Maxam–Gilbert sequencing is a method of

DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. Th ...

developed by Allan Maxam and

Walter Gilbert Walter Gilbert (born March 21, 1932) is an American biochemist, physicist, molecular biology pioneer, and Nobel laureate. Education and early life Walter Gilbert was born in Boston, Massachusetts, on March 21, 1932, the son of Emma (Cohen), a ...

in 1976–1977. This method is based on

nucleobase Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides, which, in turn, are components of nucleotides, with all of these monomers constituting the basic b ...

-specific partial chemical modification of DNA and subsequent cleavage of the DNA backbone at sites adjacent to the modified

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...

s. Maxam–Gilbert sequencing was the first widely adopted method for DNA sequencing, and, along with the Sanger dideoxy method, represents the first generation of DNA sequencing methods. Maxam–Gilbert sequencing is no longer in widespread use, having been supplanted by

next-generation sequencing Massive parallel sequencing or massively parallel sequencing is any of several high-throughput approaches to DNA sequencing using the concept of massively parallel processing; it is also called next-generation sequencing (NGS) or second-generation s ...

methods.

History

Although Maxam and Gilbert published their chemical sequencing method two years after

Frederick Sanger Frederick Sanger (; 13 August 1918 – 19 November 2013) was an English biochemist who received the Nobel Prize in Chemistry twice. He won the 1958 Chemistry Prize for determining the amino acid sequence of insulin and numerous other p ...

and Alan Coulson published their work on plus-minus sequencing, Maxam–Gilbert sequencing rapidly became more popular, since purified DNA could be used directly, while the initial Sanger method required that each read start be

cloned Cloning is the process of producing individual organisms with identical or virtually identical DNA, either by natural or artificial means. In nature, some organisms produce clones through asexual reproduction. In the field of biotechnology, c ...

for production of single-stranded DNA. However, with the improvement of the chain-termination method (see below), Maxam–Gilbert sequencing has fallen out of favour due to its technical complexity prohibiting its use in standard molecular biology kits, extensive use of hazardous chemicals, and difficulties with scale-up. Allan Maxam and Walter Gilbert’s 1977 paper “A new method for sequencing DNA” was honored by a Citation for Chemical Breakthrough Award from the Division of History of Chemistry of the American Chemical Society for 2017. It was presented to the Department of Molecular & Cellular Biology, Harvard University.

Procedure

Maxam–Gilbert sequencing requires radioactive labeling at one 5′ end of the DNA fragment to be sequenced (typically by a kinase reaction using gamma-³²P ATP) and purification of the DNA. Chemical treatment 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). For example, the

purine Purine is a heterocyclic compound, heterocyclic aromatic organic compound that consists of two rings (pyrimidine and imidazole) fused together. It is water-soluble. Purine also gives its name to the wider class of molecules, purines, which includ ...

s (A+G) are depurinated using

formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Es ...

, the

guanine Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is called ...

s (and to some extent the

adenine Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivati ...

s) are methylated by dimethyl sulfate, and the

pyrimidine Pyrimidine (; ) is an aromatic, heterocyclic, organic compound similar to pyridine (). One of the three diazines (six-membered heterocyclics with two nitrogen atoms in the ring), it has nitrogen atoms at positions 1 and 3 in the ring. The other ...

s (C+T) are hydrolysed using

hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine ...

. The addition of salt (

sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

) to the hydrazine reaction inhibits the reaction of thymine for the C-only reaction. The modified DNAs may then be cleaved by hot

piperidine Piperidine is an organic compound with the molecular formula (CH2)5NH. This heterocyclic amine consists of a six-membered ring containing five methylene bridges (–CH2–) and one amine bridge (–NH–). It is a colorless liquid with an odor de ...

; (CH₂)₅NH at the position of the modified base. 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 showing the location of identical radiolabeled DNA molecules. From presence and absence of certain fragments the sequence may be inferred.

Related methods

This method led to the Methylation Interference Assay, used to map DNA-binding sites for

DNA-binding protein DNA-binding proteins are proteins that have DNA-binding domains and thus have a specific or general affinity for DNA#Base pairing, single- or double-stranded DNA. Sequence-specific DNA-binding proteins generally interact with the major groove ...

s. An automated Maxam–Gilbert sequencing protocol was developed in 1994.

References

{{DEFAULTSORT:Maxam-Gilbert sequencing DNA sequencing Molecular biology techniques 1977 in biotechnology