| 摘要 |
DNA ladders are constructed by partial restriction digestion of plasmids with the following design properties: (a) the plasmid contains five or more sites of a restriction endonuclease; (b) the length between adjacent sites is an integer multiple of a minimal length, said minimal length is the length between the closest adjacent sites; (c) partial restriction endonuclease digestion of the plasmid generates a DNA ladder with at least five differently sized DNA fragments; and (d) said DNA ladder has the properties that (1) all DNA fragments of the ladder have lengths which are integer multiples of the minimal length; (2) the smallest DNA fragment length is the minimal length, (3) the largest DNA fragment is the length of the entire plasmid; and (4) the ladder has a DNA fragment at every integer multiple of the minimal length ranging from the minimal length to the plasmid length. This plasmid design is especially useful for making small fragment DNA ladders, such as 100 and 200 base pair ladders. In this case, the restriction sites are advantageously equally spaced from each other.
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