Translation: Ribosomes, Codons and Polypeptides

From a genetic message to an amino acid chain

A protein contains amino acids arranged in a particular order. The instructions for that order come from the sequence of bases in a gene. During transcription, these instructions are copied into messenger RNA, or mRNA. Translation is the next stage: the mRNA instructions are used to assemble an amino acid chain.

In a cell with a nucleus, mRNA leaves the nucleus through a nuclear pore and enters the cytoplasm. It then attaches to a ribosome, the cellular structure where translation takes place. The mRNA provides the instructions; it does not itself become part of the amino acid chain.

Reading the message in triplets

The ribosome reads the mRNA bases in groups of three. Each group is called a codon. An amino-acid-coding codon specifies a particular amino acid, so successive codons determine which amino acids are added and in what order.

For example, a short section of mRNA could be represented as AAA | CCC. The dividing lines show two codons, not gaps in the molecule. The first triplet specifies one amino acid and the second specifies another. Reading in triplets therefore converts a sequence of bases into a sequence of amino acids.

Some codons are stop codons. Rather than specifying an amino acid, they signal that translation should end.

tRNA brings the matching amino acids

Amino acids are brought to the ribosome by transfer RNA, or tRNA. Each tRNA carries a specific amino acid and has a triplet of bases called an anticodon. The anticodon pairs with a complementary codon on the mRNA, positioning the appropriate amino acid for addition to the chain.

In RNA, A pairs with U, and C pairs with G. For example, an mRNA codon AAA pairs with a tRNA anticodon UUU. This matching links the instructions on the mRNA to the amino acid carried by the tRNA. The anticodon is a recognition sequence; it is not the amino acid itself.

Linking amino acids into a polypeptide

At the ribosome, amino acids brought by tRNA are positioned beside one another and joined by peptide bonds. As the ribosome progresses along the mRNA, further tRNA molecules bring the amino acids specified by successive codons. Each amino acid is added to the growing chain.

The resulting chain is a polypeptide. When a stop codon is reached, translation ends and the completed chain is released. The chain can then fold into the shape needed for its function as a protein.

The key connection is that the order of bases determines the order of amino acids. DNA supplies the original instructions, mRNA carries them to the ribosome, and tRNA delivers the amino acids that are linked together.