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