Virus Classification: The World of Viruses The major virus groups The evolutionary origin of viruses
Virus classification Many viruses, infecting virtually all known life forms, have been discovered Virus classification is based on molecular architecture, genetic relatedness, and host organism Viruses are grouped into species, genera, and families Distinct naming conventions and classification schemes have developed in different domains of virology
The major virus groups Study of the major groups of viruses leads to understanding of shared characteristics and replication pathways Viruses with single-stranded DNA genomes are small and have few genes
Virus sizes Fig. 3.1 Size variation of viral capsids.
ssDNA Genomes
Virus size
Genome size
The major virus groups Viruses with double-stranded DNA genomes include the largest known viruses Fig. 3.2 Size variation of viral genomes.
The major virus groups
The major virus groups Most plant viruses and many viruses of vertebrates have positive-strand RNA genomes
The major virus groups
The major virus groups All viruses with negative-strand RNA genomes have helical nucleocapsids and some have fragmented genomes
The major virus groups
The major virus groups Viruses with double-stranded RNA genomes have fragmented genomes and capsids with icosahedral symmetry
The major virus groups
Genome lengths
The major virus groups Viruses with a reverse transcription step in their replication cycle can have either RNA or DNA genomes
The major virus groups
The major virus groups Satellite viruses and satellite nucleic acids require a helper virus to replicate Viroids do not code for proteins, but replicate independently of other viruses
The major virus groups
The evolutionary origin of viruses The first steps in the development of life on Earth: the RNA world Viroids and RNA viruses may have originated in the RNA world The transition to the DNA-based world Small and medium-sized DNA viruses could have arisen as independently replicating genetic elements in cells
The evolutionary origin of viruses Large DNA viruses could have evolved from cellular forms that became obligatory intracellular parasites
Key Terms Family Genus (plural, genera) Helper virus Lineage Order Ribozyme RNA world Satellite nucleic acid Satellite virus Species Taxonomy Viroid