Features of DNA

DNA's two strands run in opposite chemical directions - if one strand runs 5'->3', the other runs 3'->5' - a property called being antiparallel; the 5' end of a strand carries a free phosphate group, while the 3' end carries a free hydroxyl (-OH) group. The angle at which the two sugar-phosphate backbones protrude from the paired bases is unequal - about 120 degrees on the narrow side and 240 degrees on the wide side - and this asymmetry creates a narrow minor groove on one edge of the helix and a wider major groove on the other. In the predominant B-form, each base pair is spaced 0.34 nm apart along the helical axis, and one complete turn of the helix spans 3.4 nm and contains 10 base pairs; the overall helix has a diameter of about 20 angstrom. The helix's stability comes from two combined sources: hydrogen bonding between complementary base pairs, and 'stacking interactions', in which successive bases in the stack interact through their electron clouds (pi-pi interactions) roughly perpendicular to the helical axis, both contributing to the double helix's overall thermodynamic stability, along with Chargaff's rule - published by Erwin Chargaff in 1949 - which states that a purine always pairs with a pyrimidine (A = T and G = C, so total purines equal total pyrimidines, A + G = T + C, and the ratio A:T equals G:C equals 1). The strong covalent phosphodiester linkages of the sugar-phosphate backbone give the DNA helix an inherent directional polarity and provide the mechanical strength and stability of the polynucleotide chain. DNA's two strands can also be described by how they are physically wound together: in plectonemic coiling, the two strands are truly wrapped around one another, so they cannot be pulled apart without breaking the entire helical structure; i …