Are Viruses Alive?

Origin of the Viruses

Coevolution --- viruses may have evolved along with cells

Retrograde Evolution --- viruses may have come from more complex parasitic life forms through loss of “unneeded”genes (Mimiviruses)

Escaped Genes --- viruses may have originated from genesthat were part of an organism’s genome but becameassociated with an autonomous replicating element(plasmid or transposon).

Viral Diseases:--- can’t be treated with antibiotics--- most dangerous human viral infections are new to humans

Viral Life Cycle:

Attachment

Injection

Gene Expression

Genome replication

Assembly

Release

Virial Quantitation

Agglutination, many animal viruses can cause the aggregationand precipitation of red blood cells

Infectious Units, look for the effects of the virus on host cells

--- mix dilutions of the virus with cells (animal, plant or bacterial), plate, and look for infected cells

--- with bacteriophages the infections usually result in celllysis and are visible as clearing zones on aconfluent plate (Plaque Forming Units, PFU)

--- many eukaryotic viruses don’t cause cell lysis andinfection can be harder to determine

Viral Genomes & Replication

--- compared to other replicating forms most viruses have very small genomes (25 – 250 Kbp)

--- viral genomes are often characterized as sense (+, or coding) and nonsense (-, or noncoding) when single stranded

--- most viral genomes make use of overlapping reading framesto get more use out of their small genomes

To Lysogenic Cycle

In its integrated form the virus is replicated along with the cell’s own DNA.

--- the only way to “kill” an integrated virus is to kill the host cell

HIV, a Retrovirus

--- HIV has a particularlyhigh mutation rate, asmany as 1 milliondifferent viral genomesin each infected person

Even Simpler than Viruses

Viroids: plant pathogens composed of short, naked (no coat)ssRNA’s

Prions: misfolded proteins that can catalyze the misfoldingof other protiens (recruitment)

--- cause diseases that look like viral infections