SH7: Virus replication

Question 1

What does particle:infectivity ratio mean? Why do we use it?

Answer 1

It is used to describe the number of virus particles that infect a cell for a certain number of virus particles that enter a cell (a low ratio, such as 1:1000 shows that for every 1000 virus particles entering a cell, 1 virion will be infectious. It is used because many viruses are defective so the particles cannot cause an infection

Question 2

What are the 3 different infection types?

Answer 2

• Productive (permissive cell)- the virus enters a host cell that allows virus replication by supporting all the biochemistry needed for replication
• Abortive (non-permissive)- the cell does not result in virus release, as either the viral particle may be defective, the cell may not allow for viral replication or the cell does not have a receptor allowing the virus to enter
• Restringent or restrictive- the cell is transiently permissive so only a few virus particles are able to reproduce in the cell, but once a few are made then the production stops. Even though production stops, the genome remains in the cell.

Question 3

What determines the tropism of most viruses (whether or not the host cell is permissive for viral replication)?

Answer 3

The expression or absence of the glycoprotein receptors on the surface of cells. These bind too attachment proteins/ anti-receptors on the virus.

Question 4

What are the two ways viruses get into cells?

Answer 4

• Endocytosis of the entire particle resulting in the fusion of the cell membrane around the viral particle forming a vesicle (endosome).
• Fusion with the viral envelope with the lipid cell membrane and releasing the capsid straight into the cell.

Question 5

How do viruses acquire their envelope?

Answer 5

Some acquire a lipid bilayer from the nuclear envelope when leaving the nucleus. Some bud into the Golgi apparatus, either exit organelle when cell dies or Golgi transports them outside cell. Some get it from plasma membrane.

Question 6

How do virus particles leave the cell?

Answer 6

• Lysis of the cell

* Budding through the plasma membrane

Question 7

In the viral growth graph, what is the x and y axis labelled as?

Answer 7

virus titre (virus number) on the y axis and time after infection on the x axis

Question 8

In the viral growth graph, what are the different stages and what do they show?

Answer 8

Eclipse phase has low amounts of parental infectious material present. The genome replication has been initiated.
Maturation phase shows the viral material accumulating in the cell of in the surrounding medium.
Depending on the life cycle of the virus, the cell either bursts (lytic) so virus titre falls, or slowly releases viral particles but continues reproduction (non-lytic) so virus titre stays similar.

Question 9

What is a lytic infection?

Answer 9

When the cell bursts because there are so many viral proteins taking over, and the infected cell does not have enough energy and resources to keep the cell structure viable

Question 10

What is a non-lytic infection?

Answer 10

When very few viral particles are released at a time, and the host cell continues to produce virus particles