Lecture 4 Flashcards
Independent entities hypothesis
-Evolution on course parallel to that of cellular organisms
-Evolved from primitive, pre-biotic self-replicating molecules
Regressive Evolution
-Viruses degenerated from previously independent life forms
-Lost many functions
-Retain only what they needed for parasitic lifestyle
Cellular origins
-Viruses derived from subcellular functional assemblies of macromolecules that gained the capacity to move from cell to cell.
Co-evolution with host
Advantages, Disadvantages, and Characteristics
-Advantage: Prosperous host means prosperous virus
-Disadvantage: Virus shares same fate as host. Genetic bottleneck events can be fatal
-Typically used by DNA viruses
-Association of a given viral genome sequence with a particular host group or demographic
-Can be used to trace human origins
Two general pathways for virus evolution
Needs Host
Infection of multiple host species & Co-evolution with host
Relationships between viral co-evolution & fitness
-Highly virulent virus will kill the host too soon
-A virus that is too exposed will cause host to kill it
Infection of multiple host species
Advantage, Disadvantage, and where it is usually found
-Advantage: If one host species is compromised, virus can replicate in another
-Disadvantage: Cannot optimize for any one situation
-Typically used by RNA viruses
Mechanisms for viral evolution
-Mutations made by polymerase & other sources
-Recombination
-Reassortment
-High level of replication in infected host
Yeast Killer Virus example
-L-A is a metabolic parasite of host
-M is a parasite of L-a
-M confers a selective advantage on host by encoding a toxin
-Host tolerates L-A to maintain M
-L-A tolerates M to stay in good graces with host
Recombination
Where it is found
All (+) sense RNA viruses and DNA viruses
-Not much in (-) sense RNA viruses
Mutations made by polymerase and other sources
All viruses but greater with RNA viruses
Relationship of mutation and evolution
-Evolution requires mutation
-Mutations occur when nucleic acids are copied
-Error rate of human DNA polymerase is ~10^-9
-Virus RNA and DNA polymerases are much more error prone (around 4-6 fold)
Reassortment
Only segmented viruses
Quasispecies
Virus populations as dynamic distributions of nonidentical but related replicons
High level of replication in infected host
Present in all viruses
Recombination general principles
-Occurs when 2 genetically-distinct viruses co-infect the same cell and interact to generate progeny genomes containing genetic information from both parental viruses
Recombination in DNA viruses
Break-rejoining mechanism that can occur between homologous or non-homologous sequences
Affects of recombination
On host-virus interactions
Important impact on host tropism, pathogenesis, evasion of immune response, etc.
Site specific recombination
-Requires special proteins that recognize specific DNA sequences to promote recombination
-Used by some viruses to integrate into the host chromosome
Homologous recomination
Exchange of genetic information between any pair of related DNA sequences
Recombination in RNA viruses
Template switching between replication. Viral polymerase disassociates with nascent genome and re-associates with another parental genome (Switch driven by homology between 2 RNA strands so RNA made on one strand can bind to a homologous region on a second)
Consequence of retrovirus recombination
If 2 copies are genetically distinct, then recombination during reverse transcription of the (+) ssRNA genome into dsDNA can generate a new virus
Antigenetic Drift
Slow accumulation of mutations in a population. Due to copying errors and immune selection
Antigenetic Shift
A major genetic change caused by recombination or reassortment of genomes
Genetic Drift
Change in frequency of an existing gene variant in the population due to random chance
Founder effect
Reduction in genetic variation that results when a small subset of a large population is used to establish a new colony
Bottleneck
Extreme selective pressure on a small population. Results in loss of diversity and accumulation of non-selected mutations
Fitness
Replicative adaptability of an organism to its environment
Why viral populations maintain stable master or consensus sequences
Diversity limited to ability to function within certain constraints such as…
-Particle geometry
-Genomes composed of nucleic acids limits solutions to replication of decoding of viral information
-Requirement for interactions with host cell geometry
-Requirements for interactions within the host organism
