Deoxyribonucleic acid (DNA) is the blueprint that carries genetic information from one generation to the next. Every cell in your body holds 46 chromosomes—23 from each parent—that must be faithfully duplicated before the cell can divide. The replication process is remarkably efficient, yet its duration varies dramatically between organisms.

DNA Basics: Structure

DNA is a long polymer composed of alternating sugar and phosphate backbones. Each sugar bears one of four nitrogenous bases—adenine, thymine, cytosine, or guanine—forming a unique sequence that encodes proteins. These proteins dictate your physical traits and biochemical functions.

DNA Basics: Replication

Chromosomes are compact complexes of DNA and proteins. In the heart of each chromosome lies a double‑helix of two complementary strands. Replication is semiconservative: each original strand serves as a template for a new complementary strand, producing two identical DNA molecules.

Read more about the structure, function and importance of DNA.

General Steps of DNA Replication

1. Replication fork formation. The enzyme DNA helicase unwinds the helix, creating a Y‑shaped fork.
2. Primer placement. DNA primers mark the starting points for DNA polymerase, guiding the direction of synthesis.
3. Elongation. DNA polymerase adds nucleotides complementary to the template strand, extending the new strand.
4. Termination. Once synthesis is complete, exonucleases remove primers, ligase seals nicks, and mismatch repair enzymes correct errors. Finally, telomerase adds telomeric repeats to protect chromosome ends.

Bacterial Replication

In prokaryotes such as Escherichia coli, the single circular chromosome contains ~4.7 million base pairs. Replication starts at a single origin and proceeds bidirectionally, completing in roughly 40 minutes—about 1,000 bases per second. Proofreading ensures a fidelity of one mistake per billion bases.

Eukaryotic Replication

Human cells harbor 46 chromosomes, each averaging 150 million base pairs. Replicating a single chromosome at a rate of 50 base pairs per second would take more than a month. However, eukaryotes initiate replication at dozens of origins across each chromosome, allowing simultaneous synthesis and enabling the entire genome to finish in about one hour during the S phase.

All 46 human chromosomes replicate concurrently, coordinated by the cell cycle machinery.

Read more about what a chromosome is.