Editing DNA sequencing (section)

=== Maxam-Gilbert sequencing ===
{{Main|Maxam-Gilbert sequencing}}
[[Allan Maxam]] and [[Walter Gilbert]] published a DNA sequencing method in 1977 based on chemical modification of DNA and subsequent cleavage at specific bases.<ref name=Maxam77>{{cite journal | vauthors = Maxam AM, Gilbert W | title = A new method for sequencing DNA | journal = Proc. Natl. Acad. Sci. USA | volume = 74 | issue = 2 | pages = 560–64 | date = February 1977 | pmid = 265521 | pmc = 392330 | doi = 10.1073/pnas.74.2.560 | bibcode = 1977PNAS...74..560M | doi-access = free }}</ref> Also known as chemical sequencing, this method allowed purified samples of double-stranded DNA to be used without further cloning.  This method's use of radioactive labeling and its technical complexity discouraged extensive use after refinements in the Sanger methods had been made.

Maxam-Gilbert sequencing requires radioactive labeling at one 5' end of the DNA and purification of the DNA fragment to be sequenced. Chemical treatment then generates breaks at a small proportion of one or two of the four nucleotide bases in each of four reactions (G, A+G, C, C+T). The concentration of the modifying chemicals is controlled to introduce on average one modification per DNA molecule. Thus a series of labeled fragments is generated, from the radiolabeled end to the first "cut" site in each molecule. The fragments in the four reactions are electrophoresed side by side in denaturing [[acrylamide]] gels for size separation. To visualize the fragments, the gel is exposed to X-ray film for autoradiography, yielding a series of dark bands each corresponding to a radiolabeled DNA fragment, from which the sequence may be inferred.<ref name=Maxam77 />

This method is mostly obsolete as of 2023.<ref name="PubMed m584">{{cite web | title=maxam gilbert sequencing | website=PubMed | url=https://pubmed.ncbi.nlm.nih.gov/?term=maxam+gilbert+sequencing&filter=years.2013-2023&sort=pubdate}}</ref>