Molecular Genetics

Facts

Nucleic acid hybridization with strand-specific probes can identify replication start sites.

	Consider an origin of bidirectional replication (OBR, left). Since lagging strands are synthesized as a series of small Okazaki fragments and leading strands are synthesized continuously, the new strand of a duplex spanning an OBR will be continuous on one side of the OBR and in fragments on the other side.
Replicating DNA is denatured and the single strands fractionated according to size. Fractionated DNA is probed by hybridization with a series of strand-specific probes spanning a putative OBR (right). The technique has been used to identify a region of the DHFR gene of CHO cells in which replication originates.

Interpretations

• OBR's can be identified by hybridization of size-fractionated single-stranded DNA with strand-specific probes.

Further information

• Genetically defined origins of replication (ori) are cis-acting genetic elements that are necessary and sufficient for initiation of replication
• In most cases, the molecularly defined OBR's and the genetically defined oris coincide. A few cases exist, however, where an ori causes bidirectional replication to initiate in an adjacent region.
• There are other methods of identifying origins.

This is page 1321 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999, 2003