If the structure that serves as a template (the gene or virus molecule) consists of, say, two parts, which are themselves complementary In structure, then each of these parts can serve as the mould for the production of a replica of the other part, and the complex of two complementary parts thus can serve as the mould for the production of duplicates of itself.
Linus Pauling in His Own Words: Selections From His Writings, Speeches and Interviews (ed. Simon and Schuster, 1995) - ISBN: 9780684813875