This invention provides methods and compositions for assembling biological constructs (e.g., plasmids, transformed cells, etc.). In certain embodiments the methods involve encapsulating separate components of the biological construct each in a fluid droplet confined in a fluid channel; optionally mixing droplets from different fluid channels to form a sequenced order of droplets carrying different components of the biological construct in a channel or chamber; and optionally combining two or more droplets each containing different components of the biological construct to permit the components to react with each other in one or more reactions contributing to the assembly of the biological construct.