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DNA transposition is the relocation or insertional copying of a defined DNA segment through the action of transposition machinery. The DNA transposon is the mobile sequence; DNA transposition is the process. In a DNA-mediated system, transposase recognizes the relevant ends and coordinates their chemical processing and transfer into target DNA. Individual systems differ in whether a donor copy is removed or retained.^[ref]

The Tn5 system reveals how two ends can be coordinated. A 2000 structure of Tn5 transposase bound to transposon-end DNA showed two protein subunits organizing the ends into catalytic positions. Each end contacted both subunits, and the structural model explained how end recognition and cleavage could involve different subunits.^[ref]

For a bookkeeping example, label a mobile segment X. An excision assay can establish that the donor junction changed from A-X-B to a repaired arrangement lacking X. That observation alone does not locate X elsewhere. A new recipient junction containing X establishes an additional event. Measuring DNA transposition therefore requires distinguishing end recognition, cleavage, target insertion and subsequent DNA repair, rather than treating any one signal as proof of every step.

P-element transposase is the enzyme encoded by an autonomous P element that supplies the catalytic machinery for its movement within DNA. The mobile element is a DNA sequence; its transposase is a protein. An engineered non-autonomous transposable element can retain the DNA features needed for recognition while depending on a separate transposase source.^[ref]

A 1986 study linked candidate proteins to activity rather than relying only on a predicted coding sequence. Immunochemical methods identified an approximately $87\mathrm{kDa}$ polypeptide associated with a transcript lacking the third intron, and a roughly $66\mathrm{kDa}$ product associated with retention of that intron. Cells producing the larger polypeptide catalyzed precise and imprecise excision of a non-autonomous transposable element. The combined expression and activity evidence supported identification of the larger protein as P-element transposase.^[ref]

A conceptual two-component comparison explains the genetic tool. One component carries the DNA to be moved; another supplies the transposase. Losing the enzyme-producing component can remove the machinery for further movement without necessarily removing a previously integrated cargo. This is a statement about separating recognition sequences from a diffusible protein, not a claim that every insertion is automatically stable under every genetic condition.

The result also illustrates why transcript structure matters. A gene can be present and expressed yet fail to produce the same active protein if its RNA splicing differs. For P-element transposase, the evidence connects the molecular form of the transcript to the size of the protein and then to a measurable DNA-rearrangement activity.

A transposase is an enzyme that catalyzes steps in DNA transposition, the movement of a transposable element between DNA locations. The mobile DNA sequence and its enzyme are distinct objects: the DNA carries recognition features, while the protein supplies catalytic and binding functions. Different DNA transposons use different reaction pathways, so transposase is a functional category rather than the name of one universal molecular machine.^[ref]

A structural example is Tn5 transposase. A 2000 study determined a $2.3\text{A˚}$ structure of the enzyme bound to transposon-end DNA. The complex was a dimer, with both protein subunits contacting each DNA molecule. Binding by one subunit and cleavage by the other showed that recognizing two DNA ends is a coordinated assembly problem.^[ref]

This enzyme–DNA assembly is a transpososome. The structure connects the geometry of the recognized ends to the active sites that process them. It does not mean that a lone protein bound to an arbitrary DNA fragment would reproduce the entire reaction.

A conceptual two-component example separates coding capacity from mobility. One DNA element contains recognizable ends but lacks a working enzyme gene. Another source supplies the compatible transposase. The first is a non-autonomous transposable element: inability to produce its own enzyme does not by itself mean that it cannot be mobilized in that cellular setting.^[ref]

Recognition, cutting, joining to a new target and subsequent host DNA repair are experimentally distinguishable events. An assay that detects only the initial cleavage establishes one part of DNA transposition, not automatically successful integration at a new site.

A transpososome is the nucleoprotein complex that brings a transposable element's DNA ends together with transposase during DNA transposition. It is an assembled reaction complex, not another name for the free enzyme or for the mobile DNA sequence. Organizing the ends allows chemical reactions at separated positions to be coordinated.

A 2000 X-ray crystallography study determined a Tn5 transposase–DNA-end complex at $2.3\text{A˚}$ resolution. Two protein subunits bound the end DNA, with each DNA end interacting with both subunits. The structure placed the ends in the active sites and gave a framework for interpreting cleavage and strand transfer.^[ref]

For an assembly example, two ends bound separately by two enzyme molecules need not constitute the same state as two ends brought into one functional transpososome. The first observation establishes binding; the second additionally requires the appropriate organization. A mutant transposase might still bind DNA while failing to assemble or catalyze a later step. Consequently, a binding assay, an assembly measurement and an insertion assay test different properties. The Tn5 structure illustrates one molecular architecture; the definition does not require every transposition system to use precisely two subunits or the same end geometry.