Other Autonomous or Semi

Other Autonomous or Semi-Autonomously Replicating Genomes

There are a number of types of genomes which have some sort of independence from cellular genomes: these include "retrons" or retrotransposable elements, bacterial and fungal (and eukaryotic organelle) plasmids, satellite nucleic acids and satellite viruses which depend on helper viruses for replication, and viroids. A new class of agents - PRIONS - appear to be "proteinaceous infectious agents".

Retrons	Plasmids	Satellite Nucleic Acids
Satellite Viruses	Viroids	Prions

http://microbiologybytes.wordpress.com/2008/05/21/what-the-heck-are-prions-for/

What the heck are prions for? « MicrobiologyBytes via kwout

Plasmids
Plasmids may share a number of properties with viral genomes - including modes of replication, as in ss circular DNA plasmids and viruses - but are not pathogenic to their host organisms, and are transferred by conjugation between cells rather than by free extracellular particles.

Satellite Nucleic Acids
Certain viruses have associated with them nucleic acids that are dispensable in that they are not part of the genome, which have no (or very little) sequence similarity with the viral genome, yet depend on the virus for replication, and are encapsidated by the virus. These are mainly associated with plant viruses and are generally ssRNA, both linear and circular - however, a circular ssDNA satellite of a plant geminivirus has recently been found.

Satellite Viruses
There are also viruses which depend for their replication on HELPER VIRUSES: a good example is tobacco necrosis satellite virus (sTNV), which has a small piece of ssRNA which codes only for a capsid protein, and depends for its replication on the presence of TNV. Another good example is the hepatitis delta agent with its circular ssRNA genome. The adeno-associated viruses (AAVs) are also satellite viruses dependent on the linear dsDNA adenoviruses for replication, but which have linear ssDNA genomes and appear to be degenerate or defective parvoviruses.

Viroids
Viroids are small naked circular ssRNA genomes which appear rodlike under the EM, which are capable of causing diseases in plants. They code for nothing but their own structure, and are presumed to replicate by somehow interacting with host RNA polymerase, and to cause pathogenic effects by interfering with host DNA/RNA metabolism and/or transcription. A structurally similar disease agent in humans is the hepatitis B virus-dependent hepatitis delta agent, which additionally codes for a structural protein.

From Wikipedia: Putative secondary structure of the PSTV (potato spindle tuber viroid).
black - secondary structure of the viroid red - GAAAC sequence common to all viroids yellow - central conservative sequence

Author: Jakub Friedl (user kyknos), 2005

Back to "What is a Virus"?