Noncomplementary Base Pairs Have a Structural Role in tRNA

4 Noncomplementary Base Pairs Have a Structural Role in tRNA 

Just as main-chain NH 0=C hydrogen bonds are important for the stabilization of the a-helix and / -pleated sheet secondary structures of the proteins, the Watson-Crick hydrogen bonds between the bases, which are the side-chains of the nucleic acids, are fundamental to the stabilization of the double helix secondary structure. In the tertiary structure of tRNA and of the much larger ribosomal RNA s, both Watson-Crick and non-Watson-Crick base pairs and base triplets play a role. These are also found in the two-, three-, and four-stranded helices of synthetic polynucleotides (Sect. 20.5, see Part II, Chap. 16). 

The secondary clover-leaf structure of tRNA is stabilized by Watson-Crick base pairs. The tRNA s are a large family of molecules consisting of 71 to 76 nucleotides, with about 10% of rare bases (Fig. 15.2). The known nucleotide sequences of over 200 tRNA s [695J can be arranged in a characteristic clover-leaf model with four double helical stems and three loop regions. In some of the positions, the same nucleotide occurs, called invariant, in others only the type is conserved, i.e., 

The Role of Hydrogen Bonding in the Structure and Function of the Nucleic Acids 

O = oxygen, = hydrogen hydrogen bonds drawn as dotted lines. After [696] from [S22] 

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