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A DNA ladder is a solution of DNA molecules of different lengths used in agarose gel electrophoresis. It is applied to an agarose gel as a reference to estimate the size of unknown DNA molecules. In addition it can be used to approximate the mass of a band by comparing the intensity to bands in the ladder to the band of interest.
Different DNA ladders are commercially available depending on expected DNA length. The 1kb ladder with fragment ranging from about 0.5 kbp to 10 or 12 kbp and the 100 bp ladder with fragments ranging from 100 bp to just above 1000 bp are the most frequent. DNA ladders are often produced by a suitable restriction digest of a plasmid. There are special DNA ladders for supercoiled DNA and RNA.
For example, a λ DNA-HindIII Digest: a common lambda DNA ladder that has band sizes (in base pairs) of 23,130; 9,416; 6,557; 4,361; 2,322; 2,027; 564; and 125 bp