tRNA Structure

• All tRNAs have similar sequences of 73 to 93 nucleotides
• 3' end always terminates with the sequence CCA, with the 3' hydroxyl of the ribose of the terminal A being the point of covalent attachment of the amino acid
• Contain a number (7-15%) of unique/modified bases.  These are post-transcriptionally modified after synthesis by RNA polymerase.
• In particular, adenosine (A) in first or 5' position of the anticodon (corresponding to the third or 3' position of the codon) is always modified to inosine (I) which lacks the amino group on the purine ring.  Inosine can base-pair with A, U or C and thus accounts for much of the degeneracy of the Genetic Code ("Wobble Theory").
• tRNAs have cloverleaf secondary structure due to four base-paired stems
• The cloverleaf contains three non-base-paired loops:  D, anticodon, and TpsiC loop

• The tertiary structure of tRNA is best described as a compact "L" shape.
• The anticodon is a single-stranded loop at the bottom of the Figure which later base-pairs with the triplet codon
• The amino acid is attached to the terminal A on the upper right.
• The active sites (anticodon and amino acid) are maximally separated.
• As in proteins, the tertiary structure is dictated by the primary sequence.
• The tertiary structure is stabilized by base pairing and base stacking.
• Two areas (anticodon stem and acceptor stem) form double helix.

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