Adenine forms two covalent bonds with thymine; guanine forms three covalent bonds with cytosine.

Adenine forms three covalent bonds with thymine; guanine forms two covalent bonds with cytosine.

Adenine forms three hydrogen bonds with thymine; guanine forms two hydrogen bonds with cytosine.

Adenine forms two hydrogen bonds with thymine; guanine forms three hydrogen bonds with cytosine.

Adenine forms two hydrogen bonds with guanine; thymine forms three hydrogen bonds with cytosine.