Chemical synthesis of genes
As outlined above, before one can start the chemical synthesis of a gene, the structure of gene should be known. The structure of a gene earlier could not be worked out by direct chemical analysis, since there were no means for isolating a gene (techniques for isolation of genes were developed later and were described earlier in this section). The structure of gene could, therefore, be inferred only from its product. For instance, if a gene is responsible for giving rise to a polypeptide chain and the structure of this chain is known, then from the genetic code dictionary, structure of the gene could be easily inferred (although degeneracy of the genetic code creates problems). Such genes, were initially considered to be too long to be synthesized because an average gene contains about 1,500 base pairs. On the other hand, since tRNA molecules are fairly small in size (about 80 nucleotides), a gene responsible for giving rise to a tRNA molecule was within the reach for artificial synthesis.