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Allan M. Maxam, with Walter Gilbert, developed a chemical approach to DNA sequencing reported in 1977. Their primary paper describes partial cleavage of terminally labeled DNA at selected bases, then size separation of labeled fragments by polyacrylamide gel electrophoresis.^[ref] Maxam-Gilbert sequencing converts chemical selectivity into a position-dependent pattern along a pre-existing strand.

The word partial is central to the measurement logic. Different molecules contribute fragments ending at different positions, while a common labeled end supplies a reference. Extensive repeated cleavage of one molecule can remove information about more distant sites, because only the fragment still containing the label is visible in the corresponding readout.

An original probability model illustrates the tradeoff without prescribing experimental conditions. Suppose the number of relevant breaks in one molecule follows a Poisson distribution with mean $\mu$. ThenAt stipulated $\mu =0.2$, about 81.9 percent have no break, 16.4 percent have one and 1.75 percent have multiple breaks. At $\mu =2$, those fractions are about 13.5, 27.1 and 59.4 percent. More cleavage does not translate monotonically into a more interpretable set of singly terminated fragments. Actual sequence-dependent chemistry need not follow this simplified independent-break model.

The measured gel is also not the sequence itself. Reading requires calibrating fragment size, identifying the labeled end and comparing the base-selective channels. Overlapping bands or an incorrect orientation can corrupt an otherwise real chemical signal. Maxam's contribution demonstrates how a carefully designed molecular measurement can expose sequence information that bulk composition alone cannot recover.

Maxam-Gilbert sequencing determines a DNA nucleotide sequence by base-selective chemical cleavage of end-labelled molecules, followed by polyacrylamide gel electrophoresis and autoradiography. Allan M. Maxam and Walter Gilbert's 1977 paper uses fragment lengths to locate attacked bases along the labelled strand. Sanger sequencing instead creates endpoints during DNA synthesis; the chemical method starts with the strand whose sequence is wanted.^[ref]

The label is crucial: each visible fragment shares one identifiable end, making its length a positional ruler. The paper allows either a 5′ or a 3′ terminal label and demonstrates separately resolved, 5′-labelled strands. Chemical modification must be partial, so that many molecules retain different cleavage positions. Destroying every susceptible site on every molecule would erase much of the distant sequence from the labelled fragments.^[ref]

The original four displays preferentially reveal guanine, preferentially reveal adenine, reveal cytosine and thymine together, and reveal cytosine alone. The purine displays use unequal reactivity rather than four perfectly exclusive reactions. Comparing parallel lanes supplies redundant information: a signal in the cytosine/thymine display but not the cytosine display identifies thymine. A sequence of four interior cleavage positions assigned A, C, T and G in order of increasing distance from a 5′ label identifies $5^{\prime }\text{-ACTG-}{3}^{\prime }$ at those positions. The nucleotide sequence of the opposite strand requires the reverse complement and its orientation.

A subtle experimental point prevents an off-by-one interpretation: the observed fragment ends immediately before the base destroyed by the chemical attack. The base assigned to a band is the attacked base, not an intact last base carried by that fragment. The 1977 example used a 64-base-pair lac operon fragment and denaturing gels; the authors read 62 bases of each strand and cross-checked the complementary sequences.^[ref]

An original Poisson distribution model illustrates why partial cleavage matters. If the number of attacks on a molecule has mean $\lambda$, thenAt $\lambda =0.1$, about $90.48\%$ are unhit, $9.05\%$ singly hit and $0.47\%$ multiply hit. At $\lambda =1$, the multiply-hit fraction rises to $26.42\%$. This independent-site model is an illustration, not a fitted description of the paper's sequence-dependent reaction rates. Gel resolution, incomplete specificity and multiple hits can all limit the recovered sequencing read.

Walter Gilbert helped turn questions about gene regulation and DNA sequence into experimentally accessible molecular measurements. He moved from physics toward molecular biology at Harvard; the Cold Spring Harbor Laboratory archive records that transition and preserves his research collection.^[ref]

With Benno Müller-Hill, Gilbert reported isolation of the lac repressor in 1966. Their assay detected binding to an inducing small molecule using equilibrium dialysis, providing a biochemical handle on a genetically inferred control protein.^[ref] The distinction between a measurable binding activity and a fully established regulatory mechanism remains important: the assay did not assume that every detail of operator recognition was already known.

With Allan M. Maxam, he later developed the chemical DNA sequencing method now called Maxam-Gilbert sequencing. Their 1977 paper described partial cleavage at selected bases in terminally labeled DNA, followed by polyacrylamide gel electrophoresis to resolve fragment lengths.^[ref] A ladder of labeled fragment lengths converts chemical specificity into information about positions along a strand.

An original schematic inference illustrates the readout. Suppose calibrated fragment-length patterns assign A to positions 1 and 6, G to positions 2 and 5, C to positions 3 and 7, and T to positions 4 and 8. Combining the position sets gives AGCTGACT in the stated reading direction. A G-selective measurement alone would leave six positions unresolved; the different chemical channels supply complementary information.

The example abstracts away migration calibration, cleavage-site offsets and band overlap. Real interpretation requires identifying the labeled end and relating each band to the appropriate base position. This sequencing approach interrogates a pre-existing labeled strand through cleavage, so its measurement logic is different from inferring sequence from terminated synthesis products. Gilbert's work exemplifies how designing a recoverable physical signal can transform an otherwise indirect molecular question.