108 .1. HKHUKRT TAYLOR 



phosphoseriiK' reisidiu's pvv thousand nucleotides. This suggests that 

 the reguhir 3' linkers or perhaj)s the 3'R linkers might consist of two 

 l')hosphoserine residues coupled to the terminal nucleoside of DNA 

 chains by an ester linkage in the same way that amino acids are 

 coupled to transfer RXA. A diphosphate bridge could then couple two 

 chains with a reversal in i)olarity. The linkage might rei)resent an 

 activated state ready for the initiation of replication or for a change of 

 the linker if it should i)rove to be the 3'R linker. The 5' linker should be 

 formed by covalent bonds which might be as stable as the i)hosphodiester 

 linkages along the iiolynucleotide chains. Although there are several 

 possible types of molecules that could serve as linkers, it would be 

 premature to make further guesses at this time. 



Although the model presented is designed primaiily to explain the 

 replication and sorting of subunits in chromosomes, the traffic control 

 problem in template function might be mentioned. How is a chromosome 

 able to open for replication without functioning as a template for RNA 

 synthesis as ap{)ears to be true for the heteropycnotic X-chromosomc 

 and jierhaps other such genetically inactivated regions on autosomes? 

 Or what is more common, how does it oj:)en for RNA replication without 

 serving as a template for DNA synthesis when all the enzymes and 

 precursors for replication are operating at other sites in the nucleus? 

 The cell could solve this problem by having special ojierator sites which 

 are opened for RNA replication, l)ut wiiich could not serve as starting 

 ends for DNA polymerase. Let us supjjose that a DNA polymerase is 

 available to repair random breaks in DNA by adding to free 3' OH 

 ends and that the same or a similar polymerase can begin to copy at the 

 proposed operator sites for DNA replication. However, the special sites 

 opened for RNA synthesis would not be available as starting loci for 

 these DNA polymerases because of the occurrence of some odd base, or 

 other end group, for example. Such a mechanism would seem to be 

 necessary to simplify the problem of traffic control in the operation of a 

 template which can serve either for RNA synthesis or DNA synthesis. 

 The principal difference would be that DNA replication requires opening 

 and unwinding of the two chains of the DNA. RNA replication either 

 can occur without strand separation or the chains can open at special 

 sites and then close again without destroying the integrity of the DNA. 



References 



.Vll'crt, M. (1955). St/mposium on Fine Structure of Cells. Leklcn, 195.'t. Intern. I'nion 

 Biol. Set. Publ. Ser. B, 21, 157. P. Noordhoff, Groningen, Netherlands. 



Avery, O. T., MacLeod, C. M., and MrCarty, M. (1944). J. Exptl. Med. 79, 137. 



Bendich, A., and Rosenkranz, H. S. (1962). In "Progress in Nucleic Acid Research" 

 (W. E. Cohn and J. N. Davidson, eds.), Vol. I. Acaileniic Press, New York. 

 In press. 



