Molecular Biology Explained
Gene Replication Steps: A Detailed Guide to the DNA Replication Process
Gene replication steps describe how a cell copies its DNA before it divides, so each daughter cell receives the same genetic information. Cells copy genes as part of the whole chromosome, which is why most textbooks call the same series of events the DNA replication process.
Gene replication steps and the DNA replication process describe the same event: a cell copies its whole genome once before it divides, and every gene copies along with it.
Replication starts at an origin of replication and finishes when the replication forks converge and a ligase seals the last nicks.
Each DNA replication enzyme has a defined job, and the polymerases proofread the new strand as they build it.
Replication is semiconservative: every new double helix keeps one parent strand, which Meselson and Stahl showed in 1958.
Proofreading and mismatch repair cut the error rate to roughly one mistake per billion nucleotides copied.
DNA Replication Enzymes and What Each One Does
Every replication step depends on a specific protein. The DNA replication enzymes below work together at the replication fork, and a defect in any one of them stalls the copy. Researchers who study these proteins often work with recombinant proteins and antibodies.
Helicase
Unwinds the double helix at the fork. DnaB does this in bacteria, and the CMG complex does it in eukaryotes, organisms whose cells contain a membrane-bound nucleus and other specialized, membrane-bound structures. Browse our helicase proteins and antibodies.
Single-strand binding proteins
Coat exposed template strands so they stay open.
Topoisomerase
Relieves torsional strain ahead of the fork.
Primase
Synthesizes the short RNA primers that polymerases extend.
DNA polymerase α
Extends each primer with a short stretch of DNA before polymerase δ or ε takes over.
Replicative DNA polymerases
Polymerase III in bacteria and polymerases δ and ε in eukaryotes build the bulk of the new DNA and proofread as they go.
RNase H and FEN1
Remove RNA primers in eukaryotes, while DNA polymerase I does this job in bacteria.
DNA ligase
Seals the nicks between fragments.
Mismatch repair proteins
Fix errors that slip past the polymerases.
DNA Replication Steps in Order: How One Genome Becomes Two
DNA replication follows a fixed order that begins at an origin and ends when two complete copies of the genome separate.
Replication Copies Every Gene Along With the Genome
A cell does not copy a single gene by itself. It duplicates its entire genome once per cell cycle, during S phase in eukaryotes, so that every gene on every chromosome copies together. That timing gives each daughter cell a full set of genes. If you want a refresher on the molecule itself, our guide to what DNA is and our complete guide to genes cover the basics.
Semiconservative Replication and the Meselson-Stahl Experiment
Replication is semiconservative, meaning each new double helix retains one original strand and forms a new strand. Matthew Meselson and Franklin Stahl demonstrated this in 1958. They grew E. coli in a medium with heavy nitrogen (¹⁵N) and then moved the cells into a medium with light nitrogen (¹⁴N). A cesium chloride gradient let them measure the density of the DNA after each round of copying.
After one round, all of the DNA showed an intermediate density. After a second round, half of the DNA stayed intermediate, and half turned light, confirming the semiconservative model.
The Steps to Gene Replication at a Glance
The steps of DNA replication in order look like this:
Initiator proteins recognize the origin of replication.
Helicase unwinds the double helix and opens a replication fork.
Single-strand binding proteins coat the exposed template strands.
Topoisomerase relieves the torsional strain ahead of the fork.
Primase lays down short RNA primers.
DNA polymerase extends the new strands in the 5’→3′ direction.
The leading strand grows continuously, and the lagging strand grows in Okazaki fragments.
DNA polymerase replaces the RNA primers with DNA, and ligase seals the nicks.
Replication forks converge, and the cell ends up with two identical DNA molecules.
These steps generally play out as follows:
From Origin Recognition to Termination
Origin Recognition and Initiator Proteins
Replication begins at a specific sequence called an origin of replication. Most bacteria carry a single origin, such as oriC in E. coli. The initiator protein DnaA binds oriC and melts a short AT-rich stretch. DnaC then helps load the DnaB helicase onto the open DNA.
Eukaryotic chromosomes carry many origins, which lets the cell copy long chromosomes in a reasonable time. During the G1 phase, the origin recognition complex (ORC) marks each origin. Cdc6 and Cdt1 then load the MCM2-7 helicase core onto the DNA, which licenses the origin to fire. Kinases activate the licensed origins once the cell enters the S phase.
Initiation Unwinds and Stabilizes the Template
Helicase breaks the hydrogen bonds between base pairs, separating the two strands. The opened region forms a Y-shaped replication fork, and each origin produces two forks that move in opposite directions. Single-strand binding proteins (SSB in bacteria and RPA in eukaryotes) coat the exposed strands and keep them from snapping back together.
Unwinding twists the DNA ahead of the fork. Topoisomerase cuts the helix briefly and reseals it after the strain relaxes. In bacteria, DNA gyrase, a type II topoisomerase, handles this job.
Elongation Builds the New Strands
DNA polymerases cannot start a strand from scratch. Primase first synthesizes a short RNA primer that gives the polymerase a 3′ end to extend. In bacteria, DNA polymerase III then adds nucleotides to the primer in the 5’→3′ direction, and each new base pairs with its template partner (A with T and C with G). A sliding clamp, the β clamp in bacteria and PCNA in eukaryotes, keeps the polymerase attached to the template.
