When nucleotides link together in a series spiraling clockwise they form a

The nucleotides form a Double Helix (also referred to as a twisted ladder).

a physical change is being made.

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