DNA double helix

In 1953, the scientists James Watson and Francis Crick proposed a now-famous double-helix model for the overall three-dimensional structure of DNA, a model later confirmed directly by electron-microscope imaging. The salient structural features of the Watson-Crick DNA model are as follows. DNA consists of TWO separate polynucleotide strands, wound together around a common axis into a right-handed double helix. These two strands run in OPPOSITE chemical directions relative to one another -- they are described as antiparallel -- meaning that if one strand is read from its 5' end toward its 3' end, the second, partner strand running immediately alongside it is oriented the other way, from ITS 3' end toward ITS 5' end. Within this double-helical arrangement, the sugar-phosphate backbone of each strand lies on the OUTSIDE of the helix, while the nitrogenous bases of each strand point INWARD, into the interior of the helix, oriented roughly perpendicular to the helix's long central axis. The two strands are held firmly together, along the whole length of the double helix, by hydrogen bonding occurring specifically between paired bases on the two opposing strands -- this extensive, regular hydrogen bonding is what gives the DNA double helix its familiar 'ladder-like' internal structure, with the sugar-phosphate backbones as the ladder's two side-rails and the hydrogen-bonded base pairs as its regularly-spaced rungs. Crucially, this base-pairing is NOT random or arbitrary -- it follows a strict, universal complementary-pairing rule: adenine (A) always pairs specifically with thymine (T), forming exactly TWO hydrogen bonds between them, while guanine (G) always pairs specifically with cytosine (C), forming exactly THREE hydrogen bonds between them. Because of this strict A-T / G-C complementary pairing rule, the two strands of a DNA double helix are said to be complementary to one another: the complete base sequence of EITHER strand alone is enough to determine, unambiguously, the complete base …