--->>~Viruses~<<---
virus-shapes-183x300.jpg
>~ Nature of virus discovered - American biochemist Wendell Stanley.
>~ through electron microscope 1930.
>~ noncelluar particle made up of genetic material and protein that can invade living cells.

Structure:

>~ composed of core nucleic acid surrounded by protein coat called capsid.
>~ nucleic acid core = DNA / RNA - never both
>~ several genes - several hundred.
>~ Bacteriophage = more complex, viruses invade bacteria, hosts multiply quickly.
>~ tail fibers = viruses attaches itself to bacterium.
>~ all shapes = rod-shaped, tadpole-shaped, many-sided helical, cube like.
>~ size = 20 (polio) - 400 nanometers (nanometer one billionth of meter) tobacco virus = 300 nanometers
>~ specific viruses will infect specific organisms. (plant virus cannot infect human)
>~ some infect all animals, some only cold-blooded (body temperature changes with surrounding air)
Various-Viral-Shapes.jpg

Life Cycle:

>~ to reproduce, viruses must invade/infect a living host cell.
>~T4 - invade living cells, causing them to burst or lyse. (lytic viruses)

- - - > >~ Infection ~< < - - -

>~ activated chance of contact with right host.
>~ T4 tail fibers attach to surface - injects DNA into cell - complete virus particle never enters cell.

- - - > >~ Growth ~< < - - -

>~ after enter, goes into action. host cell cannot tell difference between it's DNA and Virus DNA.
>~ RNA polymerase takes RNA from cell of own DNA & virus DNA.
>~ viral RNA = molecular wrecking crew, shutting down and taking over infected cell.
>~ some turn off synthesis.
>~ one viral gene produces enzyme = destroys host DNA, but not viral DNA.

- - - > >~ Replication ~< < - - -

>~ as takes over, materials of host to make thousands of copies of itself.
>~ E coli (found in human intestine) infected by T4 = infection, growth, replication = 25 minutes.
>~ final reproduction stage - DNA molecules staring points, new particles assembled.
>~ infected cell bursts = 100s new virus particles. = lytic infection.
>~ come to town -> eliminate authority -> demands upgrade, new weapons, horses, equipment, terrorizing local merchants and businesspeople -> recruit, form gang -> leave town -> attack new cities.
bacteriophage_life_cycle.jpg

Lysogenic Infection:

>~ does not reproduce and lyse host cell. (not right away)
>~ enters cell, inserted into host DNA. viral DNA = prophage. remain part of DNA for many generations.

- - - > >~ Prophage Activity ~< < - - -

>~ blocks entrance of other viruses, may add useful DNA to host DNA. (lambda virus necessary synthesis of amino acids in DNA of E Coli)
>~ eventually DNA of prophage become active, remove itself from DNA - direct synthesis of new particles.
>~ sudden changes in temperature, availability of nutrients can turn on genes + activate virus.

- - - > >~ Retroviruses ~< < - - -

>~ contain RNA as genetic information.
>~ when infect cell, produce DNA copy of their RNA genes.
>~ backwards RNA to DNA, instead of DNA to RNA.
>~ responsible for some cancer in human/animal & AIDS
images.jpghiv-budding.jpg

Viruses & Living Cells:

>~ depend on living hosts for respiration, nutrition, and all other functions.= parasites.
>~ are not alive, even though they reproduce, regulate expression, evolve. they are not made up of cells, don't live independently
>~ used in gene therapy, possible modified viruses be apart of routine medical tools.

- - - > >~ Origins ~< < - - -

>~ could be like first living things.
>~ completely dependent on other cells for growth and reproduction, cannot live outside host.
>~ may have evolved, over billion years.