Acceptor stem

Amino acid attachment site


FIGURE 3.22 Stylized Relationship of Charged tRNA to mRNA and the Ribosome

Note: This figure does not show the correct physical arrangement— instead it illustrates the coding relationships between the tRNA and mRNA. The mRNA binds to the 30S subunit of the ribosome. The anticodons of the tRNAs carrying amino acids bind to the corresponding codons on the mRNA. In real life only two tRNAs are present on the ribosome at any given time and the codons on the mRNA are contiguous, with no gaps between.

Hydrogen bonding

Hydrogen bonding

tRNA (adaptor)

tRNA (adaptor)

codon and anticodon recognize each other by base pairing and are held together by hydrogen bonds. At its other end, each tRNA carries the amino acid corresponding to the codon it recognizes.

The small (30S) subunit binds the messenger RNA and the large (50S) subunit is responsible for making the new polypeptide chain. Figure 3.22 shows the relationship between the mRNA and the tRNAs in a stylized way. In practice, only two tRNA molecules are base-paired to the messenger RNA at any given time. After binding to the mRNA, the ribosome moves along it, adding a new amino acid to the growing polypeptide chain each time it reads a codon from the message (Fig. 3.23). A more detailed account of protein synthesis is given in Chapter 8.

Various Classes of RNA Have Different Functions 69

FIGURE 3.23 Ribosome Elongating a Polypeptide Chain

A new amino acid is added to the polypeptide chain each time a new tRNA arrives at the ribosome, bringing its attached amino acid. The anticodon of the tRNA binds to the mRNA. The large subunit cross links the incoming amino acid to the growing chain, such that the incoming tRNA ends up carrying the growing polypeptide chain. The 30S subunit of the ribosome then moves one step along the mRNA. This results in ejection of the leftmost tRNA and readies the mRNA to accept the next incoming tRNA. The polypeptide chain continues to grow until a "stop codon" is reached.


Trp tRNA fphe Incoming RNA

Growing charged with polypeptide amino acid chain

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