MCB Lecture 52 Intro to Viruses

Question 0

What is a virion?

Answer 0

A virion is a virus particle

Question 1

What are the features of viruses? (6)

Answer 1

1. Subcellular (not living cells)
2. Obligate parasites
3. Can not self replicate
4. No energy metabolism
5. Immobile
6. Can infect all sorts of cells (plant, animal, Protozoa, bacterial, fungal)

Question 2

Describe the structure of viruses

Answer 2

1. Genome:
RNA, DNA, ss or ds, + or - sense
2. Capsid
Made of capsomeres
3. Optional: envelope

Question 3

What is the general size of a virus?

Answer 3

30-170nm

Question 4

How do we visualise viruses?

Answer 4

Electron microscopy

X ray chrystallography (only those without a membrane)

Question 5

Describe the different types of capsid

What are two other features that are seen? In which viruses are these seen?

Answer 5

1. Icosahedral
Eg. Polio
2. Helical capsid
Eg. Influenza
Envelope always present 
Complex structure, eg. Poxvirus
Multi shelled capsids, eg. rotavirus

Question 6

Which types of capsid must have a membrane associated with it? What about the other type?

Answer 6

Helical capsid viruses always have an associated envelope

Icosahedral viruses may or may not have associated envelopes

Question 7

What is interesting about the rotavirus?

Answer 7

It has a multi shelled capsid, making it very hardy

It can survive the acidic stomach

Question 8

How do we classify viruses?

Answer 8

Based on :

• Structure of genome
• Morphology
• Mode of replication

We don’t classify based on the infection caused by the virus

Question 9

What is the purpose of the infectivity assay?

Describe the steps involved

Answer 9

This tells us how much virus is present in a source

1. Monolayer of cell culture
2. Add the virus
3. Layer of agar
4. Let develop
5. Remove the agar, wash and stain
6. Dead cells will not stain or be washed away
   We can thus visualise the number of viruses present in the original dilution and can calculate the number in the original source

Question 10

Describe the steps of replication of viruses

Answer 10

1. Adhesion to host
2. Entry into host
3. Uncoating
4. Genome replication
5. Assembly
6. Exit from host cell, shedding

Question 11

How does bacterial and viral replication differ?

Answer 11

Viral replication uses host machinery

Also, there is a much longer eclipse period (12 hrs) for viruses

Question 12

How do viruses get into cells? (3)

Answer 12

1. If envelope: membranes fuse

2. Endocytosis

Question 13

Where are the possible locations for genome replication of viruses? Which viruses do which?

Answer 13

Cytosol: RNA viruses
Nucleus: DNA viruses

Influenza: nucleus, because it needs splicing
Poxvirus: DNA, however it is so big that it does not rely on host
HIV: RNA, however, it is a retrovirus, so it goes to nucleus

Question 14

Which enzymes are available in the host cell for use in viral replication?
Why does this present a problem?

Answer 14

DNA dependent DNA pol
DNA dependent RNA pol

This presents a problem for RNA viruses, who require RNA dependent RNA pol
They must thus get this from somewhere else

Question 15

Describe the replication of the genome of RNA viruses using an example

Answer 15

Poliovirus
+ sense RNA virus

Once in cell and uncoated:
1. RNA strand is translated (mRNA)
2. One big protein translated
3. Auto cleavage of the big protein yielding structural proteins, proteases and RNA dep RNApol
4. Proteases cleave the proteins further
5. RNA-dep RNA pol transcribes + RNA into - RNA
6. - RNA then much transcribed into + RNA for packaging
4.

Question 16

What different types of genes are present on the polio genome?

Answer 16

Structural proteins (capsid)
Non-structural: proteases (to further cut up the single protein product) and RNA dependent RNA polymerase

Question 17

What is the Baltimore classification system?

Answer 17

This describes the ways that the genomes of different viruses can be replicated, based on whether they are ss or ds, and RNA or DNA

Question 18

What modifications are made to viral proteins?

Answer 18

Glycosylation in the Golgi

Question 19

How do viral proteins destined for the envelope get inserted into membrane that will go onto envelope the virus?

Answer 19

They are transported through the synthesis pathway and are placed in the plasma membrane of the host cell
Then, when viral shedding is occuring, viral genomes and the membrane proteins are concentrated into these budding vesicles

Question 20

Describe the process of release of viruses
(2)
Differentiate between viruses with membranes and those without

Answer 20

With envelope: budding

1. Envelope proteins transported to membrane
2. Nucleocapsid assembles near membrane
3. Nucleocapsid induces the assembly of a membrane with the proteins embedded
4. Budding, lipid destabilised

No envelope: burst out when numbers are great enough (apoptosis)