Mod 6 Quiz

Question 1

Can viruses be classified as either prokaryotic or eukaryotic microorganisms?

Answer 1

No. Viruses are neither prokaryotic nor eukaryotic.

Question 2

What is an obligate intracellular parasite?

Answer 2

As an obligate intracellular parasite, viruses cannot (obligate) replicate unless inside (intracellular) a living host cell. As a parasite, a virus depends on its living host to provide the framework (proteins, etc) to replicate—to make additional copies of itself.

Question 3

What is a viral capsid?

Answer 3

The capsid is a membrane-like protective structure that contains the genetic material, similar to the nucleus of a eukaryotic cell.

Question 4

What is the viral envelope, and what does the presence (or absence) of it tell you about the type of virus?

Answer 4

The envelope surrounds the viral capsid, is derived from the host cell membrane and serves as an additional barrier to the external environment. The overwhelming majority of animal viruses are enveloped whereas the majority of plant or bacteria-infecting viruses are not.

Question 5

What type of microscope would be used to study the shape of a viral capsid?

Answer 5

Due to its size (~30-150 nm) most viral capsids must be visualized via electron microscopy.

Question 6

How does the viral envelope both aid in cell entry as well as hide it from being detected by the immune system?

Answer 6

The viral envelope contains the necessary proteins required for interaction with host cells. As the envelope is derived from the host cell membrane, this camouflages the virus and lowers the likelihood of immune detection by making it appear similar to the normal biological host cell.

Question 7

What must happen before viral replication of its genome can occur?

Answer 7

For all viruses, the genome is replicated following attachment and entry, never before.

Question 8

Describe the basic steps of the viral life cycle.

Answer 8

1. Attachment: Viral receptors bind to host proteins on the surface of the cell.
2. Entry: The virus fuses with the host membrane and enters the cell
3. Uncoating: (where applicable) the viral capsid disassembles
4. Replication: The viral genome provides the ‘blueprint’ to make copies of itself
5. Exit: New viruses are produced and leave the cell
6. New infection: Newly produced viruses leave the host cell and go on to infect new cells. The process outlined above now restarts.

Question 9

What is a bacteriophage?

Answer 9

A bacteriophage is simply a virus (phage) that infects bacteria.

Question 10

What structural aspects make bacteriophages distinct from viruses that infect either animals or plants?

Answer 10

Bacteriophages possess an icosahedral capsid head group and a helical tail. Attached to the tail are fibrous extensions or ‘legs’ that aid in binding host cells. Notably, this particular structural arrangement has not been observed among viruses that infect either animals or plants.

Question 11

Which viral replicative process results in the destruction of the host cell?

Answer 11

The lytic replication cycle occurs when a bacteriophage replicates within a host until it ruptures, effectively destroying the host bacterial cell. The phrase lytic stems from ‘lysis’, meaning to rupture or disintegrate (breakdown) a cell.

Question 12

Describe the appearance of a bacterial culture infected with virus at the time of infection versus later once viral replication is occurring at a high rate.

Answer 12

During the initial inoculation of virus and bacteria, the media has a high degree of turbidity (cloudiness). However, as the lytic cycle progresses and more and more bacterial cells are destroyed, the culture media turns from turbid to clear, as turbidity is a function of the number of intact bacterial cells present in the media. Thus, as bacterial cell numbers decrease due to lysis, the media becomes clearer.

Question 13

To what family of viruses does rubella come from and how is it transmitted?

Answer 13

Rubella virus belongs to a family of viruses called Togaviridae. Rubella is transmitted via air-borne particles such as when a person coughs.

Question 14

What is the common name for rubeola, and is it a DNA or RNA virus?

Answer 14

Rubeola is commonly referred to as the measles and belongs to the family of viruses called Paramyxovirus. Rubeola viruses are RNA viruses with a genome ~ 15,000 nucleotides long.

Question 15

What disease state emerges as a viral reactivation of a prior infection with the varicella virus?

Answer 15

Shingles, also known as Herpes zoster, is due to the reactivation of the varicella-zoster virus (VZV). Because VZV lies dormant in inactive nerve cells, its reactivation typically results in painful blisters. However, unlike the initial chickenpox rash, the blisters are localized and limited to small areas.

Question 16

What linear, double-stranded, enveloped DNA virus is the first virus (as of October 26, 1977) to be declared eradicated?

Answer 16

Although once a major cause of death in the world, a smallpox vaccine was developed in 1796 by Edward Jenner. Global vaccination efforts have prevented smallpox from appearing in humans since October 26, 1977, making smallpox the first infectious disease ever to be eradicated.

Question 17

What are the two key glycoproteins found in the influenza virus?

Answer 17

The viral envelope contains two key glycoproteins, hemagglutinins (HA) and neuraminidase (N). Hemagglutinins are heavily involved in the entry of the viral particle into a host cell, while neuraminidase proteins are involved in the budding and release of new viral particles from the host cell.

Question 18

Describe the composition of HIV.

Answer 18

HIV contains two copies of single-stranded, linear RNA that is enclosed by a capsid. The conical capsid surrounds the two RNA strands and is comprised of ~2,000 copies of the viral protein p24. A viral envelope derived from the cell host membrane then surrounds the viral capsid. Within this envelope are two key glycoproteins, gp120 and gp41, which are essential for viral binding and entry into the host cell.