Viral Evolution, Morphology, and Classification

Free Response

The first electron micrograph of a virus (tobacco mosaic virus) was produced in 1939. Before that time, how did scientists know that viruses existed if they could not see them? (Hint: Early scientists called viruses “filterable agents.”)


Viruses pass through filters that eliminated all bacteria which were visible in the light microscopes at the time. As the bacteria-free filtrate could still cause infections when given to a healthy organism, this observation demonstrated the existence of very small infectious agents. These agents were later shown to be unrelated to bacteria and were classified as viruses.

Varicella-zoster virus is a double-stranded DNA virus that causes chickenpox. How does its genome structure provide an evolutionary advantage over a single-stranded DNA virus?


Both viruses are made of DNA, but single-stranded DNA viruses lack the ability to create the double helix. Thus, double-stranded DNA viruses have a more stable genome due to the complimentary base pairing, increasing the lifespan of the virus’s genome.

Classify the Rabies virus (a rhabdovirus family member) and HIV-1 with both the Baltimore and genomic structure systems. Compare your results. What conclusions can be made about these two different methods?


Rabies virus is a (-) strand RNA virus that transcribes mRNAs from its genome (Group V).

HIV-1 is a single-stranded RNA retrovirus that uses reverse transcriptase to create a double-stranded DNA copy of its genome which is integrated into the host human’s genome prior to making mRNAs (Group VI).

The genome structure system classifies both viruses as single-stranded RNA viruses with linear genomes.

Baltimore classification sorts Rabies virus and HIV-1 into two different groups, indicating that the two viruses have very different life cycles. However, genome structure classification does not distinguish between the two viruses. This leaves out important information regarding virus function and survival.