In school you learn that genes have a fixed order on chromosomes. McClintock followed strange speckled patterns in maize kernels and showed that some DNA pieces cut themselves out and insert elsewhere. She called them “controlling elements” and identified two kinds, Ac (Activator) and Ds (Dissociator). When Ac is present, Ds can move and disrupt a nearby pigment gene, making spotted kernels. This mechanism acts as a switch for mutations and gene activity, potentially speeding evolution. At that time the structure of DNA was not yet clear, so many scientists doubted her claims. Her patient collection of evidence was finally recognized when molecular biology advanced during the 1950s-1970s. Today similar mobile elements are found in viruses, bacteria, and every animal or plant, and they are tools in genetic engineering and medical research.

Using advanced cytogenetic techniques such as observation of the synaptonemal complex, McClintock traced the generational movement of the Ds locus across maize chromosomes. She proposed that this movement is a “cut-and-paste” process driven by a transposase encoded by Ac, and she presented the idea at the 1950 Cold Spring Harbor Symposium. Later molecular cloning showed that Ac/Ds possesses terminal inverted repeats and leaves an 8-bp target site duplication, confirming its identity as a class II DNA transposon. The discovery introduced the concept that eukaryotic genomes are dynamic and plastic rather than fixed bead-on-a-string structures. Modern LINEs, SINEs, and LTR retrotransposons—occupying about 45 % of the human genome—are now recognized as relatives of McClintock’s elements. Transposons participate in epigenetic silencing, small-RNA pathways, and immune-receptor gene rearrangements, acting as powerful engines of evolution. They have also been harnessed in CRISPR-Cas systems and transposon-based vectors, providing core technologies for gene therapy and crop improvement. McClintock’s rigorous observations and bold interpretations reshaped the entire framework of genome science.

McClintock rigorously separated the autonomous nature of Ac from the non-autonomous Ds through cytogenetic phenographs, predicting a 14-nt deletion and an 8-bp target site duplication flanking Ac insertions. Subsequent cloning revealed that Ac encodes a TnpA transposase with the conserved DDE motif, establishing it as a prototype class II cut-and-paste transposon. The Ac/Ds system, together with MuDR and P-element, became a principal model for studies of host DSB repair and chromatin remodeling. Her description of the breakage-fusion-bridge cycle now underpins our understanding of chromosomal rearrangements common in cancer genomes. RNA-dependent DNA methylation pathways and H3K9me2-mediated silencing demonstrate, at the molecular level, the “genome stress response” concept she foresaw. Polymorphism analyses show that more than 80 % of the maize genome is transposon-derived, contributing to cis-regulatory element acquisition and alterations in 3-D genome folding. Transposon engineering has re-emerged with CRISPR-associated transposase systems and improved Sleeping Beauty and PiggyBac vectors. Enhancer trapping and conditional mutagenesis using Ac/Ds are now standard in functional genomics across numerous crops beyond model plants. Studies on cell-cycle-dependent transposition frequency and higher-order chromatin accessibility are elucidating epigenetic reprogramming triggered by insertion events. High-throughput sequencing combined with barcoding enables temporal mapping of de novo insertions, advancing quantitative models of transposon dynamics. In sum, McClintock’s insight launched the modern concept of genome plasticity and continues to guide both basic and applied biology.