Virus Structure and Function

Question 1

LO 1. Define the basic properties of viruses.

Answer 1

submicroscopic, obligate intracellular parasites; not alive;

Question 2

LO 2. List the strategies viruses employ for survival.

Answer 2

1. house genetic info w/I capsids
2. genome contains all info needed to initiate and complete an infectious cycle
3. establish a relationship with a host, from benign to lethal

Question 3

LO 3. Describe 2 means of classifying viruses.

Answer 3

1. the Classical System (grouped by shared physical properties)
2. the Baltimore System (by the way they produce mRNA)

Question 4

LO 4. Understand basic methods for studying viruses.

Answer 4

electron microscopy
cell culture
animal models
sequence analysis
serology
other molecular techniques

Question 5

LO 5. Identify the main structural characteristics of virus particles.

Answer 5

capsids, envelopes, genetic info (DNA, RNA, or both)

Question 6

LO 6. Describe the 7 basic virus genomes, protein expression strategies, and replication strategies of each.

Answer 6

1. dsDNA (use the - strand as template, RNA pol II, in nucleus or cytoplasm)
2. gapped circular dsDNA (uses a protein as part of its pol machinery)
3. ssDNA (ds intermediate made in nucleus)
4. dsRNA
5. ss(+)RNA (RNA-dependent RNA pol (RdRp)
6. ss(-)RNA (also use RdRp)
7. ss(+)RNA with DNA intermediate (copy their ssRNA into DNA first using reverse transcriptase)

Question 7

LO 7. Describe a typical, generalized replication cycle of a virus.

Answer 7

1. attachment
2. entry and uncoating
3. viral gene expression
4. viral genome replication
5. assembly of new viruses and egress of these from the cell

Question 8

Viruses house their DNA or RNA genomes in small proteinaceous particles called _____.

Answer 8

capsids

Question 9

Name 2 capsid shapes.

Answer 9

1. helical

2. isosahedral

Question 10

What is the Classical System?

Answer 10

grouping viruses by their shared physical characteristics (type of genome, capsids, naked or enveloped, dimensions)

Question 11

What is the Baltimore System?

Answer 11

categorizes viruses based on how they produce mRNA

Question 12

What is the (+) strand?

Answer 12

mRNA that has an open reading frame- it’s ribosome ready and able to be translated into protein

Question 13

What is the (-) strand?

Answer 13

complementary mRNA seq that is not directly translated to mRNA, but is used to make DNA

Question 14

What are the 2 key principles of viral genomes?

Answer 14

1. Genomes serve as the template for synthesis of progeny genome (small, finite # of nucleic acid copying strategies)
2. viral genomes have to provide mechanisms for making mRNA

Question 15

What is a helical capsid?

Answer 15

has rotational symmetry; arranges irregularly-shaped proteins in a circle to form a disk

Question 16

What is an icosahedral capsid?

Answer 16

proteins arranged to form a hollow, quasi-spherical structure with genome inside it; 20 triangular faces

Question 17

What is an envelope?

Answer 17

lipid bilayers with viral glycoproteins embedded in the membrane that viruses acquire by budding thru a host membrane

Question 18

Why are envelopes important?

Answer 18

enveloped viruses don’t necessarily kill their host cells during replication, but non-enveloped viruses almost always do

Question 19

Why are viral glycoproteins important?

Answer 19

they mediate entry and to specific hosts, assembly and egress, and evasion from immune systems

Question 20

Describe the viral growth curve.

Answer 20

1. eclipse period (0-12 hours; no detectable infection inside or out of the cell)
2. latent period (16 hours; time it takes from initial infection to release of new viral particles)
3. burst (sudden increase of PFUs)

Question 21

What happens during the latent period?

Answer 21

attachment, entry, uncoating, gene expression, genome replication, assembly, and egress

Question 22

What is attachment?

Answer 22

binding of viral-attachment protein to a cellular receptor, which can be protein (specific) or carbohydrate (less-specific)

Question 23

What is entry?

Answer 23

penetration of the virus into the target cell. Energy dependent- cell must be metabolically active. Endocytosis or viral envelope fusion.

Question 24

How do enveloped viruses gain entry?

Answer 24

viral envelope fuses with the plasma membrane or endocytic membranes; requires the presence of a specific fusion protein

Question 25

What is uncoating?

Answer 25

once inside the cell, the viral particle loses its capsid and viral genome exposed

Question 26

How do DNA viruses express their genes?

Answer 26

1. fill (-) strand DNA using host's DNA pol 2 in the nucleus 2. transcribe normal mRNA * ** exception: poxviruses replicate in cytoplasm and encode their own RNA pol

Question 27

How do RNA viruses express their genes?

Answer 27

all have their own RNA-dependent RNA pol (RdRp) to produce mRNA and replicate RNA genomes

Question 28

How do (+) stranded RNA viruses express their genes?

Answer 28

direct translation by cellular ribosomes; amplification via RdRp

Question 29

How do (-) stranded RNA viruses and ds RNA viruses express their genes?

Answer 29

must transcribe (+) sense mRNA first by using RdRp

Question 30

How do retroviruses express their genes?

Answer 30

first copy ssRNA into dsDNA using reverse transcriptase; integrate new dsDNA into host genome; transcribe mRNA using host RNA pol 2

Question 31

What is assembly?

Answer 31

packaging of the genome into capsids

Question 32

Name 2 ways icosahedral capsids can package their genomes.

Answer 32

1. capsid assembles around the genome | 2. genome is fed into preformed capsids

Question 33

How do helical capsids package their DNA?

Answer 33

genome is coated with nucleocapsid protein during its synthesis

Question 34

What does CPE stand for?

Answer 34

cytopathic effect

Question 35

What is CPE?

Answer 35

capsid proteins produced in such high amounts that they are visible as cytoplasmic or nuclear inclusions on light microscopy; size and location characteristic of particular virus

Question 36

What is egress?

Answer 36

exit from the infected cell

Question 37

How do enveloped viruses egress?

Answer 37

budding (from plasma membrane = into extracellular environment; from Golgi or ER = secreted from cell)

Question 38

How do naked viruses egress?

Answer 38

cell lysis