Intro to Viruses

• Viruses are non-living particles, considered intracellular obligate parasites and causative agents of infections.
• Investigation into viruses includes, components, structures and different genomes since some contain DNA and some contain RNA.
• This has an effect on the virus life cycle.
• Bacteriophages are viruses that exclusively infect prokaryotic cells.
• By definition, a virus is a non-cellular and non-living infectious agent with its own genome.
• They are considered non-living because they cannot perform metabolic processes or build their own components; they rely on host cells.
• Viruses can infect cells of plants, animals, and prokaryotes.
• Over 5,000 viruses can infect mammals, with an estimated 320,000 more yet to be characterized.
• At least 220 viruses infect human beings.
• Viruses can range in virulence from asymptomatic to deadly, such as Ebola, which causes hemorrhagic fever.
• Virus size is a key distinguishing feature
• The small pox viral particle is 300 nanometers, bacteriophage is 225 nanometers, polio virus, is 30 nanometers in size
• Viruses were once called filterable infectious agents because they are smaller than cells.

Viral Composition

• A single virus particle is called a virion, comprising a capsid and genetic material.
• Each virion has a protective outer layer called a capsid that wraps around the virus's genetic material (DNA or RNA).
• Virus structure dictates the host cell that the virion can infect.
• Viral capsids have three different shapes: helical, icosahedral (20-sided), and complex (asymmetrical, e.g., bacteriophage with a tail).
• Some viruses have a viral envelope, dependent on how the virus replicates.
• Viruses that replicate by budding from the host cell take a portion of the host cell's cytoplasmic membrane with them.
• Virions without an envelope are called naked and replicate by lysing their host cell.
• Bacteriophages specifically infect bacterial cells and replicate by lysing their host cell, resulting in naked virions.
• Picking up plasma membrane adds an extra level of defence against being detected by the body.

Viral Spikes

• Viral spikes are glycoprotein extensions on the capsid or envelope of the virion, which serve to identify and bind to a host cell.
• They allow viruses to enter cells with the right corresponding lock and provide a means for the immune system to identify the virus or viral infected cells.
• Some vaccines mimic spike proteins.
• Viral proteins make up the capsomere - so the capsid.
• Some viruses carry their own viral enzymes.

Viral Genomes

• The human genome has about 25,000 genes and 320 billion base pairs, whereas most viruses are under 300 genes.
• Viral genomes can contain DNA or RNA.
• DNA viral genomes can be circular or linear, and either double-stranded or single-stranded.
• RNA genomes are generally linear, segmented, or in pieces.
• RNA viruses are often single-stranded but can be double-stranded.
• Viruses need a genome for their structure and replication processes, and depend on cells for replication and protein building.
• A cell has double-stranded DNA that gets transcribed into messenger RNA, and then that's translated into a protein by the cell's ribosomes.

Viral Replication

• If a virus has double-stranded DNA, it's transcribed into messenger RNA and then translated into a viral protein.
• If the viral DNA is single-stranded, complementary DNA is produced, then transcription occurs to make messenger RNA, which is then translated into a viral protein.
• For RNA genomes, there are four ways to produce a viral protein:
  • If the virus has single-stranded RNA of a positive or sense stranded type, the RNA can be directly translated into a viral protein (e.g., polio, rubella, West Nile encephalitis).
  • If the virus is single-stranded and antisense (complementary RNA), it has to be transcribed into the sense strand by a viral enzyme called RNA-dependent RNA polymerases so that the ribosomes can read it
  • Retroviruses contain viral enzymes that convert their RNA into DNA, which is then inserted into a host's genome via the enzyme reverse transcriptase (e.g., HIV).
  • Double-stranded RNA viruses need to be unwound to be transcribed by RNA-dependent RNA polymerases (e.g., rotavirus, which causes diarrhea).