Viruses, viroids, and prions Flashcards
What is a Virus?
- Non-cellular; simply genes in a protein coat; lack metabolic machinery
- Lack replication machinery, so they need the host to replicate
- Major cause of disease across all domains
- Viruses are small (range between 5-500 nm long).
Discovery of Viruses
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
- Scientists deduced via experimentation that:
- TMV was a biological agent, smaller than bacteria, and needed host to replicate
- Discovery of TMV highlights logical deductive reasoning of science.
- could not see with a microscope of the time
Viral Structure/components
Components
- Genetic material (RNA or DNA)
- Capsid – outer shell of virus (the protein coat)
a. Capsomer – functional unit of capsid
b. virally encoded glycoproteins - Membrane – host derived membrane
4 Structure Types: Helical Icosahedral Envelope Complex
Viral Classification (depends on genetic material and presence of an envelope)
- DNA (e.g. Herpes, pox, HPV)
- Single versus double stranded
- Enveloped versus non-enveloped
- Linear versus circular DNA
- RNA (e.g. SARS, hepatitis, rabies, measles, HIV)
- Single versus double RNA
- enveloped versus non-enveloped
Virus Life cycles
General - all viruses need host to replicate
Enveloped
Retrovirus
Bacteriophage
Virus Life cycles:
General
all viruses need host to replicate
- virus enters cell
- host cell replicates viral genome
- viral genes transcribed
- viral proteins / genes self-assemble
- Completed virus departs cell
Virus Life cycles:
Enveloped
- glycoproteins attach to cell
- capsid enters
- complementary RNA made
- Comp. RNA replicates
- Comp. RNA creates proteins
- Glycoprotein exported to cell membrane
- self assembly of capsid and RNA
- Release with host membrane
Completed virus takes host membrane upon departure
Virus Life cycles:
Retrovirus
- transcriptase – enzyme that reverse transcribes viral RNA genome into DNA
- integrase – enzyme that randomly inserts new DNA into cell genome
- HIV is an enveloped retrovirus
Virus Life cycles:
Bacteriophage
- 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)
- 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
Viral Origins
- Progressive hypothesis - Viruses are mobile genetic elements that have evolved the added ability to escape from the cellular genome (e.g. retro-transposons).
- Regressive hypothesis - viruses are degenerate intra-cellular parasites that have eliminated all but essential features (e.g. membrane, replication machinery, etc.)
- Virus-First hypothesis - originated independent of cells as the earliest protobionts
Viroids
- 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)
Prions
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)
Viral origins:
Progressive hypothesis
Viruses are mobile genetic elements that have evolved the added ability to escape from the cellular genome (e.g. retro-transposons).
Viral origins:
Regressive hypothesis
viruses are degenerate intra-cellular parasites that have eliminated all but essential features (e.g. membrane, replication machinery, etc.)
Viral origins:
Virus-First hypothesis
originated independent of cells as the earliest protobionts
Virus
Biological agent + smaller than bacteria + needs host to replicate = virus
Virus Structure:
Helical
- Single type of capsomer stacked helically around genetic material creating a rod shape.
Virus Structure:
Icosahedral
- Comprised of numerous equilateral triangular faced capsomers. Sphere shaped.
Virus Structure:
Envelope
- Either a helical or icosahedral virus enclosed in a host-derived membrane.
- Capsid helical or icosahedral.
Virus Structure:
Complex
- Possess aspects of both helical and icosahedral morphologies.
Virus Components:
- Genetic material (RNA or DNA)
- Capsid – outer shell of virus (the protein coat)
a. Capsomer – functional unit of capsid
b. virally encoded glycoproteins - Membrane – host derived membrane
Capsid
outer shell of virus (the protein coat)
a. Capsomer – functional unit of capsid b. virally encoded glycoproteins
Membrane
host derived membrane
Transcriptase(Retrovirus)
enzyme that reverse transcribes viral RNA genome into DNA
Integrase(Retrovirus)
enzyme that randomly inserts new DNA into cell genome
Bacteriophage:
Lytic (virulent; cell death via lysis eminent)
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)
Bacteriophage:
Lysogenic (temperate; cell death via lysis eventual)
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
