Baltimore system of Classifications of Viruses

By Acharya Tankeshwar •  Updated: 03/12/22 •  4 min read

The Baltimore system of virus classification devised by virologist & Nobel laureate David Baltimore is based on the genomic nature of the viruses. The central theme of the Baltimore system of virus classification is all viruses must synthesize positive-strand mRNAs from their genomes, in order to produce proteins and replicate themselves. The precise mechanisms whereby this is achieved differ for each virus family.

Central Dogma of Molecular Biology

Why + mRNA is placed at the center?

Baltimore classification system is based on the central role of translational machinery and placed mRNA in the center, and described the pathways to form mRNA from DNA or RNA genomes. Viruses can replicate DNA and/or RNA, synthesize RNA from DNA or vice versa, but lack a complete system to make proteins, for which they have to rely on host cell ribosomes. Host cells on the other hand can synthesize proteins only from +mRNA strands. Irrespective of the genomic nature of viruses, all viruses must synthesize viral + mRNAs to produce viral proteins”no exception to date“.

How to designate positive (+) and/or negative (−) strands?

The mRNA which can serve as the template for protein synthesis is defined as a positive (+) strand {by convention (nothing to do with charge/electricity)}. A strand of DNA of the equivalent polarity is also called the (+) strand. RNA and DNA strands that are complementary to the (+) strands are called negative (−) strands.

Viral genomes can be broken down into seven fundamentally different groups, which obviously require different basic strategies for their replication depending on the nature of their genome. When originally conceived, the Baltimore scheme encompassed six classes of viral genome. Subsequently, the gapped DNA genome of hepadnaviruses (e.g. hepatitis B virus) was discovered and incorporated in a new scheme.

Viruses can be classified into seven (arbitrary) groups:

Baltimore System of Classification of Viruses
Baltimore System of Classification of Viruses

I.    Double-stranded DNA viruses

Some replicate in the nucleus e.g adenoviruses using cellular proteins. Poxviruses replicate in the cytoplasm and make their own enzymes for nucleic acid replication. e.g.   Adenoviruses; Herpesviruses; Poxviruses, etc

II. Single-stranded (+) sense DNA viruses 

Replication occurs in the nucleus, involving the formation of a (-) sense strand, which serves as a template for (+) strand RNA and DNA synthesis. e.g., Parvoviruses

III. Double-stranded RNA viruses

These viruses have segmented genomes. Each genome segment is transcribed separately to produce monocistronic mRNAs. e.g.,  Reoviruses

IV.  Single-stranded (+) sense RNA viruses  (Picornaviruses; Togaviruses, etc)

V. Single-stranded (-) sense RNA viruses 

The virion RNA is a negative sense (complementary to mRNA) and must, therefore, be copied into the complementary plus-sense mRNA to make proteins. This group of viruses must code for RNA-dependent RNA-polymerase and also carry it in the virion so that they can make mRNAs upon infecting the cell. e.g. Orthomyxoviruses, Rhabdoviruses, etc

VI.  Single-stranded (+) sense RNA viruses with DNA intermediate in life-cycle 

RNA genome is (+) sense but unique among viruses in that it is DIPLOID, and does not serve as mRNA, but as a template for reverse transcription. e.g. Retroviruses.

Retroviruses, therefore, encodes an RNA-dependent DNA polymerase (reverse transcriptase) to make the DNA provirus which then is transcribed to genomic RNA by a host enzyme, RNA polymerase II.

VII.  Double-stranded DNA viruses  with RNA intermediate 

This group of viruses also relies on reverse transcription, but unlike the Retroviruses, this occurs inside the virus particle on maturation. On infection of a new cell, the first event to occur is a repair of the gapped genome, followed by a transcription. e.g., Hepadnaviruses

Watch video explanation about Baltimore scheme by renowned virologist Vincent Racaniello


  1. The elegance of the Baltimore system is “knowing only the nature of the viral genome, one can deduce the basic steps that must take place to produce mRNA”*.

Further Readings

  1. Expression of animal virus genomes by D. Baltimore
  2. * Simplifying virus classification: The Baltimore system by Vincent Racaniello

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please email at

4 responses to “Baltimore system of Classifications of Viruses”

  1. Usman says:

    Please shed more light on positive and negative strand viruses thank you

  2. Danielle Flower says:

    Are there viruses that can cause human disease other than Orthohepadnavirus, and Anelloviridae under the class VII?

  3. DR. Md. Mostofa Kamal DVM says:

    Please give me some idea about primer designing of infectious laryngo tracheitis virus. e-mail:

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