66 George Gamow and Martynas Ycas 



names, by twenty letters of a (reduced) English alphabet. Thus, one protein 

 molecule may look like: 



. . .arreducesugarreducesug. . . 

 and another like: 



. . . akeacoloruisionpigmentma . . . 



Just to give an example of how the sequence of amino acids in protein 

 molecules may affect their biochemical activity, we will give the example of two 

 closely related hormones: oxytocine and vasopressin. Both are formed by a 

 sequence of only nine amino acids: 



Oxytocine — Cys-Tyr-Z/ew-GIun-Aspn-Cys-Pro-Lew-Gly 

 Vasopressin — Cys-Tyr-PAe-Glun-Aspn-Cys-Pro-vlr^-Gly 



The two sequences are identical except for the substitutions in the third and 

 eighth place. However, their functions are rather different. Oxytocine has the 

 property of causing the contraction of the uterus in the process of childbirth. 

 If you inject it into the blood of a cow, even if the cow is not pregnant it will 

 go through all motions it would go through if a calf were to be born. Vaso- 

 pressin, on the other hand, has rather different properties: it contracts the 

 blood vessels and causes increased blood pressure. Thus, simply by changing 

 two amino acids out of nine, the action of the hormone is completely changed. 



Whereas replacement of some amino acids in a protein may completely 

 change its biological function, there also exist replacements which distinguish 

 the same protein taken from different species of animals. Thus, for example, 

 insulin A, which is formed by a sequence of amino acids with twenty-one 

 members, differs for cattle and swine in the eighth and tenth place. Human 

 insulin, which has not yet been analyzed, possibly differs slightly from that 

 extracted from cattle and swine. Nevertheless, the latter are successfully used 

 on human patients. 



Since there must exist a definite relation between the sequence of bases in 

 nucleic acid and the sequence of amino acids in proteins, we can ask ourselves 

 what this relation is. Here we have to return to our analogy of a factory. The 

 v/orkers from the factory do not walk into the manager's of^ce to find out what 

 to do, and the manager also does not go to the plant to instruct workers per- 

 sonally. There are people, called foremen, who get the information from the 

 manager's ofiice and tell the workers. In the cell the role of foreman is carried 

 out by RNA molecules (ribonucleic acid) which are, presumably, very similar to 

 the molecules of DNA. They are different only in that one oxygen atom is 

 missing in each sugar of DNA, and there is a slight change in one of the four 

 bases, which in RNA is called urosil instead of thymine. RNA is presumably 

 synthesized by DNA inside the nucleus and receives the set of instructions 

 carried by DNA. Then it passes out into the cytoplasm, and is incorporated into 

 the so-called microsomes, i.e. foremen's offices, where the synthesis of proteins 

 takes place. 



We do not yet have a model of the RNA molecule. It seems, however, 

 that in this case the pairing rules of adenine to thymine (urosil), and guanine 

 to cytosine do not hold, which suggests that RNA molecules are single-stranded. 



