While constructing a restriction map, it is necessary to use a proper concentration of the enzyme and follow the rule as per the conditions required for obtaining a restriction digest. The fragments obtained from restriction digestion are of predictable sizes. Suppose the DNA samples are digested with Eco RI and Bam HI. One more sample gets digested with a combination of Eco RI and Bam HI. Agarose gel electrophoresis separates the fragments as per their sizes. An electrophoretic apparatus consists of agarose gel with wells for loading the DNA samples. Five consecutive wells involve marker DNA, control sample, DNA digested with Eco RI, DNA digested with Bam HI and a digest of Eco RI+ Bam HI. The DNA fragments with short length migrate faster, thereby separating them as per the sizes. Cutting a DNA with both the enzymes is known as a double restriction. It enables mapping of three restriction sites. A large fragment consisting of two Bam HI sites again gets an enzyme treatment. Hence it synthesizes partially digested fragments with a few uncut sites. The separated DNA fragments in the gel get stained using ethidium bromide for visualizing the bands under ultraviolet light. Restriction mapping sometimes leads to fragments having the same sizes. Hence measuring such fragments is difficult. Two classes of rare cutters may be helpful. Some enzymes cut with seven to eight nucleotide sequences such as Sap I and Sgf I. Another class of enzymes involves recognition of 5’-CG-3’ sequence site. An example includes Sma I enzyme. Separation of Fragments larger than 50 Kb involves orthogonal field alteration gel electrophoresis (OFAGE).