Which of the following statements about base pairing in dna is incorrect?
purines always base pairs with pyrimidines.
adenine binds to guanine.
base pairs are stabilized by hydrogen bonds.
base pairing occurs at the interior of the double helix.

Adenine binds to guanine.

Explanation:

Rules of the base pairing are:  

A with T

Purine base, adenine (A) pairs with the pyrimidine base, thymine (T).

C with G

Pyrimidine base, cytosine (C) pairs with purine base, guanine (G).

(Option A is correct).

(Option B is incorrect).

Nitrogenous bases are positioned in the inside helix structure like the rungs of a staircase due to hydrophobic effect and are stabilized by hydrogen bonding.  

(Option C and D is correct).

Adenine pairs with Thymine

Cytosine pairs with Guanine

1. amino acid refers to the fundamental building block of proteins, comprising of an amino group and a carboxyl group.

2. catalyst refers to any substance, which increases the rate of chemical reaction without being used up in the reaction.

3. a complex molecule containing he genetic code is dna or deoxyribonucleic acid.

4. an enzyme refers to a molecule of protein, which is unique in accelerating up a particular chemical reaction in the biological systems.

5. nucleic acid refers to an sequence of sugars, phosphates, and nitrogenous bases, for example, dna and rna.

6. a protein refers to a large, composite organic molecule comprising of a huge number of amino acids associated together with the of peptide bonds.

7. rna refers to a nucleic acid that takes part in the production of proteins.

8. substrate refers to a compound, which takes part in a chemical reaction, and is chemically modified by the reaction.