Overview

The two kinds of final gene products, RNA and protein (polypeptides and peptides), are encoded polymers, and just as a gene's nucleotide sequence specifies an RNA molecule's nucleotide sequence during transcription, so does an RNA molecule's nucleotide sequence specify a polypeptide's amino acid sequence during translation, the synthesis of a polypeptide in the ribosome. The newly made polymer often undergoes enzymatic processing.

Coding in translation is less direct than in transcription, for amino acids do not pair with the nucleotides that encode them. Instead, tRNA is an intermediary, base-pairing with mRNA as it carries an amino acid to a growing polypeptide. Outside the ribosome, amino acids bond covalently to the appropriate tRNAs (aminoacylation); inside the ribosome, peptidyltransferase adds one amino acid at a time to a polypeptide chain.

This chapter breaks down protein synthesis into three phases: (1) the enzymatic aminoacylation of tRNAs; (2) translation, accomplished by ribosomes, mRNA, tRNA, and various protein factors; and (3) posttranslational processing of polypeptides (cutting, folding, covalent modification, and sometimes splicing).

Transfer of Amino Acids: A Cellular “Bucket Brigade”

A polypeptide grows by successive addition of amino acids to its carboxyl end, through the formation of peptide bonds. However, because peptides are not complementary to nucleotide chains, polypeptide synthesis requires special machinery. Each incoming amino acid is “handed off” from molecule to molecule before the peptide bond is formed. An amino acid first makes a covalent bond between its carboxyl carbon and the 5′ α phosphate of ATP.