Viruses cannot reproduce and make copies of themselves without a host; however, they can survive on their own. Viruses can remain dormant in a non-biological environment before being transferred to a biological host.

All viruses carry some form of genetic material, either in the form of DNA or RNA, and have a protein coat to contain the genetic material. Bacteriophages are viruses that specifically target bacteria. By hijacking the cellular machinery of host cells, viruses are able to replicate, essentially becoming intracellular parasites.

Different classes of viruses store genetic material in different forms.

Double-stranded RNA (dsRNA) viruses, single-stranded RNA (ssRNA) viruses, double-stranded DNA (dsDNA) viruses, and single-stranded DNA (ssDNA) viruses are all large classes of viral classification. Retroviruses can be classified as ssRNA viruses, but are often left in their own category.

The process of inactivating live viruses results in dead (noninfectious) virus. In terms of safety (if the virus is inactivated correctly), inactivated virus vaccines have no risk of inducing a viral infection from the contents of the vaccine. In terms of effectiveness of inducing protective immunity, inactivated virus vaccines are typically less effective than attenuated but live virus vaccines. Therefore, the induction of long-term protective immunity against the virus is questionable. Inactivated virus vaccines normally require larger doses, multiple doses, and adjuvants to be effective.

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.

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.

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.

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.

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.

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.

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.