The enzyme which cuts down the viral dna with no harm to bacterial chromosme is

Bacteria and restriction enzymes

,use enzymes to cut (and thereby destroy) foreign DNA (such as viral DNA), which would restrict the growth of the virus; own DNA is protected in some way (often by addition of methyl group CH3) to the sequence recognized by enzyme (thus preventing enzyme from binding there)

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature