Lecture 27: Virology II

Question 1

Based on genome, what are the 2 types of viruses?

Answer 1

DNA viruses and RNA viruses.

Question 2

What are are possible compositions of viral genomes for both DNA and RNA viruses?

Answer 2

• Double or single stranded DNA
• Double or single stranded RNA

Question 3

What are the possible shapes of viral genomes?

Answer 3

Linear, circular, or segmented

Question 4

How many genes do viral genomes typically contain?

Answer 4

4 to hundreds (generally quite few)

Question 5

Where does DNA replication take place for eukaryotic DNA viruses? Name an exception.

Answer 5

It usually takes place in the nucleus of the host cell. The exception is poxviruses, which replicate in the cytoplasm.

Question 6

Where does RNA replication take place for eukaryotic RNA viruses? Name an exception.

Answer 6

It usually takes place in the cytoplasm. The exception is retroviruses, which require an extra intra-nuclear step where its RNA gets converted into DNA and is integrated into the host DNA.

Question 7

What are the two major experiments in the virology breakthrough in the 1950s? What did they prove?

Answer 7

1. Hershey-Chase experiment
2. Fraenkel-Conrat experiment
   They proved that the viral nucleic acid genome is the genetic code.

Question 8

What did the Hershey-Chase experiment prove?

Answer 8

It proved that DNA is the genetic material of bacteriophage T2.

Question 9

Explain how the Hershey-Chase experiment worked.

Answer 9

The phage proteins from the capsid were radioactively labelled with 35S and the DNA was labelled with 32P. After allowing the T2 virus to infect the host, it was found that radioactive proteins were not inherited, but radioactive DNA was, as the virus produced from the radioactively labelled DNA was also radioactive.

Question 10

What did the Frankel-Conrat experiment prove?

Answer 10

It proved that RNA is the genetic material of the TMV virus.

Question 11

Explain how the Fraenkel-Conrad experiment worked.

Answer 11

It was based on the creation of hybrid viruses by mixing and matching TMV A and TMV B, which produce different-sized lesions on tobacco leaves. The capsid of one was mixed with the RNA of another. It turned out that the type of lesions produced correlated with the type of RMV of the RNA, not the capid. This indicated that the RNA dictates the characteristics of a virus encoded by TMV.

Question 12

What is the relationship between the structure of the genome and whether the virus has an envelope or not?

Answer 12

There is no correlation.

Question 13

How do RNA viruses challenge the traditional dogma (2 ways)?

Answer 13

• They can reverse transcribe RNA into DNA (reverse transcriptase)
• They can synthesize RNA from an RNA template (RdRP)

Question 14

What are the 2 enzymes characteristic of RNA viruses? Explain what they do and which types of RNA viruses have them.

Answer 14

Reverse transcriptase: transcribes RNA into DNA, present only in retroviruses
RNA-dependent RNA polymerase (RdRP): synthesizes RNA from an RNA template, present in all RNA viruses except retroviruses.

Question 15

Give 3 examples of attributes that can be used in the classification of viruses based on viral attributes.

Answer 15

• Nature of nucleic acid in virion
• Symmetry of protein shell (capsid)
• Presence of absence of phospholipid membrane (envelope)

Question 16

What is the ICTV classification?

Answer 16

It is taxonomy of viruses based on the classical hierarchical system: kingdom, phylum class, order, family, genus, and species.

Question 17

In the ICTV classification, the order of a virus ends with…

Answer 17

-virales

Question 18

In the ICTV classification, the family of a virus ends with….

Answer 18

• viridae

Question 19

In the ICTV classification, the sub-family of a virus ends with…

Answer 19

-virinae

Question 20

In the ICTV classification, the genus of a virus ends with…

Answer 20

-virus

Question 21

What are the 3 criteria upon which Baltimore’s system of virus classification is based?

Answer 21

1. The nucleic acid composition of the viral genome
2. The way the viral genome is replicated
3. The way the virus makes mRNAs for expression of the viral proteins (i.e. the way the viral genome is expressed)***

Question 22

What is Baltimore’s central insight about how viruses work?

Answer 22

All viral genomes must make mRNAs that can be read by host ribosomes to synthesize viral proteins. Thus, all viruses must make (+)mRNAs that can be used by host ribosomes, and the pathways used to make viral mRNAs can be used for virus classification.

Question 23

What about the RNA produced by viruses makes them legible to ribosomes in the host cell?

Answer 23

They must have a positive polarity.

Question 24

What are the 7 categories of viruses according to Baltimore classification?

Answer 24

1. Double-stranded DNA viruses
2. (+) single-standed DNA viruses
3. Double stranded RNA viruses
4. (+) single-stranded RNA viruses
5. (-) single-stranded RNA viruses
6. Single-stranded RNA (+) retroviruses
7. Double-stranded DNA retroviruses

Question 25

Explain how (+) single-stranded RNA viruses work to produce mRNA in the host cell.

Answer 25

The virus uses its RdRP to make a negative copy of the RNA. This copy is then used as a template to make many positive copies that are legible to the host ribosomes. The RNA produced may be subgenomic, meaning only a portion of the original genome.

Question 26

Are (+)ssRNA and (+)mRNA always identical? Explain.

Answer 26

No. The (+)RNA genome of a virus might not get completely transcribed into (+)mRNA - when the (+)mRNA is shorter than the genome, it is called subgenomic (+)mRNA.

Question 27

How does the Baltimore classification account for subgenomic RNA?

Answer 27

It doesn’t make a distinction.