Viral structure & replication

Question 1

What is a virion?

Answer 1

The mature infectious virus particle.

Question 2

What is the capsid?

Answer 2

The protein shell that encloses and protects the viral nucleic acid.

Question 3

What are capsomers?

Answer 3

The individual protein subunits that form the capsid.

Question 4

What is the nucleocapsid?

Answer 4

The nucleic acid and its closely associated capsid proteins (the internal part of the virus).

Question 5

What is the viral envelope?

Answer 5

The viral membrane consisting of a lipid bilayer, proteins, and glycoproteins.

Question 6

What kind of nucleic acid is found in a mature virus particle?

Answer 6

DNA or RNA, but usually not both.

Question 7

What are the three topologies (shapes) of the viral capsid?

Answer 7

Complex, Icosahedral, or Helical.

Question 8

What is the source of the lipid bilayer in the viral envelope?

Answer 8

A modified host cellular membrane (host derived phospholipid bilayer).

Question 9

What are matrix proteins (M proteins) associated with?

Answer 9

The inner layer of the envelope; they aid in viral structure.

Question 10

What is the function of viral attachment proteins and fusion proteins (F proteins) on the envelope?

Answer 10

They allow attachment and cause the viral membrane to fuse with cellular membranes.

Question 11

Which type of virus is more susceptible to inactivation?

Answer 11

Enveloped viruses, because their lipid membranes are easier to strip away (e.g., with detergents).

Question 12

What is the most important determinant of host specificity for a virus?

Answer 12

The presence of viral receptors on the cell surface.

Question 13

What are the four steps of viral infection of a host cell?

Answer 13

Attachment, Entry, Synthesis (proteins/genome), and Assembly/Release.

Question 14

How do nonenveloped viruses generally enter the host cell?

Answer 14

Binding and rearrangement of capsid proteins or receptor-mediated endocytosis.

Question 15

How do enveloped viruses generally enter the host cell?

Answer 15

Fusion of viral and cell membranes (via F protein) or phagocytosis/endocytosis with subsequent membrane fusion.

Question 16

Why is the F protein essential for enveloped viruses?

Answer 16

It mediates the fusion of the viral membrane with the host cell membrane for entry.

Question 17

In the viral growth curve, what happens during the eclipse period?

Answer 17

Nucleic acid is uncoated; no infectious particles are inside or outside the host cell.

Question 18

In the viral growth curve, what happens during the latent period?

Answer 18

Intracellular virus is present, but no extracellular virus can be detected.

Question 19

What enzyme do RNA viruses use to make mRNA and RNA genomes?

Answer 19

RNA-dependent RNA polymerase.

Question 20

Why does RNA-dependent RNA polymerase have a high mutation frequency?

Answer 20

It often lacks a proofreading function.

Question 21

What is the key characteristic of positive sense single-stranded RNA (+ssRNA)?

Answer 21

It has the same polarity as mRNA and codes for proteins.

Question 22

Why is an isolated +ssRNA genome infectious alone?

Answer 22

It is mRNA and can immediately use host ribosomes to initiate protein synthesis.

Question 23

What must accompany a negative sense single-stranded RNA (-ssRNA) genome into the host cell to start infection?

Answer 23

RNA-dependent RNA polymerase.

Question 24

Where do most RNA viruses replicate?

Answer 24

In the cytoplasm.

Question 25

Where do most DNA viruses replicate?

Answer 25

In the nucleus.

Question 26

What enzyme do most DNA viruses depend on to produce their mRNA?

Answer 26

The cellular RNA polymerase II.

Question 27

What is a polyprotein in viral synthesis?

Answer 27

A large precursor protein that is cleaved by proteases into discrete, functional viral proteins (e.g., enzymes, glycoproteins).

Question 28

How are unenveloped viruses (naked nucleocapsids) typically released?

Answer 28

By disintegration (bursting) of the infected cell.

Question 29

How are enveloped viruses typically released?

Answer 29

By slow release (budding), acquiring their envelope as they pass through modified cellular membranes.

Question 30

What is the nucleic acid and path to mRNA for Class II viruses?

Answer 30

Single-stranded DNA (+ DNA); conversion to ± DNA, then transcription to mRNA.

Question 31

What is the nucleic acid and path to mRNA for Class I viruses?

Answer 31

Double-stranded DNA (± DNA); transcription to mRNA.

Question 32

What is the nucleic acid and path to mRNA for Class III viruses?

Answer 32

Double-stranded RNA (± RNA); transcription of the negative strand to mRNA.

Question 33

What is the nucleic acid and path to mRNA for Class IV viruses?

Answer 33

Positive-sense single-stranded RNA (+ RNA); the genome is the mRNA.

Question 34

What is the nucleic acid and path to mRNA for Class V viruses?

Answer 34

Negative-sense single-stranded RNA (- RNA); transcription by viral polymerase to mRNA.

Question 35

What is the nucleic acid and path to mRNA for Class VI viruses (Retroviruses)?

Answer 35

Positive-sense single-stranded RNA (+ RNA); reverse transcription to - DNA, then to ± DNA, then transcription to mRNA.

Question 36

What is the nucleic acid and path to mRNA for Class VII viruses (Hepadnaviruses)?

Answer 36

Double-stranded DNA (± DNA) with a gap; completion to ± DNA, then transcription to mRNA.