The purine hetrocycle occurs mainly in the structure of DNA. Identify number of 'N' atoms: having localised lone pair of electron.

Three groups surround N′, causing hybridization. All nitrogen atoms in purine are hybridised.
This is due to the presence of three groups around the two ′ N ′ atoms in the six membered ring and the one ′ N ′ atom in the five membered ring.