Molecular Genetics

Characteristics used to define Origins: The position on the DNA at which replication start points (see right) are found. A DNA sequence that when added to a non-replicating DNA causes it to replicate. A DNA sequence whose mutation abolishes replication. A DNA sequence that in vitro is the binding target for enzyme complexes known to function in initiation of DNA replication.

Replication start points In synchronoulsy replicating cells, the levels of many enzymes increase in pseudo-quantum fashion (left). The timing of the leaps depend on the location of the gene in the genetic map. In synchronously replicating cells, pulse-chase and density transfer experiments can reveal which sequences replicate earliest. For small replicons, electron microsopy can identify the replication start point. Probing leading and lagging strands with strand-specific probes can also identify replication start points. Probing 2-D gels can identify restriction fragments containing replication start points.

Interpretations

• Replication start points are not necessarily the same as sequences that are required for replication initiation.
• Fiber and electron microscopy methods observe anonymous pieces of DNA and can not be used directly to identify specific replication start points.
• Quantum increases in enzyme levels are not correlated with position on the overall genetic map in organisms containing multiple replicons.