1422 All tRNAs possess a common secondary structure the cloverleaf structure The base sequence in the flattened model is for tRNAAla Credit for Fig 1422 allowing rest of line for photo credit here

The acceptor arm has no loop but contains the 5' and 3' ends of the tRNA molecule. All tRNAs have the same sequence (CCA) at the 3' end, where the amino acid attaches to the tRNA; so clearly this sequence is not responsible for specifying which amino acid will attach to the tRNA.

The T^C arm is named for the bases of three nucleotides in the loop of this arm: thymine (T), pseudouracil and cytosine (C). The anticodon arm lies at the bottom of the tRNA. Three nucleotides at the end of this arm make up the anticodon, which pairs with the corresponding codon on mRNA to ensure that the amino acids link in the correct order. The DHU arm is so named because it often contains the modified base dihydrouridine.

Although each tRNA molecule folds into a cloverleaf owing to the complementary paring of bases, the cloverleaf is not the three-dimensional (tertiary) structure of tRNAs found in the cell. The results of X-ray crystallographic studies have shown that the cloverleaf folds upon itself to from an L-shaped structure, as illustrated by the space-filling and ribbon models in Figure 14.22. Notice that the acceptor stem is at one end of the tertiary structure and the anti-codon is at the other end.

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