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 said biological construct each in a fluid droplet confined in a fluid channel; optionally mixing droplets from different fluid channels to for a sequenced order of droplets carrying different components of said biological construct in a channel or chamber; and optionally combining two or more droplets each containing different components of said biological construct to permit said components to react with each other in one or more reactions contributing to the assembly of said biological construct.