The helix contains two intertwined strands of DNA. It is…

““The helix contains two intertwined strands of DNA. It is "right-handed"-twisting upward as if driven by a right-handed screw. Across the molecule, it measures twenty-three angstroms-one-thousandth of one-thousandth of a millimeter. One million helices stacked side by side would fit in this letter: o. the biologist John Sulston wrote, "We see it as a rather stubby double helix, for they seldom show its other striking feature: it is immensely long and thin. In every cell in your body, you have two meters of the stuff; if we were to draw a scaled-up picture of it with the DNA as thick as sewing thread, that cell's worth would be about 200 kilometers long." Each strand of DNA, recall, is a long sequence of "bases"-A,T,G,and C. The bases are linked together by the sugar-phosphate backbone. The backbone twists on the outside, forming a spiral. The bases face in, like treads in a circular staircase. The opposite strand contains the opposing bases: A matched with T and G matched with C. Thus, both strands contain the same information-except in a complementary sense: each is a "reflection," or echo, of the other (the more appropriate analogy is a yin-and-yang structure). Molecular forces between the A:T and G:C pairs lock the two strands together, as in a zipper. A double helix of DNA can thus be envisioned as a code written with four alphabets-ATGCCCTACGGGCCCATCG...-forever entwined with its mirror-image code.””

— Siddhartha Mukherjee