Which enzyme is commonly isolated and used to digest DNA?

The enzyme commonly isolated and used for digesting DNA is a restriction enzyme. Restriction enzymes, also known as restriction endonucleases, are proteins that recognize specific short sequences of nucleotides in DNA and cut the DNA at these sites. This characteristic makes them invaluable tools in molecular biology, especially for applications involving cloning, genetic engineering, and DNA analysis.

Restriction enzymes are naturally occurring in bacteria, where they serve to protect against viral DNA. By cutting DNA molecules into manageable fragments, they can contribute to creating recombinant DNA molecules or facilitate the analysis of fragments by gel electrophoresis.

The other enzymes listed do not primarily function in the digestion of DNA. DNA polymerase is involved in synthesizing new DNA strands, RNA polymerase synthesizes RNA from a DNA template, and helicase unwinds the DNA double helix but does not digest it. Thus, the role of restriction enzymes in cutting and digesting DNA makes them the correct choice for this question.