32 CELL HEREDITY 



tions are of interest. Gene action can be viewed as the transfer of 

 genetic information from nucleic acids to other cell constituents. This 

 is a coding problem; the nucleic acids may be considered a tape coded 

 with signals which determine cellular activities. The only elements of 

 the nucleic acid molecule that appear suitable for coding are the nucleo- 

 tides themselves. Since there are four different nucleotides regularly 

 present, we can consider the nucleic acid alphabet as composed of four 

 letters. [One may also consider a binary code with only two letters, 

 determined by whether there is a keto or an amino group on the 6-position 

 in the nitrogen bases (see Figure 1.1).] 



Whicii cell constituents decode the tape? If we restrict our considera- 

 tions to genes which act by controlling protein specificity, then we can 

 restrict the coding problem to translation from nucleic acids (two- or 

 four-letter alphabet) to proteins, which have a twenty-letter alphabet 

 (most proteins contain twenty different amino acids). According to 

 present biochemical information, most proteins are synthesized on 

 ribonucleoprotein particles in the cytoplasm, in which, so far as is 

 known, no DNA is present. Consequently, it has been postulated widely 

 that the last steps in protein synthesis involve translation from RNA 

 to protein, and a number of codes have been proposed to solve this 

 translation step. Since DNA and RNA are so similar and the codes de- 

 vised so general, they can be considered purely as nucleic-acid-to- 

 protein codes without specifying which nucleic acid is involved. Despite 

 the ingenuity of these codes, there is insufficient information available 

 as yet for choosing among them. 



Although there are many gaps in our knowledge, we may be sure from 

 what we have already learned that genes must be fairly large. If their 

 influence is upon protein structure or the rate of protein synthesis, each 

 must contain enough information to relate it to a particular enzyme or 

 enzyme-forming site, and not to any others. Analogous considerations 

 would apply to other gene functions. In our considerations of the gene 

 we shall assume, therefore, that the functional unit consists of many 

 nucleotides, numbering perhaps in the hundreds. Later (Chapter 6) we 

 shall see that this conception has an independent experimental basis. 



BIBLIOGRAPHY 



Avery, O. T., C. M. MacLeod, and M. McCarty, 1944. Studies on the chemical 

 nature of the substance inducing transformation of pneumococcal types. Induc- 

 tion of transformation by a desoxvribonucleic acid fraction isolated from 

 pneumococcus Type III. /. Exptl. Med., 79:137-158. 



Beadle, G. W., 1945. Biochemical genetics. Chem. Rev., 37:15-96. 



