenic cycle; they're not lysed in the lytic cycle.

Cells are lysed in the lytic cycle; they're not lysed in the lysogenic cycle.

Correct answer:

Cells are lysed in the lytic cycle; they're not lysed in the lysogenic cycle.

Explanation:

In the lytic cycle, the virus simply reproduces inside the cell until the cell lyses. In the lysogenic cycle, the viral DNA (or RNA reverse transcripted into DNA using reverse transcriptase) is incorporated into the host's DNA. As the host reproduces, the viral DNA goes into each daughter cell. The virus can always switch between each cycle depending on the conditions the cell is in.

Example Question #3 : Viral Life Cycle

All viruses rely on the host cell's replication machinery to replicate, however, most viruses also contain genetic code for their own necessary enzymes to complete their life cycle. In the human immunodeficiency virus (HIV), what is the name of the enzyme responsible for incorporating viral DNA into the host genome?

Possible Answers:

Integrase

Primase

Helicase

Reverse transcriptase 

Correct answer:

Integrase

Explanation:

While understanding the process by which individual viruses replicate in a host is not required for the MCAT, we are expected to know that HIV is a retrovirus, and that retroviruses have a unique viral replication and life cycle. Unlike other viruses, the single-stranded RNA retroviruses, like HIV, contain an enzyme known as reverse transcriptase, which converts the RNA into a single stranded DNA molecule.

The viral enzyme integrase, however, is the enzyme responsible for integrating the newly synthesized double stranded DNA into the host's genome, therefore, integrase is the correct answer.

If the question asked which enzyme is responsible for converting the RNA virus into DNA, then reverse transcriptase would be correct. Primase and helicase are enzymes involved in normal DNA replication, and are both incorrect.

To reiterate, reverse transcriptase is responsible for reversing transcription of RNA to DNA, and integrase is responsible for integrating the DNA into the host's DNA.

Example Question #4 : Viral Life Cycle

Reverse transcriptase is most likely to be found in which of the following?

Possible Answers:

Streptococcal pneumonia bacterial cells

Cells infected by double-stranded DNA adenovirus

Parasitic toxoplasma protist cells

Fungal ring worm cells

Cells infected by retroviral human immunodeficiency virus (HIV)

Correct answer:

Cells infected by retroviral human immunodeficiency virus (HIV)

Explanation:

Reverse transcriptase is commonly used by retroviruses, such as human immunodeficiency virus (HIV). Reverse transcriptase is used to convert the virus RNA into DNA, which can then be inserted into the host genome.

Bacteria, protists, and fungi have DNA genomes and do not require an enzyme to convert RNA back to DNA. Double-stranded DNA (dsDNA) viruses also have no need for this conversion, as their DNA can be directly incorporated into their hosts.

Example Question #5 : Viral Life Cycle

Which term refers to a viral life cycle that leads to the immediate production of progeny, destruction of the host cell, and release of many virions?

Possible Answers:

Intracellular 

Lytic

Lysogenic

Intercellular

Latent

Correct answer:

Lytic

Explanation:

The key term in this question is "immediate." The lytic cycle is when the virus uses the host's machinery to make copies of itself, and the host cell eventually bursts to release the viral progeny. The latent/lysogenic cycle is when a virus integrates itself into the host's genome, but does not make copies of itself immediately. Lysogenic viruses will eventually become lytic, but have a period of dormancy to allow for more widespread infection before stimulating any immune response.

The other options are not terms that describe a viral life cycle.

Example Question #6 : Viral Life Cycle

You discover a virus in the lab that infects humans and incorporates its genetic material into the host's genome. After further investigation, you discover that the genetic material of the virus consists of DNA.

Which answer best describes this virus?

Possible Answers:

Lytic

Lysogenic

Bacteriophage

Retrovirus

Correct answer:

Lysogenic

Explanation:

The virus infects the host with DNA and incorporates that DNA into the host genome. This describes a lysogenic virus. Lysogenic viruses are able to infect a host cell and remain dormant for extended periods by incorporating their genetic material into the host's DNA. Once activated, the viruses are assembled and enter the lytic life cycle.

Lytic viruses do not incorporate their genetic material into the host genome. Instead, they immediately hijack host cell ribosomes and proteins and begin viral replication, eventually causing the cell to lyse.

Bacteriophages only infect bacterial organisms, and are harmless to humans. The genetic material of retroviruses is RNA. Retroviruses use the enzyme reverse transcriptase to transform their RNA into DNA, allowing them to incorporate into the host genome.

Example Question #7 : Viral Life Cycle

Human immunodeficiency virus (HIV) is a retrovirus. Retroviruses possess an enzyme called reverse transcriptase that is responsible for which of the following?

Possible Answers:

Synthesize RNA from proteins

Synthesize antisense RNA from DNA

Synthesize DNA from RNA

Synthesize mRNA from viral RNA

Correct answer:

Synthesize DNA from RNA

Explanation:

Reverse transcriptase is an essential part of the retrovirus life cycle. The genome of the virus is carried in the form of RNA. For the virus to have maximum potency, this RNA must be reverse transcribed into DNA, which can then be incorporated into the host genome and replicated by cell machinery. This allows the virus to spread to daughter cells from the original infected cell, increasing the infected region.

In order for the DNA to be synthesized, reverse transcriptase must be present. Its chief function is to produce DNA from an RNA template.

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