Microbio Module 6 Review Questions

Can viruses be classified as either prokaryotic or eukaryotic microorganisms?

No. Viruses are neither prokaryotic nor eukaryotic.

What is an obligate intercellular parasite?

As an obligate intercellular 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

What is a viral capsid?

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

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

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.

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

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

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

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.

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

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

Describe the basic steps of the viral life cycle.

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 cell3. Uncoating: (where applicable) the viral capsid disassembles4. Replication: The viral genome provides the 'blueprint' to make copies of itself5. Exit: New viruses are produced and leave the cell6. New infection: Newly produced viruses leave the host cell and go on to infect new cells. The process outlined above now restarts

What is a bacteriophage?

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

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

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.

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

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.

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.

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.

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

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

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

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.

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

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.

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

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.

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

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.

chickenpox family and length

Herpesvirus, ~125,000

mumps family and length

Paramyxovirus, ~15,000

Measles family and length

Paramyxovirus, ~15,000

Smallpox length


polio family and length

~7,700, picornaviridae

influenza family


HIV family and length

Retroviridae, ~10,000

Describe the composition of HIV.

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.