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FIGURE 8.03 Transfer RNA Recognizes Codons

Several tRNAs are seen bound to mRNA codons by their anticodons. Each tRNA carries a different amino acid at the end of the adaptor stem. This diagram is intended to show the principle of mRNA decoding. It does NOT illustrate the actual mechanism of protein synthesis. In real life, the codons are contiguous and there are no spacers in between and only two tRNAs are bound at any given time.

Each transfer RNA carries one particular amino acid.

is needed. These adapters are small RNA molecules, or transfer RNA (tRNA). At one end, the tRNA has an anticodon consisting of three bases that are complementary to the three bases of the codon on the mRNA. The codon and anticodon recognize each other by base pairing and are held together by hydrogen bonds (Fig. 8.03). At its other end, each tRNA carries the amino acid corresponding to the codon it recognizes.

Transfer RNA Forms a Flat Cloverleaf Shape and a Folded "L" Shape

Transfer RNA molecules are about 80 nucleotides in length. About half the bases are paired to form double helical segments. A typical tRNA has four short base-paired stems and three loops (Fig. 8.04).This is shown best in the cloverleaf structure, intended to reveal details of base pairing, which shows the tRNA spread out flat in only two dimensions. (Such a diagram is sometimes called a secondary structure map). The tRNA cloverleaf is folded up further to give an L-shaped 3-D structure, in which the TyC-loop (or T-loop) and the D-loop are pushed together.The anticodon and attached amino acid are located at the two ends of the L-structure. Different tRNA molecules vary considerably in sequence, but they all conform to this same overall structure. Variations in length (from 73 to 93 nucleotides) occur, due mostly to the variable loop.

anticodon Group of three complementary bases on tRNA that recognize and bind to a codon on the mRNA cloverleaf structure 2-D structure showing base pairing in a tRNA molecule transfer RNA (tRNA) RNA molecules that carry amino acids to the ribosome

Modified Bases Are Present in Transfer RNA 201

Aiiticodon loop

FIGURE 8.04 Structure of Transfer RNA

Aiiticodon loop

FIGURE 8.04 Structure of Transfer RNA

Modified Bases Are Present in Transfer RNA 201

A) A planar view (secondary structure) of a tRNA shows its cloverleaf structure comprised of the 3' and 5' acceptor stem, the T- (or TyC) and D-loops and the anticodon loop. A variable loop, which varies in length in different tRNA molecules is also found. B) The folded (tertiary structure) configuration resembles an "L."

The acceptor stem is made by pairing of the 5'-end, which almost always ends in G and is phosphorylated, and the 3'-end, which ends in CCA-OH. The amino acid is bound to the 3'-hydroxyl group of the adenosine at the free 3'-end of the acceptor stem. The anticodon is about halfway round the sequence, in the anticodon loop. This consists of seven bases with the three anticodon bases in the middle. The anticodon is always preceded, on the 5' side, by two pyrimidines and followed by a modified purine (Fig. 8.04).

The other two loops of tRNA are named after modified bases. The T yC-loop contains "y" (spelled "psi" but pronounced "sigh"), which stands for pseudouracil; and the D-loop or DHU-loop has "D" for dihydrouracil. These strange bases are required for proper folding and operation of the tRNA. The TyC-loop and the D-loop are needed for binding to the ribosome and other protein factors involved in translation (see below).

Modified Bases Are Present in Transfer RNA

As originally transcribed, RNA contains only the four bases A, U, G and C. However, some RNA molecules contain bases that are altered chemically after the RNA has been made. Some of these are shown in Fig. 8.05. This is especially true for tRNA, in which up to 15 modified bases per molecule may occur.

acceptor stem Base paired stem of tRNA to which the amino acid is attached anticodon loop Loop of tRNA molecule that contains the anticodon modified base Nucleic acid base that is chemically altered after the nucleic acid has been synthesized

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