Question

What is the standard conformation of each base, and what is the impact of the tautomeric forms?

I know that, due to steric constrains between the sugar and the carbonyl oxygen at the C-2 position of pyrimidines, the anti-conformation is favored by pyrimidine bases in nucleotides, while purine bases in nucleotides are found in both the syn- and anti-conformations. But, what is the impact of the tautomeric forms? I get the major concept, but I don't know how to explain this. Thank you!