Viral Structure & Replication

Question 1

What components make up a virus?

Answer 1

DNA/RNA, capsid, envelope (some viruses)

Question 2

How do viruses infect cells?

Answer 2

Attachment, entry and uncoating, macromolecular synthesis, assembly and release

Question 3

What are differences between RNA and DNA viruses?

Answer 3

RNA Viruses - labile genome, genome remains in the cytosol, encodes polymerasees, more prone to mutations

DNA Viruses - stable genome, genome resides in the nucleus, utilizes host polymerases, interacts with host transcription factors

Question 4

What are the functions of viral capsids?

Answer 4

Protect the genome, involved with attachment (naked viruses), entry, uncoating, and assembly

Question 5

Generally, what are the properties of naked, icosahedral viruses?

Answer 5

Survive pH of GI tract - primary cause of gastroenteritis;
Resist temperature extremes, detergents, and poor sewage treatment;
Released via cell lysis;
Transmitted via fomites

Question 6

Generally, what are the properties of enveloped viruses?

Answer 6

Must remain wet for infectivity; heat and pH labile; destroyed by detergents and organic solvents; released via budding; transmitted via droplets & secretions

Question 7

How is the viral envelope derived? What are the functions of the envelope?

Answer 7

The envelope is derived from the host lipid membrane during budding. The envelope functions in: viral attachment, virus-cell fusion, & induce neutralizing antibodies

Question 8

What are neutralizing antibodies?

Answer 8

Neutralizing or protective antibodies are produced against enveloped viruses and serve to prevent infection

Question 9

What mediates viral attachment to cells?

Answer 9

Surface glycoproteins of enveloped viruses and capsid proteins of naked viruses

Question 10

Through what methods can a virus enter a cell?

Answer 10

Through the plasma membrane or receptor-mediated endocytosis

Question 11

How does viral entry through a plasma membrane differ for enveloped and naked viruses?

Answer 11

Glycoproteins on the envelope of enveloped viruses bind to the plasma membrane. The envelope then fuses with the membrane and releases the capsid into the cell.

Naked viruses bind with receptors on the cell surface. A coated pit forms in the plasma membrane and hydrophobic interactions create a channel for the genome to be delivered into the cell.

Question 12

Describe the process of pH-dependent endocytosis viral entry.

Answer 12

The virus is internalized into the cell via an endosome. The endosome becomes acidic and fuses with a lysosome. Low pH and protease activity activate fusion peptide proteins. This process involved Clathrin-coated pits.

Question 13

Describe the process of pH-independent endocytosis viral entry.

Answer 13

The virus is internalized at invaginations of lipid rafts that contain caveolin. Caveolin vesicles fuse to form a caveosome that does not fuse with a lysosome. The viral fusion proteins activate without an acidic environment.

Question 14

Where and how do DNA viruses replicate?

Answer 14

DNA viruses are transcribed and replicated in the nucleus. The virus uses host machinery to transcribe mRNA and produce proteins from the viral DNA. DNA viruses are replicated by viral-encoded DNA polymerases.

Question 15

True/False. +ssRNA viruses are equivalent to coding stands and can function as mRNA to be directly translated.

Answer 15

True. The +ssRNA can be directly translated into viral proteins using host machinery.

Question 16

Describe the process of +ssRNA translation and synthesis.

Answer 16

The + strand serves as a template to form a complementary (-) strand. A viral-encoded RNA-dependent RNA polymerase then utilizes the (-) strand to synthesize new +ssRNA that can be assembled and dispersed.

Question 17

Describe the process of -ssRNA transcription and synthesis.

Answer 17

A viral-encoded RNA-dependent RNA polymerase utilizes that parental (-) strand to form a complementary (+) strand. The (+) strand is translated using host machinery to form viral proteins. The (+) strand also serves as the template to synthesize new -ssRNA using the viral-encoded RNA-dependent RNA polymerase.

Question 18

How are retroviruses synthesized?

Answer 18

A (+) strand is transcribed into DNA using a viral-encoded RNA-dependent DNA polymerase. DNA then travels to the nucleus where it integrates into host chromosomes. From here, host machinery can transcribe and translate viral proteins. Viral DNA will also be synthesized by the host cell during replication.

Question 19

True/False. Enveloped viruses can only bud from the plasma membrane.

Answer 19

False. Enveloped viruses may bud out through the plasma membrane or ER. Budding through the ER requires the virus be released via exocytosis.

Question 20

How do lytic and chronic viral infections differ?

Answer 20

Both infections are types of productive infections. Lytic infections result in cell death and production of a cytopathic effect. In chronic infections, the virus is synthesized and buds without immediate cell death. This results in continuous viral shedding.

Question 21

What viruses can produce a latent infection? How do latent infections present?

Answer 21

Latent infections occur in DNA viruses and retroviruses, but cannot occur with RNA viruses. Viral DNA is integrated into host chromosomes that maintain the viral genome but the virus remains dormant. There is no immune response and anti-viral therapy is difficult as there is no target for a response.

Question 22

How can viral infections eventually lead to cancer formation?

Answer 22

Viral infection can sometimes result in cell immortalization - continuous replication. These cells can accumulate mutations and other replication errors that may result in transformation to a cancer cell.

Question 23

True/False. In a latent infection, viral particles are synthesized and shed.

Answer 23

False. During a latent infection, NO viral particles are synthesized. In a chronic infection, viral particles are produced at a low level and the host may shed the virus in a carrier state.

Question 24

What characteristics are generally indicative of a disseminated viral infection?

Answer 24

Patient is infectious before the illness is recognized, incubation period during which time the virus disseminates throughout the body, non-specific or flu-like symptoms during incubation

Question 25

True/False. Patients actively synthesize virus particles and are infections during a transformation infection.

Answer 25

False. When cells have undergone transformation as the result of viral infection, cells maintain the viral genome but do not synthesize the virus.

Question 26

How does acute infections differ from other types of infections?

Answer 26

Acute infections are often localized, symptomatic (subacute is asymptomatic), but there could be shedding), and self-limiting. The immune system clears the virus and establishes short-term memory.