base pairing always occurs between two pyramidines, but never between a purine and a pyrimidine

Answer 1

The statement you wrote would be an incorrect statement. Under normal conditions, a purine will always pair with a pyrimidine, and a pyrimidine with a purine.

It helps to remember that there will always be equal amounts of purines and pyrimidines. The only way for this to be true is if they always pair with it’s opposite type.

Purines are: Adenine (A) and Guanine (G)

Pyrimidines are: Cytosine (C), Thymine (T), and Uracil (U)(RNA).

In DNA...

A binds to T

C binds to G.

In RNA all Thymines are replaced with Uracil.