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VIRUS GENETIC AND EVOLUTION (Viral Evolution (Importance (capacity to…
VIRUS GENETIC AND EVOLUTION
3 Hypothesis origination virus
Evolved with other forms of life at beginning of life
viruses evolved from components of cells that gain ability to replicate and spread to other cells
Evolved from other intercellular parasites
Diff virus cause diff harm effect
Depend on a number of factor
way of virus transmitted has large effect on level of virulence evolve
Vertical transmission (transmission to offspring of the host)
Viral mutation
viral nucleid acid very susceptible to mutations
loss or acquisition of pathogenicity
development of mechanism that allow virus to escape host immune system
Viral Evolution
Virus evolution fast in RNA viruses
lack of proofreading in RNA polymerases
Fast generation time
high rate of productivity (abundant progeny)
high rate of mutation
Virus-infected cell produce large no. progeny
infection of a single cells by poliovirus can yield up to 104 viral particleds
in person up to 109-1011 virus particles can produced per day
Cause of rapid evolution
Evo. act as variation
high mutation rates
produce numerous offspring
short generation times
Importance
capacity to replicate rapidly
capacity to replicate to higher titer
capacity to replicate in certain key tissue
capacity to be shed for long period of time
capacity to elude host defences
capacity to survive after being shed into the external environment
capacity to be transmitted vertically
Charac. of viral mutation
Rate of occurrence - occur spontaneously with average rate of 10-8 particles. this error occur during reverse transcription (RNA viruses) that are not corrected. It is lack of Proof-editing ability
Mechanism of virus Evolution
Mutation
Recombination
Reassortment
Complementation
Mutation
Point mutation (Genetic drift)
Single nucleotide base is changed
Charac. as 1. conditioned lethal mutants
temperature-sensitive mutants
do not replicate well at 36-41 C
attenuated strains in human-can be used as vaccine
Host-dependent mutants
Plaque-size mutant
induce changes in diameter of viral lysis zone seen in infected monolayer cells
effect capsid protein usually
easier absorption and penetration to permisive cells
larger plaque-size - grow faster and more virulent in vivo
Host-range mutants
infect cells that usually not infected by wild type virus
mutation involve protein that mediates absorption to host cells
Drug-resistant mutants
make the virus not susceptible to normally used antiviral agents
mutation usually affect enzyme that inhibit by an antiviral drug
enzyme lose affinity to antiviral drug and allowing the virus to replicate in the presence of antiviral drugs
Enzyme-deficient mutant
strain that lack of an enzyme / have mutant enzyme
may or may not be lethal- depending whether or not it effect an enzyme essential for replication
hot-mutant
grow well at 41C & extremely virulent
Deletion mutants
whole genome segment is lost Eg, Defective virus particle
Recombination
DNA viruses
involve the dsDNA viruses.
recombination result in creation of complete wild-type genome
RNA viruses
copy of +ve strands into -ve strands used as template for replication
the polymerase may jump from one =ve strands to another-producing hybrid DNA template
eg, Generation HIV strains with diverse genetic materials when two difference mutants strain of HIV co-infect the same cell-emergence of recombinant genome of HIV
2 types
Self recombinant
occur when two viral genome recombine by homologous crossing over
Happen for DNA viruses and common in prokaryotic viruses and RNA viruses or retroviruses
Can be evolution advantage for virus when it helps to evolve host immune defence, eg changing of protein antigenicity surface
recombinantion with host
occur when viral genome re-combine to acquire sequence from another organism
natural selection can retain acquired sequence if it gives an evolutionary advantage to virus and mutation can modify its original function.
common in large dsDNA viruses, some eukaryotic viruses even acquired many genes from bacteria