Genome Diversity and Genomic Replication Strategies

Viruses are the only organisms on this planet to still have RNA as their sole genetic material. They are also the only autonomously replicating organisms to have single-stranded DNA. The range of virus genomes as found in virions encompasses:

single-component dsDNA, linear or circular (occasionally circularly permuted linear);
single, double or multi-component circular ssDNA;
single-component linear ssDNA;
single or multi-component dsRNA;
single or multiple component ssRNA genomes which may be
- totally "positive"(or messenger) polarity,
- totally "negative" (or anti-messenger) polarity, or
- partially positive negative-sense;
"diploid" positive-sense ssRNA genomes which replicate via reverse transcription to and transcription from longer-than-genome-length dsDNA, and
nicked and/or partially dsDNAs which replicate via transcription to and reverse transcription from longer-than-genome-length positive-sense ssRNA.

In contrast, prokaryotes have only single-component circular (mainly) or linear (Streptomyces, Helicobacter) dsDNA while all eukaryotes have multi-component dsDNA, and all the genomes replicate via the classic semi-conservative route.

The Baltimore Classification of Viruses According to Their Genome Types and Their Replication Strategies

These various types of virus genomes can be broken down into seven fundamentally different groups, which obviously require different basic strategies for their replication. David Baltimore, who originated the scheme, has given his name to the so-called "Baltimore Classification" of virus genomes.

To understand the classification, one must understand how cells replicate their genomes, and express mRNAs, and proteins. This is conveniently explained by the "Watson-Crick Central Dogma", which states that

INFORMATION FLOW IN CELLS GOES FROM:

DNA TO DNA (REPLICATION)

DNA TO RNA (TRANSCRIPTION)

AND RNA TO PROTEIN (TRANSLATION)

This may conveniently be described in terms of a diagram as shown below:

Briefly, the types are classified by type of nucleic acid in virions (see also genomes.gif), and replication strategy. The types are:

I: dsDNA viruses
II: ssDNA viruses
III: dsRNA viruses
IV: ssRNA viruses with (+)-sense genomes
V: ssRNA viruses with (-)sense genomes
VI: "diploid" ssRNA viruses which replicate via greater-than-genome-length dsDNA
VII: dsDNA viruses which replicate via greater-than-genome-length ssRNA.

(return to Top)

Back to Contents