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5.3: The Double Helix of DNA

Figure 5.3.1: Double Helix of 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. It could be replicated and so passed on from generation to generation. For this epochal work, they shared a Nobel Prize in 1962.

Double Helix Features

The double helix of DNA 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 separate 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).

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