Molecular Biology
DNA Replication
Making two identical DNA copies — semi-conservative, bidirectional, with proofreading
DNA replication is the process by which a cell duplicates its genome before division. Semi-conservative — each daughter molecule has one parent strand and one new strand. Bidirectional — proceeds from origin in both directions via replication forks. Key enzymes: helicase (unwinds DNA), primase (RNA primer), DNA polymerase (synthesizes new strand 5' to 3'), ligase (joins fragments). Leading strand: continuous synthesis. Lagging strand: discontinuous (Okazaki fragments). Proofreading by polymerase reduces error rate to ~1 in 10⁹ bases. Discovered semi-conservative model: Meselson-Stahl 1958.
- MechanismSemi-conservative (each daughter = 1 old + 1 new strand)
- Proven byMeselson-Stahl experiment (1958)
- Direction5' to 3' synthesis only
- Leading strandContinuous synthesis
- Lagging strandOkazaki fragments (discontinuous)
- Error rate~1 in 10⁹ after proofreading
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why replication matters
- Cell division. Required before mitosis or meiosis.
- Inheritance. Copies genome to next generation.
- Cancer. Errors and dysregulation cause cancer.
- Antibiotics. Some target bacterial replication (quinolones).
- Diagnostics. PCR mimics replication in tube.
- Aging. Telomere shortening linked to senescence.
- Genetic engineering. Foundation of biotechnology.
Common misconceptions
- Both strands made same way. Leading continuous; lagging in fragments.
- Replication is conservative. Semi-conservative (proven 1958).
- One origin per chromosome. Eukaryotes have many.
- Replication is error-free. ~1 error per 10⁹; not zero.
- DNA polymerase starts replication. Primase makes primer first.
- Replication unlimited. Telomere shortening limits divisions.
Frequently asked questions
What's semi-conservative replication?
Each daughter DNA has one strand from parent and one newly synthesized. Proven by Meselson-Stahl (1958): grew bacteria in heavy nitrogen (¹⁵N), then switched to ¹⁴N. After one generation, all DNA showed intermediate density (one heavy + one light strand). After two generations: half intermediate, half light. Confirmed semi-conservative model over conservative or dispersive models.
How does the replication fork work?
Helicase unwinds DNA; single-strand binding proteins prevent re-annealing. Topoisomerase relieves torsional strain ahead of fork. Primase makes short RNA primers. DNA polymerase adds nucleotides to primer 5' to 3'. Two replication forks proceed bidirectionally from each origin. Forks meet to complete replication.
Why discontinuous on lagging strand?
DNA polymerase only synthesizes 5' to 3'. Both strands have antiparallel polarity. Leading strand can be synthesized continuously toward fork. Lagging strand template runs opposite — synthesis must be away from fork. Solution: synthesize in short Okazaki fragments (~150 bp eukaryotes; 1000-2000 bp bacteria). Each starts with new RNA primer; later joined by ligase.
How accurate is replication?
Extremely. DNA polymerase has 3' to 5' exonuclease (proofreading) — removes wrong nucleotides. Mismatch repair after replication catches remaining errors. Combined: ~1 error per 10⁹ bases. Without proofreading: ~10⁻⁵. With repair: ~10⁻¹⁰. Cancer arises partly from accumulated mutations when systems fail.
How does it begin?
Origin of replication. Bacteria: single origin (oriC). Eukaryotes: multiple origins per chromosome (~10⁴ in humans) — necessary because chromosomes are large. Origins recognized by initiator proteins; helicase loaded; replication fork forms. Coordinated to ensure each region replicated exactly once per cell cycle.
How fast is it?
Bacteria: ~1000 bp/sec per replication fork. Bacterial genome (~5 Mb) replicated in ~40 minutes. Eukaryotes: ~50 bp/sec per fork (slower polymerase + nucleosomes); but multiple origins parallelize. Human genome (3 Gb) replicates in ~6-8 hours via thousands of forks. Replication is rate-limiting for cell division.
What about telomeres?
End of linear chromosomes. Lagging strand can't be fully replicated (need primer; loses ~50-200 bp per division). Telomerase enzyme extends telomeres in stem cells, germ cells, cancer. Most somatic cells lack active telomerase — telomeres shorten over divisions; eventually trigger senescence. Connection: aging, cancer.