Viruses

Mader-Chapter 21

The basic structure of viruses includes a protein capsid that surroundsand protects the genetic information (genome) that can be either DNAor RNA. Viruses have a mechanism of replication that is dependent onthe host metabolic machinery to produce necessary viral components andviral genetic material. Some classes of viruses use RNA without a DNAintermediate; however, retroviruses, such as HIV, use a DNA intermediatefor replication of their genetic material. Some viruses introduce variationinto the host genetic material. When the host is bacterial, it is referred to aslysogenesis; whereas in eukaryotic cells, this is referred to as transformation.Since viruses use the host metabolic pathways, they experience thesame potential as the host for genetic variation that results from DNAMetabolism.

Essential knowledge 3.C.3: Viral replication results in genetic variation,and viral infection can introduce genetic variation into the hosts.a. Viral replication differs from other reproductive strategies andgenerates genetic variation via various mechanisms. [See also 1.B.3]Evidence of student learning is a demonstrated understanding of eachof the following:1. Viruses have highly efficient replicative capabilities that allowfor rapid evolution and acquisition of new phenotypes.2. Viruses replicate via a component assembly model allowingone virus to produce many progeny simultaneously via the lyticcycle.3. Virus replication allows for mutations to occur through usualhost pathways.4. RNA viruses lack replication error-checking mechanisms, andthus have higher rates of mutation.5. Related viruses can combine/recombine information if theyinfect the same host cell.6. HIV is a well-studied system where the rapid evolution of avirus within the host contributes to the pathogenicity of viralinfection.

b. The reproductive cycles of viruses facilitate transfer of geneticinformation.Evidence of student learning is a demonstrated understanding of eachof the following:1. Viruses transmit DNA or RNA when they infect a host cell.2. Some viruses are able to integrate into the host DNA andestablish a latent (lysogenic) infection. These latent viralgenomes can result in new properties for the host such asincreased pathogenicity in bacteria.

How are viruses different from bacteria?

… Bottom line, viruses are not alive!

Size

Viral Structure

• Vary in shape from threadlike to polyhedral but possess the same basic anatomy: – An outer capsid – Inner core of DNA or RNA– Various proteins such as polymerases– Possibly an outer membranous envelope in animal viruses

Parasitic Nature

• Obligate intracellular parasite• Host specific

Viral Replication

• Most DNA viruses enter the nucleus to initiate replication

• Some DNA virtues transform the host cell by integrating into the host DNA

• Few DNA viruses replicate in the cytoplasm and therefore must carry their own mRNA and DNA synthetic machinery

• RNA viruses replicate in the cytoplasm• Retroviruses carry the enzymes reverse transcriptase

and integrase

Viral ReproductionWhen the viral DNA is integrated into the host cell as a prophage, binary fission duplicates the Viral genome and may introduce mutations in the process

Also, transduction may occur, where the virus picks up some host cell DNA and carries it to another bacterial cell

Reproduction of a Retrovirus

Retroviruses are RNA animal viruses that have a DNA provirus stage1. Attachment2. Entry3. Reverse transcription4. Integration5. Biosynthesis6. Maturation7. Release

Not all RNA viruses are retroviruses!

HIV Life Cycle Movie

Lysis vs. Budding