Lecture 18: Replication of Viruses (Check facts)

Question 1

What are the 6 steps of viral replication?

Answer 1

• Attachment
• Penetration
• Uncoating
• Biosynthesis
• Assembly
• Release

Question 2

How do viruses bind to their cellular recptors?

Answer 2

Through their VAPs. This interaction is very specific and it determines the host cell and the species range.

Question 3

In enveloped viruses where are the VAPs located?

Answer 3

Within the viral envelope

Question 4

In naked viruses, where are the VAPs located?

Answer 4

VAPs are located within the capsid

Question 5

What is viral tropism?

Answer 5

The specificity of a virus to a specific cell or host

Question 6

Describe step 1 of the replication of viruses

Answer 6

• The virus will adhere to cellular surfaces through random collision.
• The binding is specific, but finding the right receptor is random, not specific.

Question 7

Describe step 2 of viral replication

Answer 7

• The virus attaches to a specific receptor on the cellular surface, more than one receptor may be involved.

Question 8

Describe step 3 of viral replication

Answer 8

• The viral genome is released into the cytoplasm
• (or the viral genome may need to be transported into the nucleus)

Question 9

Describe step 4 of viral replication

Answer 9

-

Question 10

How many receptors does HIV require for binding to the host cell?

Answer 10

2

Question 11

What was detected as the receptor for the influenza virus in 1985?

Answer 11

sialic acid

Question 12

Is it true that different viruses can bind to the same receptors?

Answer 12

Yes

Question 13

What are some examples of viruses that bind to the same recptor?

Answer 13

Adenovirus and Coxsackievirus B3 bind the same receptor

Question 14

Is it true that viruses of the same family may bind to different receptors?

Answer 14

Yes

Question 15

Is it true that one virus may bind to many different receptors?

Answer 15

Yes, one example of this is Herpes Simplexvirus-1 (HSV-1), which can infect different tissues and can interact with and bind to different receptors

Question 16

What are 2 ways a virus can enter a host cell?

Answer 16

Via phagocytosis and via endocytosis (phagocytosis usually involves the uptake of larger particles, whilst endocytosis involves the uptake of smaller particles, including fluid).

Question 17

Which viruses enter the host cell by fusing with the plasma membrane?

Answer 17

Enveloped viruses

Question 18

Why is fusion with the plasma membrane sometimes preferred?

Answer 18

• Easy and quick
• Requires certain fusion proteins, as well as the VAPs

Question 19

What are some examples of viruses that fuse with the plasma membrane?

Answer 19

HIV, herpes virus

Question 20

Describe the process of fusion of viruses with the host cell membrane

Answer 20

• The viral attachment proteins on the virus will attach to specific receptors on the host cell plasma membrane.
• After the binding to the receptor, fusion proteins embedded in the viral envelope are activated and undergo conformational changes that lead to changes in the fusion proteins, exposing a fusion peptide that interacts with the host membrane.
• The fusion peptide then exposing a fusion peptide that interacts with the host membrane.
• The virus and host will eventually fuse together, the viral genome will be uncoated, and released into the host cell cytoplasm.

Question 21

Describe the process of fusion of viruses with the host cell membrane within the cytoplasm

Answer 21

• For the fusion proteins to be activated, the pH has to be lowered.
• The virus binds to specific receptors on the plasma membrane and instead of fusing with thw plasma membrane, is endocytosed.
• The virus is endocytosed and an endosome is formed.
• The endosome is then transported to a low pH area of the cytoplasm.
• Fusion is initiated inside the endosome, uncoated, and the nuclear capsid is released in the cytoplasm.

Question 22

Describe non-enveloped endocytosis

Answer 22

Non-enveloped viruses bind to specific receptors on the host cell surface using their capsid proteins.

This interaction triggers the host cell to internalise the visu, and so the virus is endocytosed within an endosome.

The endosome and its environment becomes increasingly acidic

Viral proteins poke holes in the endosome through which viral particles enter the cytoplasm

Question 23

Is it true that microtubules move endosomes/viruses?

Answer 23

Yes

Question 24

What is uncoating?

Answer 24

The removal of the protein capsid

Question 25

The DNA of viruses must be delivered into the host cell nucleus. Which virus is an exception to this?

Answer 25

poxviruses

Question 26

The RNA of viruses must be delivered into the host cell cytoplasm. Which virus is an exception to this?

Answer 26

HIV, Influenza virus

Question 27

How do new virions exit the cells?

Answer 27

Through budding or lysing the cells

Question 28

Is it true that not every released virion is infectious?

Answer 28

Yes (only 10% are)