A block includes a hole provided on one side of the block, two fifths of the length of the block from one end, and a pin provided on the opposite side of the block two fifths of the length of the block from the other end. The hole is arranged to be connected to the pin of a like underlying or overlying block, so that a plurality of said blocks can be connected to form a double helix. Each end of the block includes a knob for connecting a plurality of blocks to a flexible member which follows a double helix shape to produce a model for DNA.