A Planet of Viruses

by Carl Zimmer

But Stanley had also made a small but profound mistake. The British scientists Norman Pirie and Fred Bawden discovered in 1936 that viruses were not pure protein, but only 95 percent. The other 5 percent consisted of another molecule, a mysterious strand-shaped substance called nucleic acid. Nucleic acids, scientists would later discover, are the stuff of genes, the instructions for building proteins and other molecules. Our cells store their genes in double-stranded nucleic acids, known as deoxyribonucleic acid, or DNA for short. Many viruses have DNA-based genes as well. Other viruses, such as tobacco mosaic virus, have a single-stranded form of nucleic acids, called ribonucleic acid, or RNA.