The crystal structures of several tRNA molecules have been determined. All tRNA molecules have very similar secondary structures in which the single-stranded chain is folded in a 'clover-leaf' structure that has three hairpins and an acceptor stem where the amino-acid is covalently attached. The acceptor stem is the 3' end of the chain and always terminates in the sequence 5'-CCA-3'.
This particular tRNA is specific for the amino-acid Alanine whose codon on the mRNA is 5'-GCC-3' and the anti-codon loop of tRNA reads 5'-GGC-3'. The grey circles are examples of unusual, chemically modified, bases.
The secondary structure then folds up to form a 3-dimensional structure which looks like an inverted L.
One end of one L arm (the 3' end of the chain) is the acceptor stem. The other end of the L is the anti-codon loop that has to match the codon on the mRNA. The distance between the two ends of the L is ~ 7 nm. The corner of the L is used for correct positioning on the ribosome where the protein synthesis takes place.
In the tertiary (3-dimensional) structures of RNA, bases sometimes make hydrogen bonds with more than one partner, as illustrated in the picture above. These extra hydrogen bonds compensate for the distortion in the double-stranded helical regions when the RNA folds up and help stabilize the tertiary structure.
The covalent attachment between the tRNA and its corresponding amino-acid is achieved by yet another adaptor molecule (this time a protein molecule called aminoacyl-tRNA synthetase) of which there are at least 20 varieties, one for each kind of amino-acid. The synthetases recognize the detailed shape and properties of a specific amino-acid and the detailed shape of the acceptor stem in the folded tRNA molecule and catalyze the covalent attachment between the amino-acid and its corresponding tRNA.
0 comments: