What is the function of Exonuclease I?

Exonuclease I is an enzyme that specifically degrades single-stranded DNA, and it does so by removing nucleotides one at a time from the 3' end of the DNA strand. This characteristic makes Exonuclease I particularly useful in molecular biology applications, such as purifying PCR products by eliminating residual single-stranded DNA that might interfere with downstream processes.

In the context of the other choices, the enzyme does not target double-stranded DNA, which eliminates that possibility. It also does not degrade single-stranded DNA from the 5' end, which is a crucial detail as it highlights its specific action site. Additionally, Exonuclease I does not function to degrade RNA strands, indicating its specificity for DNA. Thus, the correct answer accurately portrays the unique role of Exonuclease I in molecular diagnostics and research settings.