4.10.4. DNA BASE PAIRES AND DUPLEX STRUCTURE

The double helix of DNA has the following features:

It contains two polynucleotide strands wound around each other. The backbone of each consists of alternating deoxyribose and phosphate groups. The phosphate group bonded to the 5'-carbon atom of one deoxyribose is covalently bonded to the 3'-carbon of the next. The two strands are "antiparallel"; that is, one strand runs 5′- to 3′- while the other runs 3′- to 5′-. The DNA strands are assembled in the 5′- to 3′- direction and, by convention, we "read" them the same way. The purine or pyrimidine attached to each deoxyribose projects in toward the axis of the helix. Each base forms hydrogen bonds with the one directly opposite it, forming base pairs (also called nucleotide pairs). 3.4 Å separates the planes in which adjacent base pairs are located. The double helix makes a complete turn in just over 10 nucleotide pairs, so each turn takes a little more (35.7 Å to be exact) than the 34 Å shown in the diagram. There is an average of 25 hydrogen bonds within each complete turn of the double helix providing a stability of binding about as strong as what a covalent bond would provide. The diameter of the helix is 20 Å. The helix can be virtually any length; when fully stretched, some DNA molecules are as much as 5 cm (2 inches!) long. The path taken by the two backbones forms a major (wider) groove (from "34 A" to the top of the arrow) and a minor (narrower) groove (the one below).

4.10.4. 1. DISCUSSION OF BASE PAIRING in DNA
This structure of DNA was worked out by Francis Crick and James D. Watson in 1953. It revealed how DNA — the molecule that Avery had shown was the physical substance of the genes  — could be replicated and so passed on from generation to generation. For this epochal work, they shared a Nobel Prize in 1962.

The rules of base pairing (or nucleotide pairing) are:

• A with T: the purine adenine (A) always pairs with the pyrimidine thymine (T)
• C with G: the pyrimidine cytosine (C) always pairs with the purine guanine (G)
• This is consistent with there not being enough space (20 Å) for two purines to fit within the helix and too much space for two pyrimidines to get close enough to each other to form hydrogen bonds between them.

But why not A with C and G with T?

• The answer: only with A & T and with C & G are there opportunities to establish hydrogen bonds between them (two between A & T; three between C & G). These relationships are often called the rules of Watson-Crick base pairing, named after the two scientists who discovered their structural basis.

The rules of base pairing tell us that if we can "read" the sequence of nucleotides on one strand of DNA, we can immediately deduce the complementary sequence on the other strand.
The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same (called Chargaff's rule). Similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same.