Download Viruses - Field Natural History - Lecture Notes and more Study notes History in PDF only on Docsity! Field Natural History 4/06 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. Docsity.com
