The Baltimore Scheme

What is the Baltimore scheme of virus classification?

The Baltimore Scheme categorises viruses based on the nature of their genome (DNA or RNA, single/ double stranded) and how they produce protein

What genome type are Class I viruses?

Double stranded DNA

What genome type are Class II viruses?

Single stranded DNA

What genome type are Class III viruses?

Double stranded RNA

What genome type are Class IV viruses?

Single stranded RNA (positive sense)

What genome type are Class V viruses?

Single stranded RNA (negative sense)

What genome type are Class VI viruses?

Single stranded RNA with DNA intermediate (retroviruses)

What genome type are Class VII viruses?

Double stranded DNA with RNA intermediate

Which virus classes have a DNA genome?

Class I

Class II

Class VII?

Which virus classes have a RNA genome?

Class III

Class IV

Class V

Class VI

Class VII?

Name examples of Class I viruses

Adenoviruses

Herpesviruses

Poxviruses

Varicella zoster virus (VZV)

Human papilloma virus (HPV)

Name examples of Class II viruses

Parvovirus (only human pathogen in this group)

Some phage

Some archaeal viruses

Name examples of Class III viruses

Rotaviruses

Reoviruses

Birnaviruses (animal virus)

Name examples of Class IV viruses

Picornaviruses (e.g. Polio virus)

Coronaviruses

Caliciviruses

Name examples of Class V viruses

Influenza virus

Rabies virus

Paramyxoviruses (e.g. RSV)

Filoviruses (e.g. Ebola)

Name examples of Class VI viruses

Retroviruses

e.g. HIV, HTLV

Name examples of Class VII viruses

Hepatitis B virus

(reassignment of some Class I viruses - reversiviruses) 

Which class is this?

Class I

Which class is this?

Class II

Which class is this?

Class III

Which class is this?

Class IV

Which class is this?

Class V and VI

Which class is this?

Class VII

Name 3 other virus types

Satellite viruses

Viroid

Prions 

Describe satellite viruses

• Cannot replicate alone - do not encode enzymes but their genomes encode structural proteins

• Require a helper virus to provide enzymes

• E.g. Hepatitis D virus requires co-infection of Hepatitis B virus

Describe viroids

• Naked, self-replicating ssRNA molecules

• No known protein component

• Only in plants so far

Describe Prions

• Infectious proteins

  • Misfolded proteins that induce misfolding in normal variants of the same protein leading to cellular death → neurodegenerative disease

• No (known) nucleic acid component 

Where to DNA viruses replicate?

Nucleus

Where do RNA viruses replicate?

Cytoplasm

How do Class I viruses produce protein?

• Central dogma

• Uses host or viral DNA-dependent RNA polymerase to transcribe mRNA from a DNA template 

How do Class II viruses produce protein?

• ssDNA → dsDNA intermediate → mRNA transcribed by host RNA polymerase

• Synthesise dsDNA in first step of replication cycle then proceed in same manner as Class I viruses

How do Class III viruses produce protein?

Viral RNA-dependent RNA polymerase (RdRp) transcribes mRNA from each RNA strand

(Genome cannot serve as mRNA)

How do Class IV viruses produce protein?

• Genome acts directly as mRNA

• directly translated into protein but dsRNA is also used as a template

• (DNA not involved in the process)

How do Class V viruses produce protein?

• Viral RNA-dependent RNA polymerase makes complementary positive-sense RNA (mRNA)

• (Genome is -ssRNA - complementary to mRNA but cannot be directly translated)

How do Class VI viruses produce protein?

• Viral reverse transcriptase converts RNA → DNA → integration into host genome → host RNA polymerase II produces mRNA

• (reverse transcription - makes DNA from RNA with RNA genome) 

How do Class VII viruses produce protein?

• Gapped dsDNA → repaired to closed circular DNA → transcribed to RNA → reverse transcribed back to DNA during virion assembly

• (Makes DNA from RNA with DNA genome)