CoachA protein is a chain of amino acids, from a few dozen to several thousand long, drawn from a set of twenty kinds. The sequence — the order of the amino acids — is what the gene specifies; once made, the chain folds into a definite three-dimensional shape determined by that sequence, and the shape determines the function. Change one amino acid and the fold, and the function, may change.
Examples
Example 14.8 (Four mutations, four outcomes)
Coding strand ATG CCT GAA TTC TAG: mRNA AUG CCU GAA UUC UAG, protein Met–Pro–Glu–Phe.
ATG CCC GAA TTC TAG: CCC is still Pro — silent.ATG CCT GCA TTC TAG: GCA is Ala — Met–Pro–Ala–Phe, missense.ATG CCT TAA TTC TAG: UAA is stop — Met–Pro, nonsense, a truncated protein.ATG CCAT GAA TTC TAG: an inserted A shifts the frame:AUG CCA UGA… Met–Pro–stop — frameshift.
Example 14.10 (Speed and yield)
A ribosome adds 5 to 20 amino acids per second: a protein of 300 amino acids takes under a minute. Ribosomes follow each other on an mRNA about 80 nucleotides apart, so a message of 900 nucleotides carries a dozen at once; over its life of a few hours a single mRNA produces a thousand protein molecules. A cell of E. coli makes some protein molecules per second; a red blood cell’s precursor, some haemoglobin molecules in its lifetime.