Download LECTURE 10 Viruses I. Properties of viruses. 1. They are obligate

BIO 120
Field Natural History
Spring
LECTURE 10
Viruses
I. Properties of viruses.
1. They are obligate intracellular parasites.
2. They are incapable of independent metabolism.
3. They are smaller than the tiniest bacteria.
4. They possess one type of nucleic acid, either DNA or RNA.
5. While in the host cell, viruses undergo an “eclipse phase”.
6. Some viruses can be crystallized and still retain their infectivity.
II. Viral Structure.
1. Viral genome- DNA or RNA
2. Capsid is a protein coat that protects nucleic acids.
3. Some viruses have a membranous envelope; derived from host, modified by virus.
4. Protein coats also allow virus to attach and infect a host cell.
5. Viroids - consist of a small, single-stranded circle of RNA unprotected by protein coat.
6. Prions- consist solely of protein, but do not resemble viruses. (Source of ‘mad cow’ disease?)
III. Viral Reproduction.
1. Attachment - virus chemically recognizes and locks on to molecule groups on the cell surface.
a. Capsids function to get the nucleic acid into a susceptible host cell.
2. Penetration - Either the whole virus, or the genetic material alone enters the host cell.
3. Replication and synthesis - Viral genes are transcribed by host cell’s enzymes and ribosomes.
a. Viral nucleic acid also instructs cell to make protein coats.
4. Assembly - Units self-assemble.
5. Release - New viruses cause lysis (breakdown) of host cell, then exit cell, often killing host
cell in the process.
6. In a lysogenic pathway, host is not killed outright. Virus inserts its DNA into host DNA.
a. Virus DNA is replicated along with host DNA when host cell reproduces.
b. A latent signal may activate the host cell at a later time to synthesize new virus particles
and release them.
IV. Virus & Disease – “A virus is a piece of bad news wrapped in protein.” Sir Peter Medawar
1. In animals, viral disease can produce symptoms in a number of ways.
a. Cells are killed by virus exiting cell.
b. Toxins they produce in combating virus kill cells.
c. Fevers, aches and inflammation are the response of the host’s immune system.
d. Dead cells may not be replaced. Nerve cells do not regenerate.
2. Virus are difficult to control by chemical means (antibiotics); they lack any metabolic
machinery.
3. Vaccines mimic viruses, thereby sensitizing the immune system.
4. Prions - are small, altered proteins (a gene product). These infectious proteins cause normally
folded proteins in the brain to become misfolded.
a. Mad cow disease (bovine spongiform encephalopathy) is the result of a prion.
4/06
BIO 120
Field Natural History
Spring
5. Plants can also be infected by viruses - most have no cure. Breeding for resistance.
a. Many plants are infected by viroids- tightly folded strands of RNA with no protein coat.
V. Evolution of Viruses
1. Cannot reproduce independently, but:
a. Have a genome with same genetic code as living organisms.
b. Can mutate and evolve.
2. They depend upon cells for their own propagation; probably evolved after prokaryotes.
3. Modern view- they are fragments of escaped genetic material
a. Genetic material is more like their host than other viruses.
b. Not assigned to any Kingdom.
4/06