PCR works readily with a DNA template of up to two to three thousand base pairs in length. However, above this size, product yields often decrease, as with increasing length stochastic effects such as premature termination by the polymerase begin to affect the efficiency of the PCR. It is possible to amplify larger pieces of up to 50,000 base pairs with a  slower heating cycle and special polymerases.  These are polymerases fused to a processivity-enhancing DNA-binding protein, enhancing adherence of the polymerase to the DNA.
Other valuable properties of the chimeric polymerases [http://www.fidelitysystems.com/TopoTaq.html TopoTaq] and PfuC2 include enhanced thermostability, specificity and resistance to contaminants and inhibitors. They were engineered using the unique helix-hairpin-helix (HhH) DNA binding domains of topoisomerase V from hyperthermophile Methanopyrus kandleri. Chimeric polymerases overcome many limitations of native enzymes and are used in direct PCR amplification from cell cultures and even food samples, thus by-passing laborious DNA isolation steps. A robust strand-displacement activity of the hybrid TopoTaq polymerase helps solve PCR problems that can be caused by hairpins and G-loaded double helices. Helices with a high G-C content possess a higher melting temperature, often impairing PCR, depending on the conditions.