Answer:
Complementary pairing held by hydrogen bonds
Explanation:
A DNA molecule is polynucleotide made up of nucleotide subunits (four). The nucleotides are Adenine, Guanine, Cytosine, and Thymine. These nucleotides are arranged on the DNA in two long chains. Each of these chains is called the DNA strand. The base portions of the nucleotides are held together by hydrogen bonds. This pairing of nucleotides is responsible for the double-helical structure of DNA.
Hydrogen bonds hold together complementary base pairs i.e. each base held together is complementary to one another. Complementary base pairing is a phenomenon whereby in DNA, Adenine (A) always binds to Thymine (T), while Guanine (G) always binds to Cytosine (C).
Note that, Adenine and Guanine are purine bases while Cytosine and Thymine are pyrimidine bases. In the nucleotide base pairing in DNA, purines binds to pyrimidines and vice versa
