Virology: Virus Structure and Function

Viruses

• Viruses include H1N1, Spanish Influenza, Influenza, SARS, MERS, SARS-CoV2, Measles, HPV, Polio, and Small Pox.
• The first virus discovered was the Tobacco Mosaic Virus.
• Viruses are obligate intracellular parasites.
• Viruses cannot make proteins.
• Viruses can only reproduce in a host cell.
• Viruses rely on the host to replicate and survive.
• Host ranges vary depending on the virus, with recognition systems being key.
• Viral surface proteins and host cell receptors are key elements of recognition systems.

Virus Structure

• Viruses are small, with the smallest being 20 nm in diameter.
• The protein shell of a virus is called a capsid.
• The nuclear material of a virus can be either RNA or DNA.
• Some viruses have a viral envelope surrounding the capsid.
• Viral envelopes consist of host cell membranes and viral glycoproteins.
• Bacteriophages are the most complex viruses.
• Bacteriophages have icosahedral capsid heads and protein tails for attachment.

Viral Replication

• A virus cannot reproduce outside of a host cell
• Simple viral reproduction involves entry and uncoating, replication, transcription and manufacture of capsid proteins, and self-assembly and exit from the cell.
• The lytic cycle converts a cell into a virus factory.
• The lysogenic cycle incorporates the virus genome into the cell genome and allows replication without destroying the host.

Temperate Viruses

• Temperate viruses alternate between the lytic and lysogenic cycles, remaining dormant for extended periods.
• Temperate viruses can reactivate and reproduce.
• Various signals, such as environmental conditions, chemicals, and high-energy radiation, can trigger a switch between cycles.

Bacterial Defenses

• Bacterial defenses include different surface receptors on bacterial mutants.
• Restriction enzymes are a defense mechanism.
• The CRISPR/Cas system is a defense.

Restriction Enzymes

• Restriction enzymes protect against bacteriophages.
• These enzymes act as 'scissors' to cut up viral DNA.
• They restrict the ability of the phage to infect bacterial cells through methylation of bacterial DNA.
• Natural selection can favour phage mutants that resist restriction enzymes.
• Evolutionary flux is constant.

CRISPR/Cas System

• The CRISPR/Cas system develops a 'memory' of infection.

Animal Viruses

• Animal viruses are surrounded by a viral envelope.
• Animal viruses enable entry to the host cell.
• Animal viruses reroute the cell's resources to produce more viruses.
• Animal viruses bud off from the cell without destroying it.
• Provirus is a latent virus.
• Animal viruses produce symptoms by damaging or killing cells via lytic enzymes from lysosomes or toxins in the envelope proteins.

Animal Viruses vs. Bacteriophages

• Bacteriophages are typically DNA viruses.
• Animal viruses have several classes.
• Animal viruses sometimes have envelopes (RNA viruses and some dsDNA).
• Animal viruses can have high rates of mutation without proofreading, which leads to epidemics and pandemics.

Retroviruses

• Retroviruses use ssRNA as a template for DNA synthesis.
• They contain two identical strands of ssRNA.
• They contain reverse transcriptase.
• They contain an envelope and glycoproteins
• Retroviruses use DNA which is produced from RNA.
• Then new viruses are synthesized from DNA produced.

Reverse Transcriptase

• Reverse Transcriptase uses viral RNA as a template for DNA synthesis.

Animal Viruses: Cancers

• Viruses are responsible for 21-40% of human cancers.

Vaccines

• Vaccines use harmless variants of a pathogen to stimulate the immune system.
• Vaccines stimulate the immune system to defend against the pathogen.
• Antivirals differ to antibiotics https://www.weforum.org/agenda/2019/04/vaccines-work-vaccinations-healthcare-medicine-world-immunization-week-health-organization/

Viruses in Animals

• Antibiotics kill bacteria but have no effect on virally encoded enzymes must use antiviral drugs
• Most interfere with viral nucleic acid synthesis

Prions

• Prions are infectious proteins.
• They cause degenerative brain diseases in animals.
• Prions are transmitted through food.
• Prions act very slowly, with an incubation period of 10 years.
• Prions cause transmissible spongiform encephalopathy.
• Mad Cow Disease is an example of a prion disease.
• Creutzfeldt-Jakob Disease is another example of a prion disease.
• Scrapie in Sheep is also an example of a prion disease.
• Chronic Wasting Disease in Cervids example of a prion disease.
• Prions are transmissible between species.
• The blood-brain barrier helps prevent transmission, but not always.
• Prions are a misfolded form of protein.
• When a prion enters a cell, it converts normal proteins into misfolded proteins.
• Prions aggregate interfere with normal cell functioning.

Chronic Wasting Disease