Viruses Flashcards
Virus Structure
20-300 nm (10^-9)
MUST HAVE: Nucleic acid + protein coat (caspid)
CAN HAVE: viral envelope
Viral genome (nucleic acid)
- Can be DNA (more stable) and RNA (less stable)
- Can be linear or circular
- Can be multiple molecules / or just 1
- 4-1000 genes
2 Lifecycle types (reproductive cycles)
1) Lytic Cycle
2) Lysogenic Cycle
Lytic Cycle
Results in the release of new phages by lysis of the host cell
Lysogenic Cycle
Viral genome becomes incorporated into the host chromosome as a prophage (does not kill host)
- Becomes activated
- can become lytic cell
Prophage
viral DNA integrated in the host chromosome during a lysogenic cycle
- can alter the hosts phenotype
Animal Viruses
- mostly RNA
- most have viral envelope
- Circular, “buds”
Retrovirus
An RNA virus that reproduces by transcribing its RNA into DNA and then inserting the DNA into the chromosome
3 Evolutions of viruses
1) Regressive theory
2) Cellular origin theory
3) Coevolution theory
Regressive theory
Viruses may have once been small cells that parasited larger cells and over time they genes that were not required for the parasitism was lost
- Some bacteria can only reproduce with a host (Chlamydia)
Cellular Origin theory
viruses evolved from bits of DNA / RNA that “escaped from cells”
- Jumping genes (transposons) & plasmids
Coevolution theory
Viruses may have evolved from complex molecules of protein and nucleic acid the same time the first cells appeared from eart
- Viroids
- Prions
Viroids
Molecules of RNA that aren’t viruses because they lack protein coat
Prions
infectious proteins that don’t contain genetic material (infect by converting shape of original protein into the prion shape)
Vertical & Horizontal Transmissions
Vertical = offspring (in utero / colostrum / milk suckling)
Horizontal = Direct/Indirect infections
