Virus Classification And Description

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

Size of Viruses Ranges of sizes –20 nm to 500 nm (spherical) –12 nm to 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

Comparison Between Cellular Genome and Viral Genome Viral genome –2-1,200 Kb –Encoded proteins: 2-1,200 –Compact and economical Cellular genome –3x10 9 bp –30,000 proteins –Massive and with 90% non-coding DNA

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

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%)

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

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)

- 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

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

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

Viroids These are plant pathogens They replicate in the absence of helper virus Genome is circular and single stranded 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