Viruses lectures

Question 1

Size of virus?

Answer 1

0.1-0.3 micrometers

Question 2

What is viral tropism?

Answer 2

Each virus affects only 1/ a few cell types

Question 3

Outside the host cell what are viral particles named?

Answer 3

Virons

Question 4

Draw bacteriophage basic diagram

Answer 4

See notes for diagram

Question 5

What is a lytic virus?

Answer 5

It destroys the host cell in its life cycle

Question 6

What are stages of bacteriophage attack?

Answer 6

1) Phage attaches to E. Coli and injects chromosome
2) Bacterial chromosome breaks down
3) Expression of phage genes produces phage structural components
4) Progeny phage assemble
5) Bacterial wall lyses, phage progeny released

Question 7

Draw HIV

Answer 7

See notes for diagram

Question 8

How does HIV use host PM

Answer 8

It bursts out of cell with incorporated receptors for host infection in its viral envelope

Question 9

What does HIV infect?

Answer 9

Lymphocytes (T helper cells) and macrophages with CD4 molecule on surface

Question 10

What does viral genome code for?

Answer 10

Capsid and genome replication and viral protein synthesis in the host cell

Question 11

How does viral translation work?

Answer 11

From RNA sense strand engaging host ribosomal machinery for protein synthesis

Question 12

How do bacteriphages with double stranded DNA replicate their genome?

Answer 12

bacteriophages with ds DNA can replicate genome separately to the host genome, destroying the host genome in the process (bacteriophage can be ss or ds DNA or RNA)

Question 13

What is the lysogenic cycle? (different to lytic)

Answer 13

Bacteriophage viral genome incorporated in the host genome and replicated with the host genome, ensures viral genome propagated by healthy host cell
ssRNA –> dsDNA (via reverse transcriptase) –> genome integrated into host cell (NB reverse transcriptase uses host tRNA as primer)

Question 14

How are viral genomes economical?

Answer 14

HIV encodes many proteins on a single gene transcribed by a single RNA
Translated by host ribosomes as polyproteins which are then broken down by HIV protease into single proteins
Only need 1 promoter
Expression of a few HIV proteins requires alternate splicing of mRNA

Question 15

How can viral genomes evolve so rapidly?

Answer 15

Polymerases and reverse transcriptases for viruses are very low fidelity so virus still replicated even after lots of mutation

Question 16

What happens in influenza?

Answer 16

There is recombination between strands: 8 RNA strands can be shuffled between coinfecting species

Question 17

What are 3 strategies to get the host ribosome to translate sense viral RNA?

Answer 17

1) EIF cleavage
2) Cap stealing
3) IRES use
(see notes for details)

Question 18

How many reading frames can viral RNAs have?

Answer 18

Many, can promote ribosome skipping between alternate start AUG codons

Question 19

What is HAART?

Answer 19

High activity anti-retroviral therapy, in 1990s transformed HIV to a managable chronic disease
Targets HIV reverse transcriptase and HIV protease
Prevents replication, transcription and assembly of the viral genome

Question 20

How can HIV escape therapy?

Answer 20

Long phases of molecular quiescence

HIV infected lymphocytes (T helper cells specifically) are quiescent

Question 21

Draw diagram of how HIV gains entry into cell

Answer 21

(see notes for diagram)

Question 22

What enzyme allows HIV genetic material to be integrated into the DNA of the infected cell?

Answer 22

Integrase

Question 23

How are virons released?

Answer 23

Budding from the cell membrane without cell lysis, viral genome in the host cell can be preserved for many years due to quiescence

Question 24

How does HIV cause immunodeficiency?

Answer 24

Via continued apoptosis of infected and non-infected T-cells due to immune reaction
(or direct toxicity of viral proteins)

Question 25

Draw diagram of HIV transition between states

Answer 25

(see notes for diagram)

Question 26

What are new therapies for HIV trying to do?

Answer 26

Transcription of viral genome by host RNA pol 11 depends on HIV protein TAT (Trans-Activator of Transcription)
TAT above threshold= replication cycle
TAT below threshold= latency
New therapies either try to increase TAT variability and get it above threshold
Or dampen