[too rz duh fawrs, -fohrs; French toor duh fawrs] /ËtÊÉrz dÉ ËfÉrs, -ËfoÊrs; French tur dÉ ËfÉrs/ (Show IPA) 1. an exceptional achievement by an artist, author, or the like, that is unlikely to be equaled by that person or anyone else; stroke of genius: Herman Melville's Moby Dick was a tour de force. 2. a particularly adroit maneuver or technique in handling a difficult situation:
edit Nucleobases. Purines make up two of the four nucleobases in DNA and RNA: adenine and guanine. Pyramidines make up the other bases in DNA and RNA: cytosine, thymine (in DNA) and uracil (in RNA).
Each complementary pair of bases contains one pyrimidine and one purine base. This ensures that the combination of two bases connecting the two strands is always of equal length. Adenine (purine) and thymine (pyrimidine) form two hydrogen bonds. Guanine (purine) and cytosine (pyrimidine) form three hydrogen bonds. Only two base pairs are possible in DNA: adenine-thymine and guanine-cytosine. A strand of RNA complementary to DNA will have uracil instead of thymine opposite adenine.