Dense brushes of linear DNA polymers are assembled on a blochlp with ∼30 nm between anchorage points, amounting to a few mega-base-pairs/ μm3. In bulk solution, a barrier Incurs to conjugate more than two end-functionalized DNAs. However, such doublets bind the surface with almost equal efficiency to singlets, suggesting that extended brush buildup reduces the barrier. On-chip transcription reveals that doublets are roughly 2-fold Inefficient compared to singlets, a manifestation of the Interaction of the enzymatic machinery with the dense brush. Synthetic gene brushes made of DNA conjugates provide simple means to regulate expression on a chip.