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1
ACGTACGT ACG

Direction of electrophoretic movement

T

23

Fig. 5.38

Autoradiograph of a DNA sequencing gel. Samples were prepared using the Sanger dideoxy method of DNA sequencing. Each set of four samples was loaded in

to adjacent

tracks, indicated by A,C, G and T, depending on the identity of the dideoxyribonucleotide used for that sample. Two sets of samples were labelled with

35 S (1 and 3) and one

was labelled with

32 P (2). It is evident that

32 P generates darker but more diffuse bands than does

35 S, making the bands nearer the bottom of the autoradiograph easy to see.

However, the broad bands produced by

32 P cannot be resolved near the top of the autoradiograph, making it impossible to read a sequence from this region. The much sharper bands

produced by

35 S allow sequences to be read with confidence along most of the autoradiograph and so a longer sequence of DNA can be obtained from a single gel.