L3: The Viruses

Question 1

What is a virus?

Answer 1

An obligate intracellular parasite that requires the biochemical machinery of the host for reproduction.

Question 2

What is a prion?

Answer 2

An infectious protein. In its extracellular form it contains no nucleic acid.

Question 3

Viruses can be various shapes. Name two.

Answer 3

Icosahedral (20 faces) and filamentous (rod-shaped).

Question 4

Key features of a virus?

Answer 4

1. Obligate intracellular parasite.
2. Cannot replicate without the biochemical machinery of the host.
3. Cannot make energy/proteins without the host.
4. Genome can be either DNA or RNA.

Question 5

What is the extracellular form of a virus called?

Answer 5

Virion - this is what spreads.

Question 6

‘Naked’ virus vs. ‘enveloped’ virus.

Answer 6

Naked - just the nucleocapsid (which is the capsid + nucleic acid).
Enveloped - nucleocapsid is surrounded by an envelope.

Question 7

Is a naked virus or an enveloped virus more stable? Explain your answer.

Answer 7

Naked viruses are more stable under environemental stress. The enveloped virus is less stable because the lipid envelope can dry out. Therefore, it must stay wet to remain infectious.

Question 8

The viral genome codes for 3 classes of protein that are not provided by the host cell. Name them.

Answer 8

1. Proteins for new viruses.
2. Enzymes for genome replication.
3. Proteins to interfere with host defence mechanisms.

Question 9

Name the types of viral genome.

Answer 9

dsDNA, dsRNA, ssDNA (+ or -), ssRNA (+ or -), retroviruses.

Question 10

What does it mean to say ssDNA/RNA has a (+) or (-) sense?

Answer 10

If the nucleic acid is (+) then the sequence is the same as the mRNA that is used for protein synthesis. (-) is complementary.

Question 11

Is mRNA (+) or (-)?

Answer 11

POSITIVE.

Question 12

How does a virus particle penetrate a host cell?

Answer 12

Membrane fusion or endocytosis.

Question 13

Give a description of the HIV (retrovirus) life cycle.

Answer 13

1. The viral RNA undergoes reverse transcription using reverse transcriptase. ssRNA –> ssDNA –> dsDNA.
2. Integrated into host cell DNA using viral integrase.
3. Viral DNA is transcribed into mRNA. Exits the nucleus and translated into proteins.
4. Proteins exit cytoplasm via budding.

Question 14

Give an example of a virus with a (+) ssRNA genome.

Answer 14

Hepatisis C.

Question 15

Is the replication of hepatisis C extra nuclear or inter-nuclear? Explain this.

Answer 15

Extra-nuclear. Its genome is (+) ssRNA so can be translated straight away. Does not require the nuclear machinery. Uses viral synthase to replicate the genome.

Question 16

Give an example of a virus with a (-) ssRNA genome.

Answer 16

Influenza virus.

Question 17

Explain the function of the glycoproteins hemagglutinin and neuraminidase in the influenza vrius.

Answer 17

HA - attaches to host receptors.

N - cleaves sialic acid.

Question 18

Antigenic drift?

Answer 18

Minor mutations.

Question 19

Antigenic shift?

Answer 19

Major recombination. This is when two or more viruses exchange genetic material, forming a new subtype with a mixture of antigens on their surface. E.g. influenza virus.

Question 20

What is the key issue with with HIV reverse transcriptase?

Answer 20

Highly error prone. Every time RNA is reverse transcribed a genomic variant is made.