II. REPLICATION OF DNA IN CHROMOSOMES 



95 



rapid one that does not require high energy bonds, i.e., base pairing of 

 two single polynucleotide chains (Fig. 6). The two chains of the DNA 

 double helix were assumed to break at different points in many cases. 

 The union by base pairing could be a very rapid process. The second 

 step would presumably be the healing of the lesion by the growth of two 

 antiparallel DNA chains, each copying the other. If the growth occurs 



Fig. 6. Diagram of the steps involved in the reunion of broken chromosomes 

 according to the hypothesis described in the text. Fragments of two replication units 

 of different broken chromosomes are shown. The lines represent the subunits of 

 chromatids, presumably single nucleotide chains. (From Taylor et al.. 1962.) 



by the action of the enzyme DNA polymerase (Chapter I) only chains 

 with 3' OH groups at the end can grow. If the breaks leave a 3' phos- 

 phate group at the end, an enzyme might be required to chop the chain 

 and prepare it for growth. In addition, about one half of the unions by 

 base pairing would be expected to occur between non-growing ends, i.e., 

 those ending in a 5' phosphate or 5' OH group. In these the shorter 

 complementary chains ending in a 3' OH might be expected to grow and 

 bypass the original point of union of nongrowing ends. This could result 

 in a reopening of the break and result in new exchanges or to a union 



