Abstract: A method has been discovered for purifying a specific desired DNA sequence, starting from RNA heterogeneous in length and sequence. The steps of the method include making complementary DNA transcripts of the RNA by means of an enzyme such as reverse transcriptase, subjecting the DNA transcripts to the action of one or more selected restriction endonuclease enzymes, and fractionating the fragments produced by endonuclease action according to their length. By this method it is possible to isolate homogeneous length DNA fragments complementary to RNA sequences present in the original preparation in as low a frequency as two percent. A method is also disclosed for further purifying the homogeneous length fragments and for determining their final purity. Using the disclosed methods, a DNA fragment approximately 550 nucleotides in length coding for a portion of the peptide hormone, human chorionic somatomammotropin, has been purified to greater than 99% purity.

Abstract: A selected portion of DNA molecules having reactant ends which are capable of being joined in a ligase catalyzed reaction are pretreated so a to remove the 5'-terminal phosphate groups. Such a treatment reduces the frequency of joining an undersired combination and enhances the frequency of joining the desired combination.