1. Virus Classification And Description

2. Classification Parameters • Several Parameters Are Used for Classification • Viral classification study is referred to as Taxonomy • 73 families exist so far!! • Type of genomic nucleic acid • Size of virion and genome • Capsid structure • Host • Replication mechanism

4. Size of Viruses • Ranges of sizes • 20 nm to 500 nm (spherical) • 12 nm to 300-2000 nm (rod like) • Easily observed with electron microscope • Ex.1 Mimivirus is 500 nm • Infects algae • Ex.2 Parvovirus is 20 nm in diameter • Infects algae • Viral genomes range in size 2,000 bp to 1,200,000 bp

5. Comparison Between Cellular Genome and Viral Genome • Viral genome • 2-1,200 Kb • Encoded proteins: 2-1,200 • Compact and economical • Cellular genome • 3x109 bp • 30,000 proteins • Massive and with 90% non-coding DNA

6. ssDNA Viruses • ssDNA Viruses have the following characteristics • Small genome, 2-7 Kb • Possibly due to unstable nature of ssDNA compared to dsDNA • Circular genomes with the exception of Parvoviridae (hairpin) • No envelope • Predominantly icosahedral capsids

7. dsDNA Viruses • Examples of dsDNA viruses that infect humans • HSV, HPV and adenoviruses • Among the largest known viruses • Genome size varies from 5 to 1180 Kb • Unfragmented genomes • Both linear and circular • Large genome size attributed to stability of dsDNA • Low error rate during replication • No dsDNA virus is known to infect plants • Phages are dsDNA viruses (95%)

8. dsRNA Viruses • They utilize RNA dependent polymerase • Icosahedral capsids • Capsids stays intact inside cell. Why?Genome protection. • Transcription occurs via viral RNA polymerases • Reoviruses (dsRNA) are capable of infecting multiple species (plants, vertebrates, fungi). Not a common phenomenon. • Rhabhoviridae infect multiple species as well • The fact that they carry their own RNA replication/transcription proteins makes them more adept

9. Viruses With + strand RNA Genomes • Very common of plant viruses to be + ssRNA • Only one phage family is + ssRNA • RNA viruses have linear genomes • Similar to ssDNA viruses they are susceptible to nucleases and divalant cation degradation • Coronavirus has the largest genome of + ssRNA virus (16-30 Kb)

10. - ssRNA Viruses • This group includes some of the deadliest viruses • Ebola, rabies, influenza, measles • Only helical nucleocapsids • Nucleocapsid seems to provide stability for RNA dependent RNA polymerase to generate + ssRNA • + ssRNA=mRNA

11. Viruses With Reverse Transcription • 3 families belong to this group • Retroviridae, Ex. HIV • Hepadnaviridae, Ex. Hep B • Caulimoviridae, Ex. Cauliflower Mosaic Virus • These families utilize enzyme that uses an RNA template to make DNA template • Reverse transcriptase is packaged in capsid • Similar to + ssRNA and – ssRNA that package the RNA dependent polymerase • Retroviruses package 2 copies of their RNA genome in the capsid

12. Satellite Viruses/Nucleic Acids • These viruses require a helper virus • Their genomes encode for capsid proteins • Nucleic acid satellites are either non-coding or encode for non-capsid proteins • Mostly a plant phenomenon • In humans the Hep  virus resembles characteristics of satellite virus/viroid

13. Viroids • These are plant pathogens • They replicate in the absence of helper virus • Genome is circular and single stranded • 250-400 nt • Cellular DNA dependent RNA polymerases are used for replication • High degree of internal base pairing • RNA behaves as enzyme • i.e cut/ligate themselves • Ribozymes