Hi Thato,

the easiest way would be first to know the genome you're working on (human?) and test some cutting tools as NEBcutter (http://nc2.neb.com/NEBcutter2/) to build your restriction map. you'll thereby have length of all possible fragments.

fred

This is a strange question and do not provide enough information. But as Frederic interpret and answered, it the good method to start.

In practice, no one uses semi-log graph paper to estimate DNA sizes in an agarose gel. We simply use a purchased DNA ladder with known fragment sizes. This is a classic undergraduate laboratory exercise designed to teach you that DNA migrates as a function of size. You need to know the sizes of at least some of the bands, measure how far the migrate on an agarose gel using a ruler, and plot DNA size vs distance on semi-log graph paper. You then draw a "best fit" line on the graph and use can use the slope to calculate the sizes of other bands based on their migration distance.