Viruses
Viruses What are viruses Component assembly model of viral replication Lytic and lysogenic cycles DNA Viruses RNA, or Retroviruses Recombination of Viral genetic material in the host cell.
What is a Virus Viruses are generally considered non living but are intimately dependent on a living host for replication Protein capsule with genetic material inside Genetic Material can be DNA or RNA
Component assembly model Enter host Replicate Viral Chromosome Make viral capsule proteins Assemble Rupture cell or bud from cell Infect new host
Lytic vs. Lysogenic Cycles The Lytic cycle: rapid reproduction, can infect many new hosts. Viral genome can obtain mutations via the usual host mechanisms The Lysogenic cycle: the viral DNA integrates into the host. Host can get new traits from the latent virus genome.
DNA Viral Replication
Recombination of Viruses in the Host Related Viruses in the same host can exchange genetic material
2009 Human H1N1
Retrovirus Replication Virus enters host using receptor Reverse transcriptase copies the viral RNA genome into DNA. DNA integrates into the host genome Provirus becomes transcribed and translated for the assembly of new viral progeny.
Rapid Evolution of Retroviruses HIV is a well studied model of the rapid evolution possible in Retroviruses Rapid Evolution leads to pathogenicity of HIV
Evolution of HIV in 9 patients The tree was constructed using the NEIGHBOUR-JOINING METHOD on envelope gene-sequence data that was taken from nine HIV-infected patients48(a total of 1,195 sequences, 822 base pairs in length), with those viruses sampled from each patient depicted by a different colour. In each case, intra-host HIV evolution is characterized by continual immune-driven selection, such that there is a successive selective replacement of strains through time, with relatively little genetic diversity at any time point. By contrast, there is little evidence for positive selection at the population level (bold lines connecting patients), so that multiple lineages are able to coexist at any time point. A major BOTTLENECK is also likely to occur when the virus is transmitted to new hosts.
1. Viruses transmit DNA or RNA when they infect a host cell. 1. Viruses have highly efficient replicative capabilities that allow for rapid evolution and acquisition of new phenotypes. 2. Viruses replicate via a component assembly model allowing one virus to produce many progeny simultaneously via the lytic cycle. 3. Virus replication allows for mutations to occur through usual host pathways. 4. RNA viruses lack replication error-checking mechanisms, and thus have higher rates of mutation. 5. Related viruses can combine/recombine information if they infect the same host cell. 6. HIV is a well-studied system where the rapid evolution of a virus within the host contributes to the pathogenicity of viral infection. b. The reproductive cycles of viruses facilitate transfer of genetic information. 6. Genetic information in retroviruses is a special case and has an alternate flow of information: from RNA to DNA, made possible by reverse transcriptase, an enzyme that copies the viral RNA genome into DNA. This DNA integrates into the host genome and becomes transcribed and translated for the assembly of new viral progeny. [See also 3.C.3] 1. Viruses transmit DNA or RNA when they infect a host cell. This can be observed in the following examples: • Transduction in bacteria • Transposons present in incoming DNA 2. Some viruses are able to integrate into the host DNA and establish a latent (lysogenic) infection. These latent viral genomes can result in new properties for the host such as increased pathogenicity in bacteria.