Stanford Medical Bulletin 



^-/ 



S V/ H 



S V 



/ 



N C 



// \ 



O H " — c 



JL\ v/ 



\ 



c . 



/ 



/ 



H \ / \ 



x / 



O H " — L 



1\ v/ 



/ 



Fig. i. — Primary structure of DNA — a segment of a polynucleotide sequence CGGT. From (13). 



ose ring of deoxyribose, which is coupled as 

 aw N glvcoside to one of the nuclein bases: 

 adenine, guanine, cytosine, or thymine, sym- 

 bolized A. G, C, or T, the now well-known 

 alphabet in which genetic instructions are com- 

 posed. With a chain length of about 10,000 

 residues, one molecule of DNA contains 20,000 

 "bits of information," comparable to the text 

 of this article, or in a page of newsprint. 



Pyrophosphate - activated monomer units 

 (e.g., thymidine triphosphate) have been iden- 

 tified as the metabolic precursors of DNA 

 (31). For genetic replication, the monomer 

 units must be assembled in a sequence that re- 

 flects that of the parent molecule. A plausible 

 mechanism has been forwarded by Watson 

 and Crick (87) as a corollary to their struc- 

 tural model whereby DNA occurs as a two- 

 stranded helix, the bases being centrally ori- 

 ented. When their relative positions are fixed 

 by the deoxyribose-phosphate backbones, just 

 two pairs of bases are able to form hydrogen 

 bonds between their respective NH and CO 

 groups; these are A : T and G : C. This pairing 

 of bases would tie the two strands together for 

 the length of the helix. In conformity with this 

 model, extensive analytical evidence shows a 

 remarkable equality of A with T and of G 

 with C in DNA from various sources. The 

 two strands of any DNA are then mutually 

 complementary, the A, T, G, and C of one 

 strand being represented by T, A, C, and G, 

 respectively, of the other. The information of 

 one strand is therefore equivalent to, because 

 fully determined by, the other. The determi- 

 nation occurs at the replication of one parent 



strand by the controlled stepwise accretion of 

 monomers to form a complementary strand. 

 At each step, only the monomer which is com- 

 plementary to the template would fit for a 

 chain-lengthening esterification with the adja- 

 cent nucleotide. The model requires the un- 

 raveling of the intertwined helices to allow 

 each of them to serve as a template. This 

 might, however, occur gradually, with the 

 growth of the daughter chain — a concept em- 

 bedded in Fig. 2 which symbolizes the new 

 Cabala. The discovery of a single-stranded 

 configuration of DNA (85) makes complete 

 unraveling more tenable as an alternative 

 model. 



For the vehicle of life's continuity, DNA 

 may seem a remarkably undistinguished mole- 

 cule. Its over-all shape is controlled by the uni- 

 form deoxyribose-phosphate backbone whose 

 monotony then gives X-ray diffraction pat- 

 terns of high crystallinity. The nucleins them- 

 selves are relatively unreactive, hardly differ- 

 ent from one to the other, and in DNA intro- 

 verted and mutually saturated. Nor are any of 

 the hydroxyls of deoxyribose left unsubstituted 

 in the polymer. The structure of DNA befits 

 the solipsism of its function. 



The most plausible function of DNA is 

 ultimately to specify the amino acid sequence 

 in proteins. However, as there are twenty 

 amino acids to choose among, there cannot 

 be a one: one correspondence of nucleotide to 

 amino acid. Taking account of the code dupli- 

 cation in complementary structures and the 

 need to indicate spacing of the words in the 

 code sequence, from three to four nucleins 



s-100 



