Starting translation and building the chain

Like transcription, translation runs in three stages. In initiation, the mRNA, the first tRNA, and the two ribosomal subunits come together. In elongation, the ribosome moves toward the mRNA’s 3′ end and adds one amino acid for each codon. Termination, at a stop codon, releases the finished polypeptide.

Initiation

Assembling the starting complex takes help from proteins called initiation factors, and it takes energy. Some initiation factors supply that energy by breaking down GTP, a close relative of ATP that carries guanine in place of adenine. In bacteria, initiation runs like this:

Check yourselfA mutation changes a bacterial mRNA’s ribosomal-binding site so it no longer matches the small subunit’s rRNA. What goes wrong?

The mRNA can’t base-pair with that rRNA, so it doesn’t bind the small subunit well and is rarely translated.

How eukaryotes start differently

Eukaryotic initiation follows the same plan with three changes:

• There is no ribosomal-binding site. Instead, cap-binding proteins recognize the 5′ cap and bring the small subunit to the mRNA.
• The small subunit finds the start codon by scanning from the 5′ end toward the 3′ end. It often uses the first AUG it meets, but the bases around that AUG matter: they should fit a favored pattern (the Kozak sequence). If they stray far from it, the subunit skips ahead to a later AUG.
• The initiator tRNA carries ordinary methionine, not formylmethionine.

Once the start codon is chosen, the initiator tRNA binds it and the large subunit joins, just as in bacteria.

Elongation

During elongation, amino acids are added to the chain one at a time. Proteins called elongation factors help, again using energy from GTP. Each added amino acid takes one cycle of three steps:

The peptide bond isn’t made by a ribosomal protein. An rRNA in the large subunit catalyzes it, which makes the ribosome itself a ribozyme.

Check yourselfIs the tRNA in a ribosome’s E site still attached to the polypeptide?

No. It handed the chain off during peptidyl transfer, so it leaves empty. Only tRNAs in the A and P sites hold the growing chain.

Many ribosomes on one mRNA

A ribosome doesn’t need to finish before the next one starts. As soon as one has moved far enough along an mRNA, another can begin at the start codon behind it. A single mRNA with many ribosomes on it, each making its own copy of the polypeptide, is a polysome. Polysomes let a cell make many copies of a protein from one mRNA quickly.