Viruses, viroids, and prions

Question 1

What is a Virus?

Answer 1

1. Non-cellular; simply genes in a protein coat; lack metabolic machinery
2. Lack replication machinery, so they need the host to replicate
3. Major cause of disease across all domains
4. Viruses are small (range between 5-500 nm long).

Question 2

Discovery of Viruses

Answer 2

Tobacco Mosaic Virus (TMV) – first virus isolated

- disease caused by biological agent
- agent contained in sap
- agent likely bacteria
- tuned out the agent was a bacterial toxin
- toxin much smaller than bacteria

Biological agent + smaller than bacteria + needs host to replicate = virus

1. Scientists deduced via experimentation that:
   
   • TMV was a biological agent, smaller than bacteria, and needed host to replicate
2. Discovery of TMV highlights logical deductive reasoning of science.
   
   • could not see with a microscope of the time

Question 3

Viral Structure/components

Answer 3

Components

1. Genetic material (RNA or DNA)
2. Capsid – outer shell of virus (the protein coat)
   a. Capsomer – functional unit of capsid
   b. virally encoded glycoproteins
3. Membrane – host derived membrane

4 Structure Types:
Helical
Icosahedral
Envelope
Complex

Question 4

Viral Classification (depends on genetic material and presence of an envelope)

Answer 4

1. DNA (e.g. Herpes, pox, HPV)
   
   • Single versus double stranded
   • Enveloped versus non-enveloped
   • Linear versus circular DNA
2. RNA (e.g. SARS, hepatitis, rabies, measles, HIV)
   
   • Single versus double RNA
   • enveloped versus non-enveloped

Question 5

Virus Life cycles

Answer 5

General - all viruses need host to replicate

Enveloped

Retrovirus

Bacteriophage

Question 6

Virus Life cycles:

General

Answer 6

all viruses need host to replicate

1. virus enters cell
2. host cell replicates viral genome
3. viral genes transcribed
4. viral proteins / genes self-assemble
5. Completed virus departs cell

Question 7

Virus Life cycles:

Enveloped

Answer 7

1. glycoproteins attach to cell
2. capsid enters
3. complementary RNA made
4. Comp. RNA replicates
5. Comp. RNA creates proteins
6. Glycoprotein exported to cell membrane
7. self assembly of capsid and RNA
8. Release with host membrane

Completed virus takes host membrane upon departure

Question 8

Virus Life cycles:

Retrovirus

Answer 8

1. transcriptase – enzyme that reverse transcribes viral RNA genome into DNA
2. integrase – enzyme that randomly inserts new DNA into cell genome
   - HIV is an enveloped retrovirus

Question 9

Virus Life cycles:

Bacteriophage

Answer 9

1. Lytic (virulent; cell death via lysis eminent)
   
   • attachment
   • entry of phage DNA and degradation of host DNA
   • synthesis of viral genomes and proteins
   • assembly
   • release(lyses cell)
2. Lysogenic (temperate; cell death via lysis eventual)
   
   • Phage injects its DNA
   • Page DNA circulates
   • Lytic or Lysogenic cycle
   • Phage DNA integrates into the bacterial chromosome, becoming a Prophage
   • the bacterium reproduces, copying the prophage and transmitting it to daughter cells
   • cell division produces daughter cells of bacteria infected with the Prophage
   • Keeps going through and reproducing or enters the lytic cycle

Question 10

Viral Origins

Answer 10

1. Progressive hypothesis - Viruses are mobile genetic elements that have evolved the added ability to escape from the cellular genome (e.g. retro-transposons).
2. Regressive hypothesis - viruses are degenerate intra-cellular parasites that have eliminated all but essential features (e.g. membrane, replication machinery, etc.)
3. Virus-First hypothesis - originated independent of cells as the earliest protobionts

Question 11

Viroids

Answer 11

• No Protein coat; small circular genome; make no proteins
• Can be parasites of other viruses
  
  • cause plant diseases by interfering with gene transcription
  • delta-viruses are viral parasites(hepatitis D)

Question 12

Prions

Answer 12

A. Misfolded proteins(no nucleic acids)

B. Replicate by touching other normal proteins

• cause degenerative neural diseases
• resistant to heat and other sterilization methods
• long incubation periods(~10 years)
• convert normal proteins by physical contact
• form plaques in the brain(causing cavaties)

Question 13

Viral origins:

Progressive hypothesis

Answer 13

Viruses are mobile genetic elements that have evolved the added ability to escape from the cellular genome (e.g. retro-transposons).

Question 14

Viral origins:

Regressive hypothesis

Answer 14

viruses are degenerate intra-cellular parasites that have eliminated all but essential features (e.g. membrane, replication machinery, etc.)

Question 15

Viral origins:

Virus-First hypothesis

Answer 15

originated independent of cells as the earliest protobionts

Question 16

Virus

Answer 16

Biological agent + smaller than bacteria + needs host to replicate = virus

Question 17

Virus Structure:

Helical

Answer 17

• Single type of capsomer stacked helically around genetic material creating a rod shape.

Question 18

Virus Structure:

Icosahedral

Answer 18

• Comprised of numerous equilateral triangular faced capsomers. Sphere shaped.

Question 19

Virus Structure:

Envelope

Answer 19

• Either a helical or icosahedral virus enclosed in a host-derived membrane.
• Capsid helical or icosahedral.

Question 20

Virus Structure:

Complex

Answer 20

• Possess aspects of both helical and icosahedral morphologies.

Question 21

Virus Components:

Answer 21

1. Genetic material (RNA or DNA)
2. Capsid – outer shell of virus (the protein coat)
   a. Capsomer – functional unit of capsid
   b. virally encoded glycoproteins
3. Membrane – host derived membrane

Question 22

Capsid

Answer 22

outer shell of virus (the protein coat)

    a. Capsomer – functional unit of capsid
b. virally encoded glycoproteins

Question 23

Membrane

Answer 23

host derived membrane

Question 24

Transcriptase(Retrovirus)

Answer 24

enzyme that reverse transcribes viral RNA genome into DNA

Question 25

Integrase(Retrovirus)

Answer 25

enzyme that randomly inserts new DNA into cell genome

Question 26

Bacteriophage: Lytic (virulent; cell death via lysis eminent)

Answer 26

Lytic (virulent; cell death via lysis eminent) - attachment - entry of phage DNA and degradation of host DNA - synthesis of viral genomes and proteins - assembly - release(lyses cell)

Question 27

Bacteriophage: Lysogenic (temperate; cell death via lysis eventual)

Answer 27

Lysogenic (temperate; cell death via lysis eventual) - Phage injects its DNA - Page DNA circulates - Lytic or Lysogenic cycle - Phage DNA integrates into the bacterial chromosome, becoming a Prophage - the bacterium reproduces, copying the prophage and transmitting it to daughter cells - cell division produces daughter cells of bacteria infected with the Prophage - Keeps going through and reproducing or enters the lytic cycle