If a section of DNA experiences base mutation at a predictable rate, then .

your answer would be These external agents of genetic change are called mutagens. Exposure to mutagens often causes alterations in the molecular structure of nucleotides, ultimately causing substitutions, insertions, and deletions in the DNA sequence.

i don't know if this is correct or not let me know please.

Explanation:

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products