Translation and Protein Function

Objective: Use a codon table with correct direction and reading frame.

A ribosome reads an mRNA in the 5′ to 3′ direction as consecutive nonoverlapping three-base codons in a chosen reading frame. Each codon specifies an amino acid or a translation stop signal in the standard code. AUG commonly serves as a start codon and specifies methionine; UAA, UAG, and UGA are stop codons and do not encode amino acids in the standard code. The code is redundant because multiple codons may specify the same amino acid, but a codon in a specified code has one conventional assignment. Variations in genetic codes exist, for example in some organelles. A DNA template triplet should not be mistaken for the matching mRNA codon.

Objective: Explain how anticodons and amino acids relate to a growing chain.

Transfer RNAs carry amino acids and present anticodons that recognize complementary mRNA codons in the ribosome. Aminoacyl-tRNA synthetases attach the appropriate amino acid to a tRNA, contributing greatly to translation accuracy. The ribosome coordinates tRNA positioning and catalyzes peptide bond formation, extending a polypeptide from its amino end toward its carboxyl end. A release factor recognizes a stop codon and helps terminate translation; no tRNA bearing an ordinary amino acid is assigned to a standard stop codon. The order of amino acids reflects the mRNA reading frame and genetic code, subject to special biological exceptions.

Objective: Distinguish sequence, folding, modification, and context.

A polypeptide is a linear sequence of amino acids; interactions among residues and the surrounding environment help it fold into structures associated with function. Many proteins also require modifications, cofactors, or assembly with other subunits. A sequence change may alter function, have little measurable effect, or affect stability depending on position and context. Cells regulate protein abundance through synthesis and degradation, so mRNA level is not a perfect proxy for protein amount. Protein structure can change dynamically, and one protein may play different roles depending on cell location or partners. Sequence-to-phenotype claims require evidence beyond simply identifying a codon.