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Molecular Biology · Process

DNA Replication Diagram

How a replication fork copies DNA: helicase opens the helix, primase lays primers, and DNA polymerase builds a leading and a lagging strand.

Before a cell divides, it copies every chromosome. At each replication fork the two strands of the double helix are pulled apart, and each becomes the template for a new one. This is the fork as it works in a bacterium such as E. coli; eukaryotes do the same with their own versions of most of these enzymes.

Also called Replication fork or DNA synthesis.

A DNA replication fork. The parent double helix on the left is unwound by helicase, with topoisomerase ahead of it. The upper template is copied continuously towards the fork as the leading strand by DNA polymerase III; the lower template is copied away from the fork in Okazaki fragments, each started by an RNA primer from primase, with DNA polymerase I replacing primers and DNA ligase joining the fragments. Single-strand binding proteins hold the opened strands apart.5′3′3′5′5′HelicaseTopoisomeraseSingle-strandbinding proteinsPrimaseDNApolymerase IIIDNApolymerase IDNA ligaseDNApolymerase IIILeading strandLagging strandOkazaki fragmentParent DNAFork movesTemplate (parent) strandNew DNARNA primer
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Step 1 of 10

The fork at a glance

DNA is copied at a replication fork, where the double helix is opened and each parent strand becomes the template for a new one. DNA polymerase can only build a strand 5′ to 3′, and the two templates run in opposite directions, so one new strand is made in one piece and the other in short fragments.

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Step 1 of 10: The fork at a glance

Download the diagramChecked against its sources on 25 October 2026.

Step by step

The steps of the DNA Replication

  1. 01The fork at a glance

    DNA is copied at a replication fork, where the double helix is opened and each parent strand becomes the template for a new one. DNA polymerase can only build a strand 5′ to 3′, and the two templates run in opposite directions, so one new strand is made in one piece and the other in short fragments.

  2. 02Helicase opens the double helix

    Helicase moves along the DNA and unwinds it, using ATP to break the hydrogen bonds between base pairs. The two strands part into the arms of the fork.

  3. 03Topoisomerase relieves the strain

    Unwinding one part of the helix overwinds the part ahead of it. A topoisomerase, in bacteria mainly DNA gyrase, cuts the DNA backbone, lets the twist out and joins it again, so the fork can keep moving.

  4. 04Single strands are held apart

    Single-strand binding proteins coat the separated strands, so they do not snap back together or fold up on themselves before they are copied.

  5. 05Primase lays RNA primers

    DNA polymerase cannot start a strand from nothing; it needs an end to add to. Primase makes a short RNA primer, about 10 nucleotides long, on the template. The leading strand needs one; the lagging strand needs one for every fragment.

  6. 06The leading strand is made in one piece

    On the upper template, DNA polymerase III adds nucleotides to the 3′ end of the primer in the same direction as the fork moves. It can follow the fork all the way, so the leading strand grows continuously.

  7. 07The lagging strand is made in fragments

    The lower template runs the other way, so its new strand has to be built away from the fork. DNA polymerase III makes it in short Okazaki fragments, each from its own primer, starting again as the fork opens more template.

  8. 08Primers are swapped for DNA

    When a fragment reaches the primer of the one before it, DNA polymerase I takes over. It removes the RNA primer a nucleotide at a time and fills the gap with DNA.

  9. 09DNA ligase joins the fragments

    A nick is left in the backbone where two fragments meet. DNA ligase seals it by forming the missing phosphodiester bond, so the lagging strand becomes one continuous strand.

  10. 10Two helices, each half old and half new

    Each new double helix has one parent strand and one new strand, so replication is semiconservative. The polymerases proofread as they go, and mismatch repair checks the new strand afterwards; together they leave about one error in a billion bases.

Parts

What each part of the dna replication does

Parent DNA
The double helix being copied. Its two strands run in opposite directions (antiparallel), and each is read as a template for a new strand.
Replication fork
The Y-shaped region where the helix is being opened and copied. Forks move away from each origin of replication in both directions.
Helicase
Unwinds the double helix at the fork, using energy from ATP to break the hydrogen bonds between the base pairs.
Topoisomerase
Works ahead of the fork, where unwinding twists the DNA tighter. It cuts the backbone, lets the strain out and joins it again. In bacteria it is mainly DNA gyrase.
Single-strand binding proteins
Coat the opened single strands so they do not pair up again or fold back on themselves before they are copied.
Primase
Makes the short RNA primers that every new strand starts from, since DNA polymerase can only add to an existing end.
RNA primer
A short stretch of RNA, about 10 nucleotides long, that gives DNA polymerase a 3′ end to build from. Each is later replaced with DNA.
DNA polymerase III
The main copying enzyme of bacteria. It adds nucleotides to the 3′ end of a strand, so it builds 5′ to 3′, and it proofreads as it goes.
Leading strand
The new strand made in the same direction as the fork moves, so it can be built continuously from a single primer.
Lagging strand
The new strand whose template runs the other way. It is built away from the fork, a fragment at a time, and joined afterwards.
Okazaki fragment
One short piece of the lagging strand: about 1,000 to 2,000 nucleotides in bacteria and 100 to 200 in eukaryotes.
DNA polymerase I
Removes each RNA primer, a nucleotide at a time, and fills the gap with DNA. In eukaryotes other enzymes, such as RNase H and FEN1, take the primers out.
DNA ligase
Seals the nick left between two fragments by forming the last phosphodiester bond in the backbone.

Dictionary

The terms to know

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In 3D

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Sources

Where the facts come from

Drawn and written by biofideX from these sources.

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