(Section A: Virology) Lecture 03: Viral Genomes

Question 1

What experiment first proved that viral genomes were key?

Answer 1

Hershey-Chase Experiment

Question 2

Describe:

Hershey-Chase Experiments

Answer 2

Two parallel experiments using radioactivity
1. Radioactive label on the shell of the virus
2. Radioactive label in the genome of the virus

Question 3

Describe:

Process of Hershey-Chase Experiment

Answer 3

1. The viruses were allowed to infect the cell
2. Blended using a blender
3. Virus were separated during the process of blending
4. Radioactivity was shown

Question 4

What purpose did centrifuging the virus/bacteria mix have?

Answer 4

• Bacteria are larger (thus heavier and forming pellets at the bottom)
• Viruses are lighter/smaller, and float on top (supernatent)

Question 5

Describe:

Results of Hershey-Chase Experiment

Answer 5

Experiment 1
* Supernatent was radioactive
* Pellets were not radioactive

Experiment 2
* Supernatent was not radioactive
* Pellets were radioactive

Showed that it was the genome that was key in the process

Question 6

What is difference in modern-day Hershey-Chase Experiment?

Answer 6

1. Uses fluorescent dye on viral genomes
2. Ability to see the genome slowly enter the host

Question 7

Key concept:

1. Viral genomes must make —-
2. This —- must be read by —- ribosomes
3. mRNA is read in the —— direction

Answer 7

1. mRNA
2. mRNA, host
3. 5’ to 3’

Question 8

What is the “dogma”?

Answer 8

Double strand DNA is transcribed to single stranded mRNA, and then translated to protein

Question 9

True or False:

Viruses does dogma

Answer 9

False, virus does not go through this process in a linear fashion

Question 10

What is the common goal of all viruses?

Answer 10

Production of mRNA to be translated by host cell machinery

Question 11

What is the (+)/(-) terminology for mRNA and DNA strands?

Answer 11

• mRNA by convention, is the (+) strand (ready to be translated)
• DNA (+) is ready to be transcribed
• RNA or DNA complements of (+) strands are considered (-) strands

Question 12

What are the 7 types of viral genomes according to the Baltimore Classification Scheme?

Answer 12

RNA:
1. Single stranded (ss) (+) RNA
2. Single stranded (ss) (-) RNA
3. Double stranded (ds) RNA
4. Single stranded (ss) (+) RNA with DNA intermediate

DNA:
5. Single stranded (ss) DNA
6. Double stranded (ds) DNA
7. Gapped double stranded (ds) DNA

Question 13

True or False:

DNA viruses are older and existed for a longer period of time

Answer 13

False, RNA viruses came first

Question 14

Describe:

Double stranded (ds) DNA genomes

Answer 14

• Requires DNA-dependent RNA polymerase transcription
• Similar to “dogma”

Question 15

Do double stranded (ds) DNA genomes require host machinery to transcribe? Give examples

Answer 15

Some do, some don’t

For small genomes (ex. 5 Kbps):
* Ex. Polyomaviridae - JC virus
* Small and simple
* Uses host DNA-dependent RNA polymerase

For large genomes (ex. ~100-370 Kbps):
* Ex. Poxviridae - Smallpox (Variola)
* Big, makes more viral proteins
* Expresses viral DNA-dependent RNA polymerase

Question 16

Describe:

Gapped double stranded (ds) DNA genomes

Answer 16

• Uneven length strands of DNA
• Need to make double stranded DNA
• Viral associated DNA polymerase “fills in gaps”
• Then, uses host DNA dependent RNA polymerase transcription

Question 17

Give an example of:

Gapped double stranded (ds) DNA genomes

Answer 17

Hepadnaviruses (Hepatitis B Viruses)
* Treatable, may cause liver damage
* Spread through sexual contact
* 100 times more infectious than HIV

Question 18

Describe:

Single stranded (ss) DNA genomes

Answer 18

• Single strands CANNOT be copied to mRNA
• Host DNA polymerase makes (ds) DNA
• Host RNA polymerase makes mRNA

Question 19

Give an example:

Single stranded (ss) DNA genomes

Answer 19

Parvoviruses
* Infects cats/dogs
* Can infect humans too

Question 20

What is unique about Viral RNA genomes?

Answer 20

Host doesn’t have enzyme that is an RNA-dependent RNA polymerase

Question 21

What has to be done with (-) RNA strands in Viral RNA genomes?

Answer 21

Requires a virally encoded RNA-dependent RNA polymerase to make mRNA readable by host translation machinery

Question 22

Describe:

Double stranded (ds) RNA genomes

Answer 22

• Double stranded RNA cannot get translated
• (+) RNA strand is made using the viral RNA dependent polymerase and (-) RNA strand as a template
• Newly made (+) RNA strand becomes mRNA

Question 23

Give an example of:

Double stranded (ds) RNA genomes

Answer 23

Reoviridae
* E.x. Rotavirus
* Causes gastroenteritis

Question 24

Describe:

Single stranded (ss) (+) RNA genomes

Answer 24

• Ready to go!
• No need for a virally encoded RNA dependent polymerase

Question 25

Give an example of:

Single stranded (ss) (+) RNA genomes

Answer 25

Picornaviridae
* E.x. Poliovirus
* Causes paralytic polyomyelitis

Coronaviridae
* E.x. SARS-2 CoV-2

Question 26

Describe:

Single stranded (ss) (+) RNA genomes with DNA intermediate

Answer 26

• Reverse transcriptase makes DNA from RNA (because ssDNA cannot be transcribed)
• Host DNA polymerase makes dsDNA
• DNA transcription to make RNA

Question 27

Give an example of:

Single stranded (ss) (+) RNA genomes with DNA intermediate

Answer 27

Retroviridae
* E.x. HIV

Question 28

Describe:

Single stranded (ss) (-) RNA genomes

Answer 28

• (-) RNA needs to be copied by viral polymerase to (+) RNA

Question 29

Give examples of:

Single stranded (ss) (-) RNA genomes

Answer 29

Paramyxoviridae

Orthomyxoviridae
* E.x. Influenza Virus

Filoviridae
* E.x. Ebola Virus