Where Natural Peptides Come From: Genes to Chains

Module 3 · Lesson 3 of 3 · 4 min read · Beginner

Cells make peptides and proteins by following instructions stored in DNA. The process runs in two main stages.

First comes transcription. A stretch of DNA, a gene, is copied into a matching molecule of messenger RNA. In cells with a nucleus, this RNA is processed and then moves out to the cytoplasm.

Next comes translation. Ribosomes read the RNA three letters at a time. Each three-letter group is called a codon. Most codons specify one amino acid, and a few mean stop. Transfer RNA molecules bring the matching amino acids, and the ribosome joins them with peptide bonds, one by one, from the N-terminus to the C-terminus.

Many natural peptides are not made in their final form. The ribosome first builds a longer precursor chain. Enzymes then cut that precursor at specific sites to release the finished peptide or peptides. This lets a cell store an inactive form and produce the final molecule in a controlled way.

Insulin is a classic example. The human gene encodes a precursor of 110 amino acids. A short signal sequence is removed first, leaving proinsulin. Enzymes then cut out a middle segment called the C-peptide. What remains are two chains, an A chain of 21 residues and a B chain of 30 residues, held together by disulfide bonds. UniProt lists each of these segments and their positions in the precursor.

References

1. Alberts B et al. Molecular Biology of the Cell, 4th ed., From DNA to RNA (2002)
2. Alberts B et al. Molecular Biology of the Cell, 4th ed., From RNA to Protein (2002)
3. UniProt Consortium, UniProtKB entry P01308: Insulin, Homo sapiens (database entry, accessed 2026)

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