Understanding Viruses and Prions - AP Biology

Viruses are unique organisms in that they have a relatively simple cell structure. They have an outer protein coat, which sets them apart from other organisms. Peptidoglycan is found in bacteria, lipid bilayer is found in eukaryotic cells, and chitin is found in fungi. Membrane-bound orangelles are found in more complex cells, such as eukaryotic cells, and are not unique to viruses.

Reverse transcriptase is the epitome of viral evolution, as showcased by the HIV virus. Reverse transcriptase enables the HIV virus to create a DNA template from an RNA segment already encapsulated in the virus, but using the host cell's own DNA replication machinery.

When you think of viruses, think of them as parasites: they prey on host cells and take advantage of their resources. Viruses require the reproductive machinery of host cells in order to reproduce and use the energy in the cells to drive the synthesis processes; however, it is possible for viruses to remain virulent outside of a host for long periods of time. When not infecting a host, viruses are refrred to as virions. Essentially, viruses can survive outside of a host, but cannot replicate.

At no point in the reproductive cycle of a virus does it contain ribosomes. Instead, it uses the ribosomes found in the host cell to synthesize the proteins needed for its reproduction.

Influenza mutates rapidly, and can mutate into a new virus and evade previous vaccines. As a result, new vaccines must be created in order to provide the best defense against the influenza virus for that year. The vaccine you get one year is not going to be the same vaccine you get the next year.

Vaccines allow the body to produce antibodies to the exposed pathogen. They do not provide antibodies.

Lysosomes are structures within cells which contain digestive enzymes to eliminate waste and debris.

Neutrophils are the most abundant of the leukocytes making up 40-70% of white blood cells.

Basophils are the least common, representing only about 0.5% of all white blood cells.

Natural killer (NK) cells are lymphocytes, not leukocytes. They compose a small fraction of lymphocytes within the innate immune system.

Because the question refers to a bacteriophage, we know that the two answers beginning with endocytosis are incorrect; endocytosis begins the multiplication cycle of an enveloped virus. Bacteriophages are naked viruses, contained within a protein capsule with no outer membrane.

During adsorption, the virus encounters a susceptible bacterial host cell and adsorbs (binds) to receptor sites on the cell membrane like a key fitting into a lock. The flexible cell membrane of the host cell is then penetrated by the viral nucleic acids. During synthesis, the viral nucleic acids take control over the host’s metabolic processes, much like the machinery of a factory assembly line. This causes the host cell to replicate and assemble new viral particles. Once the new viruses are assembled, the host cell ruptures (lyses) and the viruses are released to find new host cells.

The non-specific host response includes fever, inflammation, positive chemotaxis, and phagocytosis. These responses are directed at any perceived invader, and therefore are non-specific.

The specific host response targets a specific invader, and includes T-cells, B-cells, and antibodies.

Eukaryotic cells have a nuclei; prokaryotic cells have no nuclei.

Eukaryotic cells are considerably larger and more complex. Eukaryotic cells possess membrane-bound organelles; prokaryotic cells, which are smaller and less complex, do not. Both eukaryotic and prokaryotic cells can have flagella, but not necessarily in all cases.

Note that ribosomes are not membrane-bound, and are found in both prokaryotes and eukaryotes.

Some viruses have a DNA genome, while others use RNA. While DNA may be a crucial component to determining life, it cannot be used to differentiate viruses from living organisms because some viruses do carry their own DNA.

All viruses can be differentiated from living organisms because they do not "eat" or absorb nutrients, they do not have cells, they do not respond to stimuli, and they do not reproduce independently. All living organisms absorb nutrients, have cells, respond to stimuli, and are capable of reproduction, thus making viruses different.

A virus may contain any of these four types of genetic material. The type of nucleic acid, and whether it is single- or double-stranded, determines how the virus integrates with the host organism. Different classes of viruses have different mechanisms for infection.

Note that retroviruses, which contain single-stranded RNA that is later converted to DNA, are often classified as a fifth type of virus.

There are a multitude of reasons as to why viruses are not classified as living organisms. In addition to the reasons given, viruses require host reproductive organelles and proteins in order to multiply. They also are not separated from the external world by a membrane or cell wall.

Human immunodeficiency virus (HIV), which is the causative agent of AIDS, primarily attacks the white blood cells. More specifically, it infects helper T-cells, macrophages, and dendritic cells. By damaging the cells of the immune system the virus creates an "immunodeficiency," weakening the body's ability to fight infection.

Virus particles, otherwise known as virions, always contain proteins. Whether they contain DNA or RNA depends solely upon the type of virion; different types of viruses will carry different types of nucleic acids. The capsids of virions contain proteins that surround the nucleic acids, whether it be DNA or RNA.

Viral genomes are incredibly diverse. Different types of viruses use different genetic materials. Viruses are capable of using double-stranded DNA, double-stranded RNA, single-stranded DNA, or single-stranded RNA.

The question states that HIV is a RNA virus; therefore, the virus must possess an enzyme that transforms the RNA genome into DNA. This enzyme is called reverse transcriptase. Since it transforms RNA to DNA it is also called an RNA-dependent DNA polymerase.

DNA-dependent RNA polymerase (required for transcription) is not present in HIV because it is not a DNA virus.

Finally, HIV does contain DNA-dependent DNA polymerase. This enzyme is required for DNA replication, which is an important process in HIV replication.

Plus-strand RNA viruses can directly translate their RNA into proteins, however minus-strand RNA viruses require an additional step before translation. The minus-strand RNA is a complement to mRNA; therefore, minus-strand RNA must be transcribed into plus-strand RNA before translation. This step is the key difference between plus-strand and minus-strand RNA viruses.

You can eliminate the answer choices involving introns because most viruses do not contain introns. The viruses that contain introns could be either plus-strand or minus-strand RNA viruses (or DNA viruses).

Remember that reverse transcriptase is an enzyme that facilitates the generation of DNA from RNA. A virus containing reverse transcriptase must initially possess an RNA genome. Once inside the host cell the virus generates complementary DNA from the RNA template. You can eliminate the choices involving DNA.

A very long latent period of three years suggests that the virus undergoes the lysogenic cycle. The lytic cycle involves immediate lysis of the host cell and spread of infection after viral replication. In contrast, the lysogenic cycle causes the virus to become dormant. One of the main characteristics of a virus in the lysogenic cycle is that it has a long latent period.

The capsid of a virion is the region that contains the viral DNA or RNA and the necessary enzymes that facilitate infection of a host cell. When a virion infects a cell, the contents of the capsid are injected into the host cell. This marks the onset of infection. A capsid contains nucleic acids (DNA or RNA) and amino acids (enzymes).

A virion does not contain ribosomes or any other organelles. Instead, the virus must use host cell machinery to translate the proteins required for replication.