28 THE BIOSYNTHESIS OF PROTEINS 



4. The Structure of DNA 



DNA as isolated from animal tissues or from micro-organisms is a macro- 

 molecule with a molecular weight in the range of 6-10^ (Sadron, 1959). 

 Chemical studies (Brown and Todd, 1952; Michelson and Todd, 1953; 

 Dekker et al, 1953; Carter, 1951) showed that DNA is made of a linear 

 backbone in which phosphoric groups and deoxyribose residues follow 

 each other in a quite regular manner. Each phosphate group is bound to 

 the C5 position of a deoxyribose and to the C3 of the next one. These long 

 chains of two alternating elements are identical in all the DNA samples 

 studied, they are perfectly monotonous and probably void of information. 

 But each deoxyribose residue of the chain carries on the Ci a purine or a 

 pyrimidine bound in N-glycosidic linkage. Adenine, guanine, thymine and 

 cytosine account for all but a small percentage of the bases. 5-Methylcyto- 

 sine (Laland et al., 1952; Wyatt, 1951), 6-methylaminopurine (Dunn and 

 Smith, 1955, 1960) have been found in smaller amounts in DNA. 



In a few exceptional but very interesting cases, large amounts of an 

 unusual pyrimidine enter in DNA composition, thus T-even bacterio- 

 phages contain 5-hydroxymethylcytosine and a large part of it carries one 

 or two glucose residues on the CH2OH (Wyatt and Cohen, 1952, 1953; 

 Sinsheimer, 1956; Loeb and Cohen, 1959). 



The base composition of DNA proved quite constant within each 

 species, and no significant dift'erences were found between DNA samples 

 obtained from different organs of a given species (Chargaff and Lipshitz, 

 1953). On the contrary, DNA specimens from various organisms were 

 found to differ considerably in base composition (Chargaff, 1950, 1958; 

 Wyatt, 1952) and in their chromatographic behaviour (Brown and Watson, 

 1953 ; Bendich et al. , 1 956) which indicates that DNA preparations are hetero- 

 geneous populations of molecules of different composition. It has not been 

 possible so far to unravel the arrangement of the purines and the pyrimi- 

 dines in the chains, but some indications were obtained by the study of 

 breakdown products formed by controlled degradation of DNA. Purines 

 and pyrimidines do not follow each other at random: for instance clusters 

 of three or more pyrimidines are more frequent than expected from a com- 

 pletely random distribution (Shapiro and Chargaff", 1957; Burton and 

 Peterson, 1960). Differential distribution analysis of the split products, 

 moreover, showed marked differences between DNA specimens of different 

 origins which had almost indistinguishable gross composition (Shapiro and 

 Chargaff, 1957, 1960). 



DNA of different species thus differ in the proportions and in the 

 arrangement of the bases along the common phosphate— carbohydrate 

 backbone. If genetic information is a modulation of the DNA constitution, 

 the arrangement of the bases along the backbone must be the language in 

 which this information is recorded. 



