Primer-directed synthesis of a molecular weight marker.

Marker primer-directed synthesis (MPDS) is a technique that allows the easy synthesis of DNA base pair (bp) ladders. This method involves the amplification of a DNA target with two different sets of primers: a set of range primers, which will define the absolute length of the ladder, and a set of spacer primers which will determine the size difference between each one of the fragments of the DNA ladder. The ladders obtained with this technique can be used as molecular weight standards to analyse the size of DNA fragments. In this way, we have synthesized a 6 bp ladder ranging from 90 to 204 bp which results particularly useful in the typing of microsatellites in ethidium bromide (EB) or silver nitrate (SN) stained polyacrylamide gels.