Pepsin, a common enzyme found in the stomach, is most active at ph values between 0.5 and 2.5. it works slowly between ph values of 2.6 and 3.4. beyond a ph of about 3.5, it becomes completely inactive. why does this happen?

Enzymes work best within a narrow range of pH  called the optimum range and it is specific to each enzyme. Outside of this optimum range, they begin to slow down in activity and stop working altogether when the change  is extreme.

At pH 2.3  - 3.4, pepsin's activity begins to slow down because this is slightly outside its optimum range.  pH3.5  and beyond is an extreme change and pepsin becomes denatured which is the breaking of the hydrogen bonds within  protein molecules responsible for the highly ordered structure of the protein. Once pepsin loses its structure or shape it stops working.

c.) polymerase chain reaction

- "unlike the original dna fingerprinting method, dna profiling does not use restriction enzymes to cut the dna. instead it uses the polymerase chain reaction (pcr)? to produce many copies of specific str sequences. pcr is an automated procedure that generates lots of copies of a specific sequence of dna."

not only is it colder, it also becomes darker, less salty, and more dense.