Biology - MCAT Biological and Biochemical Foundations of Living Systems

Prokaryotic cells can reproduce either sexually and asexually. In a bacterial cell, sexual reproduction occurs via three different methods: conjugation, transformation, and transduction. Conjugation involves the exchange of genetic material (plasmids) between bacterial cells through a bridge called the sex pilus. Transformation involves the incorporation of extracellular DNA from the environment. Transduction involves the transmission of genetic material from one bacterial cell to another by a viral vector. These three processes are considered forms of sexual reproduction because all of them involve genetic recombination.

Prokaryotic cells can reproduce asexually only through binary fission. Binary fission is the process by which a bacterial cell splits into two identical daughter cells. This is a very fast process and contributes to the rapid growth and replication of bacteria.

Mitosis and meiosis are only found in eukaryotic cells. Mitosis is a form of asexual reproduction and meiosis is a form of sexual reproduction, since it involves genetically unique daughter cells. Binary fission is the prokaryotic analog of mitosis, but is a distinctly different process.

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.

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.

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).

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.

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.

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.

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.

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

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.

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.

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.

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.

Although there are recommended intake values for vitamin D, this "vitamin" can be synthesized by the body with the help of UV radiation from the sun.

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.