Organization and Replication of DNA in Vivo 



271 



being turned over, so that the number 1 

 atoms of both face each other. This ar- 

 rangement has an important consequence 

 for the orientation of the two chains rela- 

 tive to each other, as represented in the two- 

 dimensional diagram, Figure 20-6. The 

 bases in the chain at the right all face the 

 accustomed way; those in the left chain are 

 all turned over. For each base to join to 

 its sugar in the same three-dimensional way, 

 the sugars must be arranged as shown. No- 

 tice, in proceeding downward from the top 

 of the right chain, the P0 4 ~ linkages to sugar 

 read 3'5', 3'5', and so on; reading down in 

 the same way, however, the left chain is 

 5'3', 5'3', et cetera, so that the member 

 chains in a double helix run in opposite 

 directions, as indicated by the arrows. 



The X-ray diffraction results, which led to 

 the double helix hypothesis, do not tell us 

 that all DNA in chromosomes is two- 

 stranded, or that a double strand is never 

 single-stranded at certain places or at cer- 

 tain times. Such data prove only that, in 

 the wide variety of organisms studied, a very 

 appreciable part of the chromosomal DNA 

 is not single-stranded. The base content 

 and organization of DNA in viruses attack- 

 ing bacteria have also been studied by chem- 

 ical analysis and by X-ray diffraction. In 

 the varieties T 2 and T 7 , for example, the 

 data are entirely consistent with DNA's 

 being present in the Watson-Crick double- 

 helix configuration. In the mature bacterial 

 virus particles of two other smaller varieties 

 (called 0X174 and 0S13), however, the 

 DNA is definitely single-stranded. This is 

 reflected in the nonequivalence of A and T 

 and C and G and the absence of those pat- 

 terns indicating a secondary structure in the 

 X-ray diffraction photographs. 



Whenever the DNA is in the double-helix 

 configuration, we can consider one strand is 

 the complement of the other, so that if the 

 sequence of bases in one strand is known, 

 the composition of the other strand can be 



determined. Thus, if one strand has the 

 base sequence ATTCGAC, the other strand 

 would have to contain TAAGCTG in the 

 corresponding region. 



If DNA is genetic material, we expect 

 DNA to be replicated just as accurately as 

 genetic material. Since the base sequence 

 in one strand is complementary to the se- 

 quence in the other, we immediately see a 

 simple way in which the double helix might 

 be replicated: ] the two strands separate, 

 and then each strand builds its complement. 

 In this explanation, called the strand separa- 

 tion hypothesis of DNA replication, each 

 strand is visualized as a mold or template. 

 We know that complex surfaces (like 

 statues) can be copied exactly by making 

 a mold which, in turn, can be used to make 

 a second mold which is an exact copy of the 

 original configuration. In the present case, 

 the two complementary strands of DNA can 

 be viewed as molds, or templates, for each 

 other. One strand or both strands act as 

 a mold on which the complementary strand 

 is synthesized. Figure 20-7 shows one pos- 

 sible sequence of events. At the top of 

 this figure, the two strands are coming apart 

 due to rupture of the H bonds. At the 

 center the two single chains exist in the 

 presence of single nucleotides or their pre- 

 cursors. When the complementary free nu- 

 cleotide approaches the single strand, its base 

 is H-bonded. Then, after two or more nu- 

 cleotides have bonded to the single strand, 

 they are linked — perhaps by an enzyme — to 

 start the new complementary strand. The 

 bottom diagrams show sections of the com- 

 plementary strands whose synthesis is al- 

 ready completed. 



Experiments can be designed - to simul- 



1 Based upon the hypothesis of J. D. Watson and 



F. H. C. Crick (1953b, c). 



- Based upon those of M. Meselson and F W 



Stahl. 



