There are 64 codons but only 20 amino acids, so most amino acids are spelled by several synonymous codons that usually differ only in the third base. That redundancy is what makes silent mutations possible: one letter changes and the protein is untouched. The ribosome reads mRNA in consecutive triplets, so adding or deleting a single base shifts the reading frame and garbles every codon downstream. A missense mutation is far gentler: it swaps exactly one amino acid in the chain.
There are 64 codons but only 20 amino acids, so most amino acids are spelled by several synonymous codons that usually differ only in the third base. That redundancy is what makes silent mutations possible: one letter changes and the protein is untouched. The ribosome reads mRNA in consecutive triplets, so adding or deleting a single base shifts the reading frame and garbles every codon downstream. A missense mutation is far gentler: it swaps exactly one amino acid in the chain.
Related topics in Cell & Genetics