A (Adenine) combines with T (thymine) by two hydrogen bonds (A = T) and G (guanine) combines with C (cytosine) by three hydrogen bonds (G = C).
Watson and Crick in their model of DNA concluded that:
Polynucleotide chain of DNA has a regular helix.
The helix has a diameter of about 20 Å, and
The helix makes a full turn at every 34 Å along its length and contains a stack of ten nucleotides per turn because the inter-nucleotide distance is 3.4 Å.
According to this model, the DNA molecule consists of two strands which are connected together by hydrogen bonds and helically twisted.
Thus, a strand of DNA molecule is a polymer of four nucleotides i.e. A (Adenine), G (Guanine), T (Thymine), C (Cytosine).
The two strands join together to form a double helix.
Bases of two nucleotides form hydrogen bonds i.e. A combines with T by two hydrogen bonds (A = T) and G combines with C by three hydrogen bonds (G = C).
However, the sequence of bonding is such that for every ATGC on one strand there would be TACG on the other strand.
Therefore, the two chains are complementary to each other i.e. sequences of nucleotides on one chain are the photocopy of sequences of nucleotides on the other chain.