What are Okazaki fragments?

Short stretches of DNA synthesised discontinuously on the lagging strand; they are later joined by DNA ligase into one continuous strand.

Why is a primer needed?

DNA polymerases can only extend an existing base-paired 3' end, so a short RNA primer made by primase provides the starting point for synthesis.

DNA Replication Mechanism

The step-by-step molecular choreography that unwinds the double helix and builds two faithful copies, from origin firing to the joining of the final Okazaki fragment.

Definition

The DNA replication mechanism is the coordinated set of enzymatic steps by which a cell separates the two strands of a DNA duplex at a replication fork and uses each as a template to synthesise a complementary strand in the 5'→3' direction, producing two semiconservative daughter molecules.

Scope

This topic describes how a replication fork is established and how DNA is synthesised on both template strands. It covers origin recognition and melting, primer synthesis, the antiparallel constraint that forces continuous leading-strand and discontinuous lagging-strand synthesis, the components of the replisome, and the joining of fragments into intact daughter strands. Fidelity and repair are treated only insofar as they are intrinsic to the elongation step.

Core questions

How is replication started at a specific origin rather than anywhere on the chromosome?
Why must one strand be made continuously and the other in short fragments?
Which proteins form the replisome and what does each contribute?
How are the discontinuous lagging-strand fragments joined into a continuous strand?

Key theories

Antiparallel template constraint: Because the two strands run in opposite directions and polymerases extend only 5'→3', one strand (leading) is copied continuously toward the fork while the other (lagging) is copied as discrete Okazaki fragments away from the fork.
Semiconservative fork progression: Each separated parental strand templates one new strand, so the fork yields two duplexes each containing one old and one new strand, as established experimentally by Meselson and Stahl.

Mechanisms

An initiator protein binds the origin and, with a helicase, melts the duplex to form a bidirectional fork. Single-strand binding proteins coat the exposed strands and primase synthesises short RNA primers. Replicative DNA polymerases, held to the template by sliding clamps loaded by a clamp-loader, extend the primers: the leading strand is made continuously, and the lagging strand is made as Okazaki fragments, each begun by a new primer. The primers are then removed, the gaps filled, and DNA ligase seals the remaining nicks to produce two continuous daughter duplexes.

Clinical relevance

Because the replication machinery is essential and rapidly active in dividing cells, several of its components are targets of antimicrobial and anticancer research; errors at the fork are a source of mutation. This is educational context, not clinical advice.

History

The semiconservative model, implied by the 1953 double-helix structure and confirmed by Meselson and Stahl in 1958, set the framework; Reiji Okazaki's discovery of short lagging-strand fragments and Kornberg's enzymology then revealed the discontinuous, multi-protein nature of the fork now described in textbooks.

Key figures

Arthur Kornberg
Reiji Okazaki
Matthew Meselson
Franklin Stahl

Seminal works

meselson1958
watson2013

Frequently asked questions

What are Okazaki fragments?: Short stretches of DNA synthesised discontinuously on the lagging strand; they are later joined by DNA ligase into one continuous strand.
Why is a primer needed?: DNA polymerases can only extend an existing base-paired 3' end, so a short RNA primer made by primase provides the starting point for synthesis.