System for DNA sequencing with resolution of up to 600 base pairs.

A system capable of resolving about 500 bases is of interest for sequencing of longer DNA molecules. Studies on further optimization of resolution on DNA sequencing gels were carried out. The effect of physico-chemical properties of gels and buffers on resolution were tested, e.g. ionic strength and pH of buffers, different buffer systems, acrylamide concentration, crosslinker concentration, type of crosslinker, temperature of polymerization, denaturing conditions, gel length and thickness. Tested were as well different running conditions like electric field, gel temperature, dimension of sample slots. Gels 0.1-0.2 mm thick and up to 1.2 m long were cast and tested routinely. Gel lengths of 60-70 cm (for sequencing up to 350-400 bases) to about 100 cm (above 400 bases) are practicable. Little is gained in resolution by increasing the gel length from 1 to 1.2 m. Resolution was improved using 0.1 mm thick gels, at a higher pH value of 8.6-8.8, and molarity increased to 0.2 M. The sequencing pattern in the region of higher bases could be better resolved on a twice-magnified picture of that region on the autoradiogram. With the long gels (70-120 cm), it is advantageous to obtain the sequence overlap by running in parallel gels of different concentrations, without re-application of samples, all loaded at the same time. Buffer chamber for running of two of three gels and thermostating plates up to 1.2 m long were designed. In this way four to six thermostated gels can be run from a power supply with two inputs. Three 1 m long gels (concentrations: 4%, 6%, 12-16%) are loaded with several samples of DNA to be sequenced and run in parallel without re-application of the samples. With good samples, the sequence overlap from the gels could be counted up to 500 base pairs, with exceptionally good samples closer to 600 bases. At present this number seems to be near the limit of the resolving power of the polyacrylamide gels.