1. Virus Classification: The World of Viruses
The major virus groups
The evolutionary origin of viruses

2. 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

3. 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

4. Virus sizes
Fig. 3.1 Size variation of viral capsids.

5. ssDNA Genomes

6. Virus size

7. Genome size

8. The major virus groups
Viruses with double-stranded DNA genomes include the largest known viruses
Fig. 3.2 Size variation of viral genomes.

9. The major virus groups

10. The major virus groups
Most plant viruses and many viruses of vertebrates have positive-strand RNA genomes

11. The major virus groups

12. The major virus groups
All viruses with negative-strand RNA genomes have helical nucleocapsids and some have fragmented genomes

13. The major virus groups

14. The major virus groups
Viruses with double-stranded RNA genomes have fragmented genomes and capsids with icosahedral symmetry

15. The major virus groups

16. Genome lengths

17. The major virus groups
Viruses with a reverse transcription step in their replication cycle can have either RNA or DNA genomes

18. The major virus groups

19. 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

20. The major virus groups

21. 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

22. The evolutionary origin of viruses
Large DNA viruses could have evolved from cellular forms that became obligatory intracellular parasites

23. Key Terms Family Genus (plural, genera) Helper virus Lineage Order
Ribozyme
RNA world
Satellite nucleic acid
Satellite virus
Species
Taxonomy
Viroid