DNA ladder

A nucleic acid "ladder" or molecular weight standard sample is a mix of DNA or RNA fragments with known lengths. It is used as a scale for determining the lengths of unknown nucleic acid fragments when performing gel electrophoresis experiments. The size of the fragments is determined by running a gel with the ladder in a well next to the samples with unknown lengths. The bands showing from the DNA ladder have predetermined lengths such as "100 bp", "500 bp" and others. If a band from an unknown sample has run the same distance in the gel as the 500 bp band, it can be assumed that the unknown fragment has a length close or equal to 500.

An example of a DNA ladder, using a 10 microliter sample on two percent agarose gel. The ladder appears as 8 light bands against a black background. The largest and heaviest fragment consists of 2000 base pairs and appears at the top of the ladder, the smallest fragment consists of 50 base pairs. The number of base pairs in a fragment decreases as the bands move towards the anode.

Figure 1: Figure 4: DNA ladder.

A gel electrophoresis is usually run with a "ladder" in one of the wells. Fragment lengths are given in base pairs. In this image also the concentration of the particular fragment in the "ladder" in displayed in ng/10uL.