Viruses

Question 1

What are the characteristics of viruses?

Answer 1

Acellular infectious particles
Obligate intracellular pathogens
Reproduce only inside of living cells
Lack independent metabolism

Question 2

What two things are viruses made of?

Answer 2

Nucleic acid genome (DNA or RNA) and protein coat (capsid)

Together = Nucleocapsid

Question 3

What is a capsid?

Answer 3

Protein coat that surrounds the genome

Question 4

What are the characteristics of a capsid?

Answer 4

Allows transfer of viral genome between host cells
Made of identical polypeptides – protomers
Helical capsids
Protomers form a spiral cylinder
Nucleic acid genome coiled inside
Ex) Tobacco mosaic virus capsid is made of ~ 2100 identical protomers

Question 5

What is a Icosahedral capsid? Give an example.

Answer 5

Regular geometric shape with 20 triangular faces
Exhibit symmetry
Protomers aggregate to form capsomeres
Ex. Human papillomaviruses have form their capsids from pentamers (clusters of 5)

Question 6

What is a binal capsid? Give an example.

Answer 6

Geometric head with an attached helical tail
Ex. T4 bacteriophage of E. coli
Genome is carried in a polyhedral head, helical tail is used to inject DNA into a host cell

Question 7

What is a Nucleocytoplasmic large DNA virus? What is special about it? Give an example.

Answer 7

Viruses with complex multi-layered structure
Ex. Mimivirus (infects amoebae)
0.75 µm in diameter, 1200 kbp DNA
Larger than some bacteria

Question 8

What is the structure that is a a lipid bilayer surrounding the nucleocapsid that was acquired from the host membrane?

Answer 8

Envelope

Question 9

What are the characteristics of an envelope?

Answer 9

Consists of host lipids and viral proteins – spikes
Ex. Influenza virus
Flexible helical capsid, surrounded by an envelope
Two major spikes:
-hemaglutanin (H)
-neuraminidase (N)

Question 10

True or False: viruses infect all domains of life.

Answer 10

True

Question 11

What is the term that describes viruses that infect bacteria like T4 infects E. coli?

Answer 11

Bacteriophage

Question 12

What is an animal virus? Give an example.

Answer 12

infect and multiply only inside of animal cells
Ex) Human papillomavirus – infects human epithelial cells
-Causes benign tumors (warts)

Question 13

True or False: Most viruses are non-specific when it comes to hosts.

Answer 13

False: Most viruses are specific when it comes to hosts.

Question 14

What must a virus attach to on the surface of its host?

Answer 14

A specific receptor

Question 15

Give an example of a specific receptor on a host cell.

Answer 15

HIV binds to CD4
Chemoreceptor on surface of some human immune system cells
HIV infects only humans

Question 16

Give an example of a virus that infects more than one species.

Answer 16

Influenza attaches to a glycoprotein found on surface of several animal cells
-Infects humans, pigs, chickens, seals etc.

Question 17

What are the steps of the viral replication cycle?

Answer 17

1. Adsorption – attachment to the host cell
2. Penetration and uncoating – entry into the host cell
3. Synthesis of viral nucleic acids and protein
4. Assembly of new virions
5. Release of new virions

Question 18

During the viral replication cycle, what does Step 1- Adsorption include?

Answer 18

Involves specific receptors on the host cell surface

Ex) LPS, outer membrane proteins or glycoproteins

Question 19

During the viral replication cycle, how does the virus enter the host cell in step 2?

Answer 19

The bacteriophage injects their nucleic acid into the cell, and leaves the capsid outside the cell wall like a ‘ghost’. For the bacteriophage to inject its contents, it must first fuse with the plasma membrane, so it can be Endocytosed (Binding to specific receptors triggers normal endocytic activity ) to enter the host cell.

Question 20

During the viral replication cycle, what do the viral genes do inside the cell in step 3?

Answer 20

Viral genes are expressed and viral proteins are synthesized (by the host’s own ribosomes)
Viral genome is replicated (by the host’s replication machinery)

Question 21

During the viral replication cycle, how do the viral genes assemble in step 4? How do they not replicate?

Answer 21

Viral proteins are assembled into capsids, and then genomes are packaged into nucleocapsids
-Viruses do not reproduce by division

Question 22

During the viral replication cycle, what two strategies are used to release the new virions in step 5?

Answer 22

Naked viruses usually accumulate, eventually lysing the host cell to release progeny
– lytic infection
Enveloped viruses are usually released by budding
-Virions push through the cytoplasmic membrane without killing the host cell – persistent infection

Question 23

What is budding?

Answer 23

Release of enveloped viruses •
Viral proteins inserted into the host membrane – Spikes Nucleocapsid associates with the spikes, and buds through the membrane to form the envelope

Question 24

Give two example of a virus that goes through the viral replication cycle.

Answer 24

Ex) Influenza
-Neuraminidase Allows new virions to exit the host cell
Ex) Hemagglutanin
-Allows viruses to adsorb to the next host