viruses
features of viruses in general:
Viruses differ from rickettsiae in that they all must convert to a non-infective 'eclipse phase' form during replication & that the synthetic processes during replication take place in the host cell cytoplasm & not within the organism itself as in all other classes of organisms.
A single virus particle, called a virion, is made up of a core consisting of a single molecule of nucleic acid, & a surrounding protein shell called a capsid.
Some viruses are also ensheathed in one or more outer membranes, or envelopes, composed predominantly of lipid, derived in part from the host cell or nuclear membrane prior to the release of the virus. Enveloped viruses are vulnerable to fat solvents such as ether or bile salts.
Larger viruses (esp. pox viruses), also contain carbohydratess, co-enzymes & even some enzymes (eg. lipase, catalase, phosphatase & polymerases) but none contain all enzymes necessary for its own metabolism thus all viruses are obligate intracellular parasites.
Plant viruses all contain RNA, whereas nearly all bacteriophages contain DNA.
Most RNA viruses contain a single strand (except reoviruses) whilst most DNA viruses contain a double strand (except parvoviruses).
Transmissibility:
Taxonomy:
Viruses may be classified into 20 mammalian families by mode of replication:
DNA viruses:
Class I:
Double-stranded DNA → RNA polymerase → mRNA
Replicate via DNA polymerase;
Use host RNA polymerase (most):
Have own RNA & DNA polymerase:
Class II: Single-stranded DNA
RNA viruses:
unique as the only organisms that use RNA instead of DNA.
replication is complex & depends on whether contains single- or double- stranded RNA.
RNA that acts as mRNA is termed a positive strand.
Double-stranded RNA viruses:
Class III:
Double-stranded RNA → viral RNA polymerase → mRNA
reoviridae - reovirus, rotavirus, orbivirus (Colorado tick fever)
Single-stranded RNA viruses:
Typical Viral Genome Parts & Their Protein Products:
POL - reverse transcriptase (polymerase)
GAG - group antigen protein
protease
integrase
ribonuclease
ENV - viral coat protein
trans… activation
regulatory parts:
virokines:
esp. DNA viruses;
act on non-infected cells to modulate progress of infection on body as whole:
negate interferons/TNF;
decrease inflammatory response;
block complement pathway;
stimulate metab. activity &/or prolif. nearby cells;
decr. MHC I on cell surface (eg. adenovirus, poxvirus);
NB. DNA viruses tend to have 10-100's of genes whereas most RNA viruses have < 12, thus for vaccinia (DNA virus) 56 out of 198 Open Reading Frames are not required for replication in cultured cells & presumably they confer survival advantage by sabotaging host immune system;
Thus, typical RNA retrovirus genome: