Why do a and t (and g and
c. pair in a dna double helix?

A and T pair with each other because the adenosine molecule structure has one proton acceptor, a nitrogen, and one proton donor, a hydrogen bonded to a nitrogen; the thymine molecule has one proton acceptor, an amide functional group, and a proton donor, a hydrogen bonded to a nitrogen. The proton donors and acceptors on the adenosine and thymine need to form a covalent bond with each other. 
An adenosine or a thymine cannot bond with a cytosine or a guanine because cytosine and guanine because cytosine and guanine has two proton acceptors and one proton donor, and two proton donors and one proton acceptor, respectively.

The main role of dna in the  cell is the long-term storage of information. it is often compared to a blueprint, since it contains the instructions to construct other components of the  cell, such as  proteins  and rna molecules.the main function of rna is to carry information of amino acid sequence from the genes to where  proteins  are assembled on  ribosomes  in the  cytoplasm. mark brainliest, my work is good.

Idk little ngr try paying attention in school and you would know ngr