Apologies.. my answer was me jotting down my thought process since even I was confused upon reading your question but let me elaborate-
Let’s take an example, adenine is a purine nitrogenous base, its heterocyclic, double ringed with nitrogen at position 1,3,7,9. I just defined the structure of a nitrogenous base.
Now, a nucleotide is the unit of DNA made of sugar, Phosphate and THIS nitrogenous base.
Now let’s take an example of Sickles cell anaemia- It is caused by single base substitution (GAG to GUG). This is a mutation in the nucleotide sequence because the chemical nature of the bases uracil and adenine are completely intact.
What is changes however is the sequence of base —- keynote sequence because fundamentally the nature of the bases are very much intact. But the nucleotide sequence is now mutated resulting in the attachment of Valine instead of glutamic acid