528 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
There is an important similarity between the structure of a protein 
and that of a nucleic acid. Both have a specific linear sequence of 
different units on a long chain,> and both have a helical structure. 
Nucleic acids contain mainly 4 different kinds of bases; proteins 
contain mainly 20 different kinds of amino acids. The sequence of 
these units (adenine, guanine, thymine, and cytosine) determines the 
specificity of the nucleic acids, just as the particular sequence of let- 
ters specifies a word. In addition, it has been assumed by some that 
the sequence of the bases in the DNA-RNA chain determines the 
amino acids along the protein chain. 
Thus far the reader may not see the tremendous significance of these 
“basic science” observations. However, consider sickle-cell anemia. 
This disease is due to an abnormal hemoglobin which harmfully 
affects the properties of the red cell. Very recently, Ingram (7) dis- 
covered that the abnormality consists only in the replacement of one 
glutamic acid residue by a valine—one of about 300 such amino-acid 
molecules in one of two identical halves of the hemoglobin molecule. 
This seemingly minor chemical change suggests a single mutation in 
the hemoglobin gene. 
How much more lies just ahead in what is molecular biology today 
and medicine tomorrow ? 
VIRUSES AND CANCER 
That viruses and cancer may be related is an old observation. Bor- 
rel suggested this in 1903 (8), and Rous (9) proved it a few years 
later. Viruses are now known to be found in such diverse neoplasia 
as human warts, fowl leukemia, rabbit papilloma, and milk-trans- 
mitted mammary cancer of mice (10). 
Another interesting observation is the transformation of normal 
cells in tissue culture to tumor cells after introduction of sarcoma 
virus (11). 
Many temporarily effective anticancer agents have been shown to 
affect nucleotide metabolism, and it appears that the rates of DNA 
synthesis and mitotic activity are parallel (12). Indeed, Cohen and 
Barner (13) have shown that cells (including bacteria) can be killed 
when DNA synthesis is blocked, even though RNA and protein 
metabolism may be unaltered. 
Because relatively pure nucleic acid preparations which possess 
virus activity have been obtained, Stanley suggested (14) that “it 
would now appear wise to revise the generally accepted definition 
of a virus to include nucleic acids and perhaps also to include repli- 
cating structures which do not evidence infectivity in the usual sense, 
5 The 1958 Nobel Prize for Chemistry was awarded to Frederick Sanger, of Cambridge, 
England, for working out the amino-acid sequence of insulin. 
