Viruses Mader-Chapter 21

The basic structure of viruses includes a protein capsid that surrounds and protects the genetic information (genome) that can be either DNA or RNA. Viruses have a mechanism of replication that is dependent on the host metabolic machinery to produce necessary viral components and viral genetic material. Some classes of viruses use RNA without a DNA intermediate; however, retroviruses, such as HIV, use a DNA intermediate for replication of their genetic material. Some viruses introduce variation into the host genetic material. When the host is bacterial, it is referred to as lysogenesis; whereas in eukaryotic cells, this is referred to as transformation. Since viruses use the host metabolic pathways, they experience the same potential as the host for genetic variation that results from DNA Metabolism.

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 and generates genetic variation via various mechanisms. [See also 1.B.3] Evidence of student learning is a demonstrated understanding of each of the following: 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. Evidence of student learning is a demonstrated understanding of each of the following: Viruses transmit DNA or RNA when they infect a host cell. 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.

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

Reproduction of a Retrovirus Retroviruses are RNA animal viruses that have a DNA provirus stage Attachment Entry Reverse transcription Integration Biosynthesis Maturation Release Not all RNA viruses are retroviruses! HIV Life Cycle Movie

Lysis vs. Budding