Translation is the process by which the information in mRNA (derived from DNA) is used to specify and link a specific sequence of amino acids, producing a polypeptide. Like transcription, translation occurs in three steps: initiation, elongation, and termination.

INITIATION The translation of mRNA begins with the formation of an initiation complex, which consists of a charged tRNA and a small ribosomal subunit, both bound to the mRNA (Figure 14.13).

In prokaryotes, the rRNA of the small ribosomal subunit first binds to a complementary ribosome binding site (AGGAGG; known as the Shine–Dalgarno sequence) on the mRNA. This sequence is less than 10 bases upstream of the actual start codon but lines up the start codon so that it is adjacent to the P site of the large subunit:

mRNA 5′.........A G G A G G......(start codon).....3′

rRNA 3′.......... U C C U C C.........(P site)..........5′

Eukaryotes load the mRNA onto the ribosome somewhat differently: the small ribosomal subunit binds to the 5′ cap on the mRNA and then moves along the mRNA until it reaches the start codon.

Recall that the mRNA start codon in the genetic code is AUG (see Figure 14.5). The anticodon (UAC) of a methionine-charged tRNA binds to this start codon by complementary base pairing to complete the initiation complex. Thus the first amino acid in a polypeptide chain is always methionine. However, not all mature proteins have methionine as their N-terminal amino acid. In many cases, the initial methionine is removed by an enzyme after translation.

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After the methionine-charged tRNA has bound to the mRNA, the large subunit of the ribosome joins the complex. The methionine-charged tRNA now lies in the P site of the ribosome, and the A site is aligned with the second mRNA codon. These ingredients—mRNA, two ribosomal subunits, and methionine-charged tRNA—are assembled by a group of proteins called initiation factors.

ELONGATION A charged tRNA whose anticodon is complementary to the second codon of the mRNA now enters the open A site of the large ribosomal subunit (Figure 14.14). The large subunit then catalyzes two reactions:

Because the large ribosomal subunit performs these two actions, it is said to have peptidyl transferase activity. In this way, methionine (the amino acid in the P site) becomes the N terminus of the new protein. The second amino acid is now bound to methionine but remains attached to its tRNA at the A site.

How does the large ribosomal subunit catalyze peptide bond formation? Harry Noller and his colleagues at the University of California at Santa Cruz did a series of experiments and found that:

The experiment showed that rRNA is the catalyst. The purification and crystallization of ribosomes has allowed scientists to examine ribosome structure in detail, and the catalytic role of rRNA in peptidyl transferase activity has been confirmed. These findings support the hypothesis that RNA, and *catalytic RNA in particular, evolved before DNA.

After the first tRNA releases its methionine, it moves to the E site and is then dissociated from the ribosome, returning to the cytosol to become charged with another methionine. The second tRNA, now bearing a dipeptide (a two-amino acid chain), is shifted to the P site as the ribosome moves one codon along the mRNA in the 5′-to-3′ direction. The elongation process continues, and the polypeptide chain grows, as these steps are repeated. Follow the process in Figure 14.14. All these steps are assisted by ribosomal proteins called elongation factors.

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TERMINATION The elongation cycle ends, and translation is terminated, when a stop codon—UAA, UAG, or UGA—enters the A site (Figure 14.15). Stop codons do not correspond with any amino acids, nor do they bind any tRNAs. Rather, they bind a protein release factor, which allows hydrolysis of the bond between the polypeptide chain and the tRNA in the P site. The newly completed polypeptide thereupon separates from the ribosome. Its C terminus is the last amino acid to join the chain. Its N terminus, at least initially, is methionine, as a consequence of the AUG start codon. In its amino acid sequence, it contains information specifying its conformation, as well as its ultimate cellular destination.

Question

If there is no stop codon, translation continues because there are more nucleotides at the end of mRNA past the stop codon location. The protein is not properly released from the ribosome.

Table 14.3 lists the nucleic acid signals for initiation and termination of transcription and translation.