Secondary DNA Structure the Double Helix

The problems of laboratory DNA synthesis are similar to, but even greater than, those of peptide synthesis, mainly because nucleotides have more complicated structures than do amino acids. Each nucleotide has several functional groups that must be protected, and later deprotected, during synthesis. Despite these difficult chemical problems, methods for oligonucleotide synthesis have been developed in various laboratories. Using these methods, Khorana and coworkers were able to synthesize a gene for the first time, by a combination of chemical and enzymatic means. 

More recently, automated gene synthesizers have been developed that operate on principles similar to the Merrifield solid-phase technique for peptides. A protected nucleotide is covalently bonded to a polymer. Other protected nucleotides are then added sequentially to the chain, using a coupling reagent. Eventually, the protecting groups are removed, and the synthetic oligonucleotide is then detached from the solid support. 

Let us now proceed from the primary DNA structure, the specific base sequences, to the secondary DNA structure the double helix and the genetic code. 

The meaning of these equivalences was not evident until 1953, when Watson and Grick, working together in Gambridge, England, proposed the double helix model for DNA. They received simultaneous supporting x-ray data for their proposal from Rosalind Franklin and Maurice Wilkins in London. The important features of their model follow  

DNA consists of two helical polynucleotide chains coiled around a common axis. 

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