Biological Synthesis of Polynucleotides

The cyclic product is extremely sensitive to moisture (hydrolysis) and was more readily isolated as the cyclic diester. An interesting correlation has been observed the phenyl triesters of high energy cyclic phosphates (cyclic nucleotides, ethylene phosphate) are very reactive species, susceptible to decomposition. On the other hand, the phenyl triesters of low energy cyclic phosphates are stable, crystalline compounds. Hence, for example, the phenyl triester of trimethylene phosphoric acid is a crystalline compound readily prepared in high yield by reaction of 1,3-propanediol with phenyl phosphorodichloridate. 

A similar preparation of the phenyl triester of cAMP has not yet been reported. 

Of course, as the new bases are added to the template strand, they must undergo polymerization to form the complementary chain. This requires the formation of a 3, 5 -phosphodiester linkage between two monomeric units. The enzyme that catalyzes this polymerization reaction is DNA 

The 3 -hydroxyl of the primer will attack the a-phosphorus of an appropriate (as dictated by the template) triphosphate, thus ejecting pyrophosphate and forming a phosphodiester linkage. The 3 -hydroxyl of the newly added monomer is now in a position to attack another molecule of triphosphate and so the polymerization can continue along the template. The chain will terminate such that the template will end with a 5 -phosphate but the new strand will end with a 3 -hydroxyl. 

DNA polymerase exists in different forms in order to carry out different polymerization functions. While perhaps suggestive to the contrary, these different forms are not the result of subunit structure, at least in the case of the bacterial enzyme. Three forms of the enzyme from the bacterium E. coli have been characterized and are simply designated polymerase I, II, and III. DNA polymerase I functions principally as a repair enzyme, while DNA polymerase III is a replicating enzyme. The function of DNA polymerase II is not yet certain. The mammalian enzyme also exhibits multiple forms. 

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