INVADERS

Chapter 24: Viruses

Objectives:

• Summarize the discovery of viruses

• Describe why viruses are not considered living organisms

• Describe the basic structure of viruses

• Compare the lytic and lysogenic cycles of virus replication

• Summarize the origin of viruses

Virus-nonliving particle made of nucleic acid surrounded by a protein or lipoprotein coating

- not made of cells, do not have cytoplasm, ribosomes, or mitochondria

-Can replicate only by using host cell “machinery”

Structure:

Capsid - protein coat surrounding nucleic acid (DNA or RNA)

Envelope

– protective coating some viruses have.

- made of lipids, proteins or carbohydrates.

- usually has projections (attachment sites) which attach to the cell they will infect

Classification criteria:

Identification of nucleic acid: RNA or DNA

4 basic shapes –

polyhedral - multifaceted geometric appearance

mosaic (helical) – tightly coiled like a spring

filovirus – no distinct shape; some thread-like

binal – has two distinct regions; capsid and tail

Replication in DNA viruses

1. enters host cell

2. makes mRNA

3. virus inserts its DNA into the host cell’s chromosome (provirus)

4. Host cell transcribes provirus into mRNA and translates RNA into viral proteins

5. New viral particles are assembled

Replication in RNA viruses

• RNA viruses invade and then make proteins

• Retroviruses contain enzyme (reverse transcriptase) to make DNA from RNA

Classification cont.

- Different viruses infect bacteria, plant and animal hosts

- due to receptor site forming to specific proteins on cell wall or membrane

Bacteriophages – viruses that infect bacteria

Lytic Cycle – followed when virus is virulent (active)1. virus particle attaches to a

host cell (ADSORPTION). 2. The particle injects its genetic

instructions (DNA or RNA) into the host (ENTRY).

3. injected genetic material ‘hi-jacks’ the cell’s machinery and recruits the host’s enzymes.

4. enzymes make parts for the new virus particles (REPLICATION).

5. new particles assemble the parts into new viruses (ASSEMBLY).

6. Cell explodes (lyses) releasing new viruses which search for a new host cell (RELEASE).

Lysogenic Cycle - followed when virus is in latent or temperate state

The steps of the lysogenic cycle:1) Viral nucleic acid enters cell 2) Viral nucleic acid attaches to host cell nucleic acid, creating a prophage 3) Host cell enzyme copies viral nucleic acid4) Cell divides, and virus nucleic acid is in daughter cells 5) At any moment when the virus is "triggered", the viral nucleic acid detaches from the host cell's DNA and enters stage 2 of the lytic cycle.

Common symptoms that appear to "trigger" the viral DNA are hormones, high stress levels (adrenaline), and free energy within the infected cell.

Viral Diseases

• Vector – intermediate host that transfers a pathogen or parasite to another organism– Ex: humans, mosquitos, ticks, fleas.

• Human viral diseases - chickenpox, shingles, viral hepatitis, AIDS, etc…

• Protease inhibitors – drugs that block virus reproduction

• Oncogenes – viral genes that cause cancer by messing with cell division checkpoints

• Proto-oncogene – controls cell growth

Prevention and Treatment

Vaccine – weakened sample of virus or virus parts which triggers your body’s immune system

Attenuated virus – weakened virus that cannot cause disease

Inactivated virus – unable to replicate in host

Natural immunity - antibodies are specialized proteins formed in B cells of immune system

- antibodies block attachment sites of viruses

http://www.pbs.org/wgbh/nova/bioterror/vaccines.html

Disease causing particles even smaller than viruses

Viroids

– short single strand of RNA without a capsid

- interfere with cell processes and cause new viroids to be made

- are found only in plants

Prions

- glycoprotein particle

- able to reproduce in mammalian cells

- Prion diseases: mad cow disease; Creutzfeldt-Jakob disease & kuru (degenerative nerve diseases of the brain)

Genetic engineering

Genetic engineers use viruses to carry desirable genes from one cell to another

Improved agriculture – herbicide tolerant soybeans; rot resistant tomatoes; fast growing fish; meatier chickens

Correcting genetic disorders - experimental; only done in animal with success; not allowed in humans

Pest control – insert genes of plants that create chemical to resist insects into crop plants such as corn

Manufacturing of medicine - human insulin &clotting factors 8 & 9

http://www.pbs.org/wgbh/harvest/engineer/transgen.html