Step by Step
I-III
Groups I-III — DNA and dsRNA viruses
Group I is dsDNA (herpes, adenovirus, poxvirus). Group II is ssDNA (parvovirus). Group III is dsRNA (rotavirus).
Example: herpesviruses being classified as Group I specifically because they carry double-stranded DNA as their genome.
IV-V
Groups IV-V — positive vs. negative sense ssRNA
Group IV is positive-sense ssRNA (+ssRNA), meaning the genome itself can act directly as mRNA (poliovirus, HCV, rhinovirus). Group V is negative-sense ssRNA (−ssRNA), meaning it needs an RNA-dependent RNA polymerase to first convert it into usable mRNA (influenza, rabies, measles, RSV).
Example: poliovirus's +ssRNA genome being translated directly by host ribosomes immediately upon cell entry, since it already functions as mRNA — no extra conversion step needed.
VI-VII
Groups VI-VII — retroviruses
Group VI is ssRNA retrovirus (HIV), which uses reverse transcriptase to convert its RNA genome into DNA. Group VII is dsDNA retrovirus (HBV), an unusual case that also uses reverse transcription despite having a DNA genome.
Example: HIV (Group VI) using reverse transcriptase to convert its RNA genome into DNA before integrating into the host cell's chromosome.
Applied Walkthrough
1
A virus is found to have a negative-sense ssRNA genome, meaning it cannot be directly translated by host ribosomes.
2
Ask: what does this virus need to do before it can make functional mRNA? It needs an RNA-dependent RNA polymerase to first convert its negative-sense genome into a usable positive-sense mRNA template.
3
This places the virus in Baltimore Group V, alongside other negative-sense RNA viruses like influenza, rabies, measles, and RSV.
4
Contrast: a virus with a positive-sense ssRNA genome (Group IV, like poliovirus or HCV) can skip this extra conversion step entirely, since its genome already functions directly as mRNA once inside the host cell.
Exam Application
Exams test whether you can place a described virus into the correct Baltimore group based on its genome type (dsDNA, ssDNA, dsRNA, +ssRNA, −ssRNA, or retrovirus) and whether you understand the core organizing principle — classification based on how each group ultimately produces mRNA.
⚠ Common Trap
The most common trap is confusing positive-sense (+ssRNA, Group IV) with negative-sense (−ssRNA, Group V) RNA viruses. The key distinguishing question is whether the genome itself can be directly translated as mRNA (positive-sense, no extra step needed) or whether it first requires an RNA-dependent RNA polymerase to create a usable mRNA template (negative-sense, extra step required).
✓ Quick Self-Check
1. What organizing question does the Baltimore classification system answer?
How does the virus make mRNA?
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2. What genome type defines Group I, and give an example virus.
dsDNA; herpesvirus, adenovirus, or poxvirus.
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3. What is the key difference between Group IV (+ssRNA) and Group V (−ssRNA) viruses?
+ssRNA genomes can act directly as mRNA; −ssRNA genomes require an RNA-dependent RNA polymerase to first create usable mRNA.
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4. What genome type and mechanism defines Group VI, and give an example virus?
ssRNA retrovirus, using reverse transcriptase to convert RNA into DNA; HIV is the classic example.
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5. What is unusual about Group VII (like HBV)?
It has a dsDNA genome but still uses reverse transcription, unlike typical DNA viruses.
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