Lecture Seventeen - Viruses Flashcards
What are viruses?
Acellular = non-living.
Unique class of infectious agents (very small).
Obligate intracellular parasites.
Infect bacteria, plants, fungi, insects and animals.
Most have limited host range - infect specific hosts and cannot infect others.
What is the structure of a virus?
Virions - virus particles (interchangable terms) = genome surrounded by repeating protein subunits (coat proteins).
Genertic material surrounded by a protein coat = capsid.
Some viruses have a second layer of a lipid envelope (usually animal proteins).
Replicate/multiply inside host cell (parasite).
Lack complete enzyme structures.
Replicte inside cells (use host cell factors to replicate).
What are some of the names of structures viruses can take?
Helical - e.g. TMV.
Rod shaped coat of repeating units.
Single protomer associates with nucleic acid (spiral or helical).
Enveloped - E.g. Influenza.
Icosahedral or helical (nucleo)capsid surrounded by envelope.
Envelope is host derived lipid.
Envelope contains virus encoded proteins.
Envelope may be flexible.
Icosahedral - E.g. Adenovirus.
Have twelve vertices and 20 equilateral triangular faces.
Consists of repeating units of protein (capsomers), which may be made of protomer subunits.
Whole coat is capid.
Surrounds the core of nucleic acid and protein (nucleocapsid core).
Complex - E.g. λ phage.
Large viruses.
Mix of shapes or no sonsisten symmetry.
E.g. Bacteriophages:
- T4 phage infection tail contracts to inject genome into host cell.
Virus uses host cell machinery to replicate.
Describe viral genomes.
Viral genomes vary according to:
Genetic material - nucleic acid (RNA or DNA) and whether they are single or double stranded.
Size - Number of bases long.
DNA:
Double stranded = linear or circular.
Single stranded = circular.
RNA:
Double stranded = several linear segments.
Single stranded = one or several linear fragments.
Always the saem for a gives virus species.
Explain viral replication.
Replication can take minutes (bacteriophages) to hours (animal viruses).
Take over host cell machinery to replicate.
Mode of replication depends on genome type (single stranded or double stranded RNA/DNA).
Steps of replication:
Attachement to cell surface.
Entry of viral nucleic acid (bacteriophage).
Penetration and uncoating (animal viruses).
Viral mRNA transcription -> Viral protein translation.
Replication of genome.
Assembly of progeny virions.
Mturation and release (cells may or may not lyse) - lyses or budds.
How are viruses classified?
Viruses of the same general share all makor features of their life cycles and niches.
Nucleic acid type (strandedness).
Casid symmetry, structure.
Precence or absence of an envelope.
Host range (bacteria, animal, plant etc.).
Disease caused.
Describe the characteristics of Influenza.
Single stranded RNA genome in eight fragments.
Pleomorphic (no fixed structure).
Lipid envelope.
Fragmented genome promotes genetic diversity.
New strains can arise from reassorted genome fragments.
Seasonal epidemics or global pandemics.
Describe the characteristics of smallpox (Variola).
Double stranded linear genome.
Spread by infected cells and by repiratory route.
Lipid envelope.
Highly virulent.
Visible pustules.
Systemic spread.
Vaccinia (cowpox) protects against smallpox.
Describe the characteristics of poliovirus.
Single stranded RNA genome.
Icosahedral capsid, no envelpe.
Causes intestinal infection, usually without sysmptoms.
0.1-2% develop paralytic polio.
Disease targeted by WHO for global erradication.
What are viroids?
Viroids - infectious agents of plants that consist only of RNA.
Smallest infectious agents.
Circular RNA only a few 100 nucleotides long, lack a protein coat.
Do no encode any proteins but can replicate in host cells.
Can cuase regulatory problems.
Cause disease in plants (abnormal development and growth).
What are prions?
Infectious proteins.
Prion protein alters healthy proteins conformation.
Highly resistant to heat - therefore very hard to denature.
Slow acting agents.
Degenerative brain disease in animals.
Spread through ingestion of contaminate mead/feed.
E.g. Mad Cow Disease.
