Oligonucleotides base-pair-directed synthesis

DNA polymerase enzymes all synthesize DNA by adding deoxynucleotides to the free 3 -OH group of an RNA or DNA primer sequence. The identity of the inserted nucleotide is deterrnined by its abiHty to base-pair with the template nucleic acid. The dependence of synthesis on a primer oligonucleotide means that synthesis of DNA proceeds only in a 5%o V direction if only one primer is available, all newly synthesized DNA sequences begin at the same point. 

This scheme (Fig. 2.5) thus shows how oligonucleotides can direct the synthesis of polypeptides in the absence of protein or ribosomal machinery and, as such, is an appealing bioorganic model for the origin of prebiotic protein synthesis (see Section 3.7.2.1). Indeed, it seems most probable that primitive biosystems used a similar concept to carry out primitive protein synthesis where Watson-Crick base pairing provided the intrinsic mechanism for achieving fidelity of replication and direction of protein synthesis. In time, the carrier oligonucleotides could have evolved into more efficient species such as the present-day tRNA molecules. 

The base pairing in DNA needs little introduction Guanine is paired with cytosine (22.47) and adenine with thymine (22.48). Figure 22.38 shows a standard double helix with the major and minor grooves marked. It is now possible to synthesize short oligonucleotide chains by a solid-phase synthesis method similar to that used to produce peptides. The chemical synthesis proceeds from 3 to 5, opposite to the direction of biosynthesis. Because of side reactions, the syntheses are limited to about 200 residues, and in most cases, the best results are obtained in making 20-25 residue units. ... 

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