Translation and the Genetic Code

Core Concept

Translation is the decoding of an mRNA sequence into a protein: the ribosome reads the mRNA in three-base groups called codons, and tRNA molecules deliver the matching amino acids, which are linked into a polypeptide chain. The genetic code is the set of rules mapping each codon to an amino acid (or stop signal); it is nearly universal, redundant, and read in a fixed reading frame.

Key Concepts

Codons and the genetic code

mRNA is read in codons — consecutive groups of three nucleotides. There are 4³ = 64 possible codons: 61 specify amino acids and 3 are stop codons (UAA, UAG, UGA). The code is redundant (degenerate) because most amino acids are specified by more than one codon (e.g., six codons for leucine). The code is also nearly universal — the same in almost all organisms — which is why a human gene can be expressed in bacteria.

The start codon and reading frame

AUG is the start codon: it signals the beginning of translation and codes for methionine (so new proteins begin with Met). Because codons are read in non-overlapping triplets, the mRNA has a reading frame set by the first AUG. A shift of one or two bases (a frameshift) changes every subsequent codon, usually producing a completely different, nonfunctional protein.

tRNA and the anticodon

Each tRNA carries a specific amino acid at one end and bears an anticodon — a three-base sequence complementary to an mRNA codon — at the other. By base-pairing its anticodon with a codon, the tRNA delivers the correct amino acid. An enzyme (aminoacyl-tRNA synthetase) attaches each amino acid to its matching tRNA, a step that enforces the code.

The ribosome: A, P, and E sites

The ribosome is the molecular machine that coordinates translation; it has a large and a small subunit and contains rRNA plus proteins. It has three tRNA-binding sites: the A (aminoacyl) site holds the incoming tRNA with the next amino acid; the P (peptidyl) site holds the tRNA carrying the growing polypeptide; and the E (exit) site holds the now-empty tRNA before it leaves. Peptide bonds form between the amino acid in the P site and the incoming one in the A site, catalyzed by the ribosome's rRNA (a ribozyme).

Initiation, elongation, termination

Initiation: the small ribosomal subunit, initiator tRNA (carrying Met), and mRNA assemble at the start codon; the large subunit then joins. Elongation: tRNAs enter the A site, the polypeptide is transferred to the new amino acid (forming a peptide bond), and the ribosome translocates three bases so the tRNA moves P→E and A→P, exposing the next codon. Termination: a stop codon enters the A site; a release factor binds and causes the ribosome to release the completed polypeptide and disassemble.

How It Works

The ribosome moves along the mRNA codon by codon, 5′→3′. At each codon, a tRNA whose anticodon is complementary enters the A site carrying its amino acid. The growing chain is transferred from the P-site tRNA onto the new amino acid, forming a new peptide bond. The ribosome shifts one codon, the empty tRNA exits through the E site, and the cycle repeats until a stop codon triggers release. The chain folds (often with help from chaperones) into its functional three-dimensional shape.

How it works

The ribosome moves along the mRNA codon by codon, 5′→3′. At each codon, a tRNA whose anticodon is complementary enters the A site carrying its amino acid. The growing chain is transferred from the P-site tRNA onto the new amino acid, forming a new peptide bond. The ribosome shifts one codon, the empty tRNA exits through the E site, and the cycle repeats until a stop codon triggers release. The chain folds (often with help from chaperones) into its functional three-dimensional shape.

Common confusions

• "AUG always codes for tryptophan." Wrong — AUG codes for methionine and is the start codon.
• "There is one codon per amino acid." Wrong — the code is degenerate: most amino acids have multiple codons (some have six).
• "The anticodon is the same sequence as the codon." Wrong — it is complementary (and antiparallel) to the codon.
• "The A site holds the growing chain." Wrong — the growing chain is in the P site; the A site holds the incoming aminoacyl-tRNA.
• "A stop codon codes for an amino acid." Wrong — stop codons signal termination and match no tRNA; a release factor binds instead.

Quick review

• mRNA → protein via codons (triplets).
• 61 amino-acid codons + 3 stop codons (UAA, UAG, UGA).
• AUG = start (Met); sets the reading frame.
• tRNA anticodon pairs with codon; aminoacyl-tRNA synthetase loads amino acids.
• Ribosome A, P, E sites coordinate elongation.
• Code is redundant and nearly universal.
• Frameshift = shift of reading frame (usually catastrophic).