 Viruses (2) Biology 11 Mr. McCallum

What do viruses do?  Replicate, replicate, replicate!  Harmful as this leads to the death of the host cell  Attachment: Attaching to cell specific « structures » on the cell’s surface via specific « structures » on the virus’ surface  Penetration: Entire virus enters the host cell, OR the genetic material is injected into the cell  *ultimate result of viral infection – exposure of the virus’ genetic material inside the host cell

What do viruses do? (continued)  Transcription and Replication: Virus takes over the machinery of the cell, making nothing but viral parts  Assembly: Viral parts being produced are assembled into complete viruses  Release: Exit cell through « budding » - few viruses at a time leaving the cell, OR through lysis – cellular membrane ruptures, releasing all viral particles at once

Infection  Everything living that we know today is susceptible to viral infection.  Plants, animals, bacteria  Multi-cellular, single-celled  Specificity  Smallpox (humans), influenza (humans and a few other animals), tobacco mosaic virus (particular plants), lambda bacteriophage (E. coli)

Viral Taxonomy  Variety of shapes, sizes, and organization of basic genetic material  Arrangement and type of genetic material used for sub-classification  ALL: Capable of replicating within a living cell and can produce offspring that are usually identical to the original virus

Mutation  During replication, mutation can occur.  Can render virus no longer functional  Volume of offspring negates these effects  Mutations can benefit the virus  New strains (Influenza virus)  Difficult for vaccinations

Protection  When viruses infect our cells, our body responds by creating and secreting inteferons  Inteferons: Proteins that interact with adjacent cells to aid in their resistance to viral infection  If this is not enough, we experience disease  Our immune system targets and kills cells infected by viruses  They need a living host to replicate!

Protection (2)  Usually, our immune system removes the virus  HIV is an exception – they use immune cells as their living hosts  Agents isolated from natural sources are capable of killing a virus  Acyclovir – inhibits Herpesvirus replication  AZT and HIV protease inhibitor inhibit HIV replication  Plants have substances that coat leaves and stems, closing off systems – walling off the infection  Bacteria use enzymes within their cells  Bacteriophages often suceed at killing bacterial cells in short time frames

Viral Infection - Lytic  Results in the destruction of the infected cell and its membrane (cell lyses)  Viral DNA exists separately, and is replicated independently from the host cell DNA  Many viruses produced and released into the environment simultaneously

Viral Infection – Lysogenic  Does not lyse the host cell, immediately  Integration of viral DNA into the host genome  Transmitted to daughter cells at each division  Remain “dormant” – host living normally until later event initiates viral release  Changes in temperature, available nutrients, UV raditation, certain chemicals, etc.

Lysogenic and Lytic Cycles