Answer:
DNA contains four types of nucleotide. These are adenine, thymine, guanine, and cytosine. Uracil is the fifth and final nucleotide but it is only present in certain forms of RNA. So back to the question nucleotides are the basic structural unit of nucleic acids.
Oxygen would be the best answer
Answer: The lock-and-key mechanism is usually associated with the complementary shapes of an enzyme with a single substrate wherein the lock that is being referred to is the enzyme and the substrate is the key. One right sized substrate (key) fits into the active site (key hole) of the enzyme (lock).