Eukaryotes share the same logic with different parts. DNA polymerase α works with primase to extend each primer with a short stretch of DNA. Current models assign DNA polymerase ε to the leading strand and DNA polymerase δ to the lagging strand.
The two template strands run in opposite directions, but polymerase only builds 5’→3′. The leading strand, therefore, grows continuously toward the fork. The lagging strand grows away from the fork in short pieces called Okazaki fragments, and each piece needs its own RNA primer. Bacterial fragments usually span 1,000 to 2,000 nucleotides, while eukaryotic fragments run about 100 to 200 nucleotides.
Termination Finishes and Seals the DNA
Once synthesis reaches the end of a segment, the cell removes every RNA primer and fills the gaps with DNA. In bacteria, DNA polymerase I handles both jobs. In eukaryotes, RNase H and FEN1 cut away the primers, and polymerase δ fills in the DNA. DNA ligase then seals each remaining nick with a phosphodiester bond and joins the fragments into one continuous strand.
Replication forks eventually meet. In E. coli, the Tus protein bound to Ter sites halts the approaching fork, and topoisomerase IV then separates the two interlinked circular chromosomes. Linear eukaryotic chromosomes need telomerase to rebuild their ends, which the lagging strand would otherwise shorten with each division.
Proofreading and Error Correction Protect Replication Fidelity
DNA polymerase picks the correct nucleotide with high accuracy, but base selection alone leaves roughly one error in every 10,000 to 100,000 nucleotides. Two more layers close that gap.
The first layer is proofreading. When a polymerase adds a wrong base, its 3’→5′ exonuclease activity removes the mismatched nucleotide before the polymerase continues. The second layer is mismatch repair. MutS and MutL scan the new strand in bacteria, and the MSH and MLH proteins do the same in eukaryotes. They spot the distortion and cut out the new-strand segment that holds the error, and polymerase then refills the gap.
Together, these layers bring the final error rate to about one mistake per billion nucleotides copied. That accuracy protects genomic integrity, because a human cell copies billions of base pairs each time it divides, and even a tiny error rate adds up.
Fidelity also shapes lab work. Polymerases with 3’→5′ exonuclease activity copy DNA more accurately in PCR, and our polymerase chain reaction guide explains how enzyme concentration and dNTP balance affect accuracy.
How To Replicate Genes in the Lab
Researchers copy genes in the lab by borrowing the logic that cells use. PCR swaps the helicase for heat. Synthetic primers replace primase, and a heat-stable DNA polymerase carries out extension.
The steps to gene replication in a typical PCR run look like this:
Design primers that match the sequences flanking your gene.
Mix the template DNA with a primer pair and a master mix.
Denature the DNA at approximately 94°C to separate the strands.
Anneal the primers to the template.
Extend the new strands, typically at 72°C.
Repeat the cycle, commonly 25 to 35 times. In the ideal case, each cycle doubles the amount of target sequence.
Some projects call for a plasmid instead of a PCR product. Researchers can insert an amplified gene into a vector and grow it in bacteria, which replicate the plasmid with the same machinery described above. Each of our cDNA clones undergoes gene sequencing after cloning, so you can start from a plasmid instead of building one yourself. For a broader look at how gene copying fits into biotechnology, read our guide to genetic engineering.
Find Research Reagents for Your DNA Replication Studies
Studying replication machinery or amplifying a gene? Browse our PCR kits, or search our catalog for the replication and repair proteins your project needs.
Frequently Asked Questions About Gene Replication Steps
QWhat happens if errors occur during DNA replication?
Cells repair most errors, and any that escape repair become permanent mutations. When a cell detects damage or a stalled replication fork, checkpoint signaling pauses the cell cycle so the cell can finish repair, and a cell with excessive damage may undergo apoptosis (programmed cell death). If a mismatch survives to the next division, one daughter cell inherits the original sequence and the other inherits the mutation. Inherited defects in mismatch repair genes cause Lynch syndrome, which raises cancer risk.
QWhy does the lagging strand need Okazaki fragments?
The lagging strand needs Okazaki fragments because DNA polymerase can only add nucleotides to a 3′ end, so it can only build DNA in the 5’→3′ direction. The two template strands run antiparallel, so only one of them lets the polymerase follow the fork as it opens. The polymerase copies the other template away from the fork in short stretches, and each stretch needs its own RNA primer. Reiji and Tsuneko Okazaki discovered these fragments in 1968.
QHow do eukaryotic and prokaryotic DNA replication differ?
Prokaryotic and eukaryotic DNA replication follow the same steps but differ in scale and machinery. Prokaryotes, which are single-celled organisms that do not have a distinct, membrane-bound nucleus, copy one circular chromosome from a single origin in the cytoplasm. Eukaryotes, on the other hand, copy multiple linear chromosomes from many origins inside the nucleus. A bacterial fork moves about 1,000 nucleotides per second, while a human fork moves about 50. Eukaryotes also copy DNA wrapped around histones. Eukaryotes need telomerase to protect their chromosome ends, which circular bacterial chromosomes do not require.