Okazaki Experiment

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Okazaki Experiment – Discovery of Okazaki Fragments

Background

Before the experiment, scientists knew that DNA replicates in a 5′ → 3′ direction, but they did not understand how the lagging strand was synthesized, since it runs 3′ → 5′.

In 1968, Reiji Okazaki and Tsuneko Okazaki conducted experiments in E. coli to understand this process. Their findings proved that the lagging strand is synthesized discontinuously as short fragments, now called Okazaki fragments.

⭐ Aim of the Experiment

To determine whether DNA replication on the lagging strand is:

  • Continuous, or

  • Discontinuous (in short segments)

⭐ Method / Procedure

The Okazakis used:

1. Pulse-labelling with radioactive nucleotides

They grew E. coli cells and briefly exposed them to radioactive ³H-thymidine (tritiated thymidine).

  • Short pulse = labeled newly synthesized DNA

2. Isolation of DNA after the pulse

DNA was extracted immediately after the pulse and analyzed using sucrose density gradient centrifugation.

3. Observation

Two types of DNA fragments were detected:

  • Small radioactive fragments (~1000–2000 nucleotides)

  • After allowing time for replication, larger fragments appeared.

When the cells were allowed to continue replication for longer, the small fragments joined to form long continuous DNA strands.

⭐ Results

  1. Immediately after pulse → only small fragments were radioactive.

  2. After a chase (time allowed to extend replication) → the small fragments became joined into longer DNA molecules.

This proved that:

  • The lagging strand is synthesized as short fragments.

  • These fragments are later joined by DNA ligase.

⭐ Conclusion

The Okazaki experiment demonstrated that:

  • DNA replication is continuous on the leading strand.

  • DNA replication is discontinuous on the lagging strand, producing Okazaki fragments.

  • DNA ligase joins these fragments to form a complete strand.

⭐ Significance

✔ Clear evidence of semidiscontinuous replication
✔ Explained mechanism of lagging strand synthesis
✔ Foundation for understanding DNA polymerase function, primase activity, and ligase action

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