Use of partial digests, end labelling and hybridization in restriction mapping

The above technique of individual and double digests can be supplemented with other techniques for actual construction of maps. For instance, by permitting incomplete digestion, fragments longer than those obtained by complete digestion 'may be obtained.

These will be called partial digests. In the example used in the last section, in partial digests with enzyme A, instead of getting A-I 000, A-2100, A-1400, A-500, we may ,get A-3100, A-1400 and A-500, which will suggest that A-l000 and A-21oo lie in adjacent regions. In another partial digest if we get A-l000, A-3500 and A-500, this will suggest that A-2l00 and A-1400 are adjoining, which will mean that. A-l000 and A-14oo are found on the two sides of A-2100.

The above technique will help in accurate completion of a restriction map, but will require that we have a complete set of restriction fragments which make the entire DNA region being mapped. Once the restriction map (which is a physical map) is ready, it can be compared with the genetic map.

Although large changes, already located on the genetic map, can be easily located on the restriction map, but point mutations can not always be easily located on the restriction map, since the restriction sites often do not change due to mutation. In such a situation, one may like to determine the nucleotide sequences of individual fragments and compare them in normal and mutant individuals.