II. REPLICATION OF DNA IN CHROMOSOMES 



99 



LINKER 



LINKER 



r^ 



^V/ 



^X/ 



r%j 



/VJ 



Fig. 7. A portion of a chromosome preceding replication. The plectonemically 

 coiled strands represent molecules of DNA joined in tandem array by means of 

 special sites or linkers in each polynucleotide chain. These linkers are assumed to 

 occur in pairs at sites in which there is a reversal in polarity so that two 3' and two 

 5' ends of chains are linked by 3' linkers and 5' linkers, respectively. 



replication would be the enzymatic opening of 3' linkers. However, a 

 necessary' preliminary step would be the establishment of H linkers 

 (Fig. 8) which are assumed to stabilize the chromatid by holding it in 

 a folded ladder-like arrangement (Taylor, 1962). In the model the H 

 linkers are shown as some type of polymer joining alternate 5' linkers 

 along two axes. These would represent the half-chromatids, but during 



