Maxam-Gilbert Sequencing

In the Maxam-Gilbert, or chemical cleavage method, the template DNA is first denatured and radiolabeled at the 5' end with a Phosphorus 32. 

The sequencing reaction is run in four different reaction tubes with four different chemicals with the ability to modify different nucleotides in the DNA. 

For example, formic acid will only attack the purines, adenine or guanine, while hydrazine will only attack pyrimidines - cytosine or thymine. Similarly, chemicals like dimethyl sulfate will only attack a single nucleotide.

Piperidine is then used to cleave the DNA at the modified bases generating radiolabeled DNA fragments of different lengths.

The four reactions are then run parallelly on a gel and visualized by autoradiography. The sequence of the template DNA can then be deciphered by reading the autoradiograph from bottom to top