Virology Chapter 1: General Features of Virus Structure

Question 1

How do viruses differ from cells?

Answer 1

• simple structure
• genetic info is encoded by DNA or RNA
• dependent on a living cell in order to replicate themselves

Question 2

In what forms can a virus exist?

Answer 2

• extracellular form

- intracellular form

Question 3

What is the extracellular form of a virus called? What does it do?

Answer 3

virus particle or virion

transfers viral genome from one cell to another

Question 4

What is a viral infection?

Answer 4

when viral genome is introduced into a host cell and REPLICATED

Question 5

What do viruses need to replicate?

Answer 5

the virus particle (virion) must bind to host cell, and genome + required proteins must enter the cell

Question 6

What is the intracellular form of viruses? What do they do?

Answer 6

primarily are replicating nucleic acids

use host cell to provide energy, enzymes, metabolic components (amino acids), and structural machinery (ribosomes) to replicate the genome and synthesize viral proteins that make up the virus coat

Question 7

What is encoded in the viral genome?

Answer 7

genes that allow for:

• replication of viral genome
• production of viral structural proteins and enzymes require for replication (that they can’t adapt or access from host cell)
• proteins of capsid

Question 8

What is the capsid?

Answer 8

protective cover for the nucleic acid genome of all viruses

Question 9

What is the capsid composed of?

Answer 9

multiple capsomeres – individual protein subunits

Question 10

What are the 3 types of capsids?

Answer 10

• helical
• icosahedral (spherical)
• complex

Question 11

What is the envelope?

Answer 11

additional layer – lipid bilayer

only in some viruses

Question 12

What is packaged inside the capsid?

Answer 12

enzymes needed for replication of their genome

only in some viruses

Question 13

How are helical capsids assembled?

Answer 13

• capsid protein has affinity for the viral genome and for other copies of the capsid protein
• proteins attach to virus genome as genome is being synthesized
• length of capsid is as long as the genome

Question 14

How are icosahedral capsids arranged?

Answer 14

• 20 identical equilateral triangular structures joined to form icosahedrons
• each triangular structure can be made of a single polypeptide or several polypeptide subunits – subunits could be the same or different polypeptides

Question 15

What is the structure of complex capsids?

Answer 15

consists of multiple layers of proteins and other structures

Question 16

How is the envelope (lipid bilayer) acquired?

Answer 16

acquired as the virus particle leaves the host cell – derived from nuclear, ER, or plasma membranes

Question 17

How might envelopes be modified?

Answer 17

viral proteins are added that are required for the virus to infect the next host cell

Question 18

What are factors that may cause pathological effects of viral diseases?

Answer 18

• viral gene products (proteins) may have toxic effects on host cell metabolism
• host’s immune system may react and destroy infected cells expressing viral genes
• expression of host genes may be modified by structural or functional interactions with genetic material of the virus
• host cells may be destroyed directly by infecting virus as a consequence of the virus replication cycle

Question 19

Can viruses be used for gene therapy?

Answer 19

potentially – can be used to transfer genes into cells to correct genetic defects

Question 20

Do all viruses use host cell’s ribosomes for protein synthesis?

Answer 20

yes

Question 21

What is a nucleocapsid?

Answer 21

capsid + genome packaged inside it

Question 22

Why are enveloped viruses typically less stable than naked viruses when outside of the host cell?

Answer 22

lipid structures are not durable (no covalent bonds holding adjacent lipid molecules together) and are highly susceptible to disruption such as dehydration

lipid layers often contain proteins that the virus uses as antireceptor to bind to host cell, and lipids themselves are often required for entry into cell via fusion of viral envelope to the host cell membrane

non-enveloped viruses have a protein coat as their most exterior unit – proteins are far
less susceptible to disruption than lipids

Question 23

What are some of the functions of the protein coat that surrounds the genome of a naked virus particle?

Answer 23

• transport genome from host cell to host cell
• protect genome from environmental agents that could damage genome (ie. UV light, proteases, molecules that could oxidize nucleic acid)
• facilitate binding to susceptible host cells