fluid flow conduit for reaction with the growing polymer chains prior to addition of nucleotide for chain extension steps there may also be included a re cycle arrangement whereby nucleotide reagent from the column may be re cycled to its appropriate reagent storage vessel for this purpose it is preferred to provide a single re cycle line extending from the reaction column to a drain this re cycle line contains two way valve connections arranged in series each valve being of the form previously described and arranged to return a nucleotide base to its appropriate reservoir during chain extension step with that specific nucleotide reagent most conveniently the two way valves are solenoid operated they are conveniently all coupled to a microprocessor into which the sequence of reactions can be programmed along with their duration so that the valves are operated automatically according to a predetermined sequence so as to produce the polynucleotide chains in the reaction column of the desired predetermined sequence with reference to fig1 the apparatus generally comprises a supporting frame 10 with an upper shelf 12 and a lower shelf 14 the upper shelf 12 carries nine reagent storage vessels in the form of sealed bottles each having a lower outlet conduit bottles 16 18 20 and 22 each contain a different nucleotide reagent and are provided wity refrigeration jackets to maintain the contents therein at low temperature 35 u00b0 c bottle 24 contains a capping reactant bottle 26 contains a deprotecting reagent bottle 28 contains an oxidizing reagent bottles 29.