Suppose we split this chain in two. How can we make another one just like it? If, in the substances of the cells, there is a manufacturing department which brings up phosphate, sugar, and A,B,C,D units not connected in a chain, the only ones which will attach to our split chain will be the correct ones, the complements of BAADC . . . , namely, ABBCD . . . Thus what happens is that the chain splits down the middle during cell division, one half ultimately to go with one cell, the other half to end up in the other cell; when separated, a new complementary chain is made by each half-chain.
richard p. feynman, “six easy pieces”