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Transcript Part_1-_Viruses

Chapter 24: Viruses
• 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
-nonliving particle made of nucleic
acid surrounded by a protein or
lipoprotein coating
- not made of cells, do not have
cytoplasm, ribosomes, or
-Can replicate only by using host
cell “machinery”
Capsid - protein coat surrounding nucleic acid (DNA or RNA)
– 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
• 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
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 acid
4) Cell divides, and virus nucleic acid is in daughter
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
• 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
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
Disease causing particles even smaller than viruses
– short single strand of RNA without a capsid
- interfere with cell processes and cause new viroids
to be made
- are found only in plants
- 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