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virus=often associated with human, plant, and animal disease (ex: polio, rabies, AIDS, measles, chickenpox, mumps)

Viruses are a biological enigma and have a DNA or RNA genome. They can only reproduce using the host cell’s metabolic machinery. They’re noncellular so they don't fit into the current classification systems

1st virus knowledge was in 1884 by louis pasteur when he suggested that there was something smaller than bacteria causing rabies. In 1892, Dimitir Ivanowsky studied tobacco leaf disease and he saw that even when it was filtered through a fine porcelain filter, the disease was still caused. 

By 1950 virology was an active field of research

Virus size can be compared to a big macromolecule (10-400 nm)

Many viruses can be purified and crystalized and the crystals can be stored like chemicals

Virus anatomy=outer capsid made of protein subunits and an inner core of nucleic acids-either RNA or DNA but not both. Viral genome has 3-100 genes and human cells have tens of thousands. Viral capsid may be surrounded by an outer membrane envelope but if not it’s naked

Viruses are categorized by: type of nucleic acid (single or double stranded), their shape and size, and if they have an outer envelope or not

Viruses are obligate intracellular parasites meaning that they cant reproduce outside a living cell

Viruses are host specific (only live/infect a certain species/host) Ex: bacteriophages infect bacteria, tobacco mosaic infects certain plants, rabies infect only mammals

Some viruses even specialize in a specific tissue. Ex: HIV enters blood cells, polio reproduces in spinal nerve cells, hepatitis infect liver cells

Some scientists hypothesize that viruses are derived from the cells they infect

Viruses mutate and evolve so vaccines may not always be effective (flu mutates a lot)

Viruses are microscopic pirates, taking control of a host cell’s metabolic machinery

Viruses get into a host cell bc portions of the capsid (or spikes of an envelope) adhere in a lock and key manner with a receptor on the host cell’s surface. Then the nucleic acid enters the cell and codes for the protein units in the capsid. 

A virus relies on a host’s enzymes, ribosomes, tRNA, and ATP for reproducing

Bacteriophages=viruses that parasitize bacteria

2 types of bacteriophage life cycles: lytic and lysogenic

Lytic cycle=viral reproduction occurs and the host undergoes lysis (breaking open of cell to release viral particles)

Lysogenic cycle=viral reproduction doesn’t immediately happen but may in the future

Virulent=undergoes both lytic and lysogenic cycles

Lytic cycle stages: attachment (parts of the capsid combine with a receptor on the rigid bacterial cell wall in lock and key manner), penetration (viral enzyme digests away part of the cell wall and viral DNA is injected into the bacterial cell), biosynthesis (virus takes over machinery in order to carry out viral DNA replication and production of multiple copies of the capsid protein subunits), maturation (viral DNA and capsids assemble to produce many viral particles and lysozyme is procured which disrupts the cell wall), release (new viruses release)

Lysogenic cycle: bacterium doesn’t immediately produce phage so meantime, the phage is latent (not replicating). After attachment and penetration, integration occurs (viral DNA incorporates into bacterial DNA with no destruction of host DNA). While latent, the viral DNA is called a prophage and is replicated along with the host DNA and all lysogenic cells carry a copy of prophage. Then it may reenter the lytic cycle followed by maturation and release

Animal viruses reproduce similar to bacteriophages. Once it enters the host cell, uncoating releases viral DNA or RNA and reproduction occurs. During release by budding, the viral particle acquires a membranous envelope

Retroviruses=RNA animal viruses that have a DNA stage

A retrovirus contains an enzyme called reverse transcriptase which carries out RNA->cDNA. The DNA is called cDNA bc its a copy of the viral genome. A resulting double-stranded DNA is then integrated into the host genome and it replicates when the host replicates.

Emerging viruses=diseases caused by viruses that only recently have caused widespread diseases in humans (ex: AIDS, West Nile encephalitis, HPS, SARS, ebola, bird flu)

Viruses may “emerge” when they’re transported form 1 part of the world to another 

Viruses are known for their high mutation rates. Sometimes viruses can “jump” species and start infecting humans bc of a change in their spikes

viroids=naked strands of RNA (not covered by a capsid). Viroids direct the cell to produce more viroids

Prions=term coined for proteinaceous infectious particles

Prions were discovered when members of a Papua New Guinea tribe died from a disease called kuru after eating a deceased person’s brain. The causative agent was smaller than a virus-it was a rogue protein

Prion diseases: mad cow disease, CJD, chronic wasting syndromes in animals, etc