Virus Classification and Description Lec. 3


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 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

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


– 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


• 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