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transposon

(Science: molecular biology) small, mobile dNA sequences that can replicate and insert copies at random sites within chromosomes. They have nearly identical sequences at each end, oppositely oriented (inverted) repeats and code for the enzyme, transposase, that catalyses their insertion.

bacteria have two types of transposon, simple transposons that have only the genes needed for insertion and complex transposons that contain genes in addition to those needed for insertion.

eukaryotes contain two classes of mobile genetic elements, the first are like bacterial transposons in that dNA sequences move directly. The second class (retrotransposons) move by producing rNA that is transcribed, by reverse transcriptase, into dNA which is then inserted at a new site.