What is significant about the way the nucleotides pair? how does the pairing affect the structure of the dna double helix?

Cytosine (C), thymine (T), adenine (A), guanine (G) these are the nucleotides found in DNA.

C=G and A=T

the specific name for the base pair ruling is call chargaffs rules

--The first rule states that in DNA the number of guanine units equals the number of cytosine units, and the number of adenine units equals the number of thymine units. This hinted at the base pair makeup of DNA.

The second rule was that the relative amounts of guanine, cytosine, adenine and thymine bases varies from one species to another

also C and G create a 3 hydrogen bond and A and T create a 2 hydrogen bond. these hydrogen bonds allow the double helix to form.