A researcher is studying two DNA samples: a linear DNA fragment and a closed circular plasmid. Upon treatment with Enzyme X, the linear fragment is completely degraded into nucleotides, while the circular plasmid remains intact. Conversely, treatment with Enzyme Y cleaves both the linear and circular DNA into smaller fragments. Identify the categories of Enzyme X and Enzyme Y.

Step-by-step solution

Exonucleases are enzymes that remove nucleotides only from the free ends of a DNA molecule. Because a closed circular plasmid lacks free ends, an exonuclease (Enzyme X) cannot act on it, but it will successfully degrade a linear DNA fragment from its ends. Endonucleases (Enzyme Y) make cuts at specific internal positions within the DNA duplex. They do not require free ends to function, thus they can cleave both closed circular plasmids and linear DNA into smaller fragments. Therefore, Enzyme X must be an exonucleas