What is one of the main actions helicases perform on DNA?

Helicases play a crucial role in the process of DNA replication and other cellular activities involving DNA by unwinding the DNA double helix. This unwinding is essential because the DNA double helix consists of two strands that are bound together by hydrogen bonds between the nitrogenous bases. To facilitate processes such as replication, transcription, and repair, these strands need to be separated.

The action of helicases is to impose mechanical tension on the DNA, which allows the two strands to separate, thus creating two single-stranded templates that can be used for the synthesis of new DNA strands. This activity is foundational for the proper functioning of the molecular machinery that synthesizes DNA, ensuring that each strand can be copied accurately and efficiently.

Other options, while related to DNA metabolism, describe different functions. Joining DNA fragments involves ligases, adding methyl groups pertains to the process of DNA methylation typically performed by methyltransferases, and the digestion of nucleic acids is primarily the job of nucleases, which cleave the phosphodiester bonds within the nucleic acid backbone. Thus, unwinding the DNA double helix is the sole function of helicases, making it the correct choice.