Molecular Genetics

Local unwinding (melting) of the DNA duplex must precede replication initiation to expose a single-stranded region of the DNA.

Facts

One way that melting can be achieved is by winding double-stranded DNA around a protein core. E. coli DNA is naturally negatively supercoiled (underwound). In E. coli, a protein complex has the ability to bind to a specific region (ori). It is thought to wrap DNA around it, causing a local shift from underwinding to unwinding. The complex also has affinity for other proteins. These proteins are responsible for creating primers for DNA synthesis. The minimal E. coli origin of replication consists of 245 bp and contains 11 repeats of GATC.

Interpretations

• Binding induces positive supercoils in the DNA. To compensate for the increased positive supercoiling, adjacent areas of DNA experience an increased density of negative supercoiling. That increase provides enough energy to melt the duplex locally.

Further information

• An unwinding complex has been shown to be involved in SV40 and ARS replication origins.

This is page 1326 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999