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Some of our finest [ about ] Approved Articles Developed Articles (over 800) Share your knowledge. It's a way to achieve immortality. —Often attributed to the Dalai Lama Article of the Week [ about ] Three-dimensional model of the structure of part of a DNA double helix. Deoxyribonucleic acid (DNA) is a very large biological molecule that is vital in providing information for the development and reproduction of living things. Every living organism has its own DNA sequence that is like a unique 'barcode' or 'fingerprint'. This inheritable variation in DNA is the most important factor driving evolutionary change over many generations. But, beyond these general characteristics, what "exactly" is DNA? What are the precise physical attributes of this molecule that make its role so centrally imposing in understanding life? DNA is a long polymer of simple units called nucleotides, held together by a sugar phosphate backbone. Attached to each sugar molecule is a molecule of one of four bases; adenine (A), thymine (T), guanine (G) or cytosine (C), and the order of these bases on the DNA strand encodes information. In most organisms, DNA is a double-helix (or duplex molecule) consisting of two DNA strands coiled around each other, and held together by hydrogen bonds between bases. Because of the chemical nature of these bases, adenine always pairs with thymine and guanine always pairs with cytosine. This complementarity forms the basis of semi-conservative DNA replication — it makes it possible for DNA to be copied relatively simply, while accurately preserving its information content. [more...] New Draft of the Week [ about ] The Battle of Gettysburg, which took place July 1-3, 1863, on the outskirts of the town of Gettysburg, Pennsylvania, was the turning point in the American Civil War, and, combined with the simultaneous loss of the Mississippi River in the west, foreshadowed Confederate exhaustion and ultimate defeat. Confederate General Robert E. Lee led his Army of Northern Virginia on a raid into Pennsylvania designed to capture supplies and destroy the political will of the Union to continue the war. He unexpectedly encountered the main Union army under General George Meade. In an intensely fought three-day battle, Lee had advantages on the first two days but lost badly on the third. He was, at that point, trapped but Meade's dilatory pursuit allowed Lee to escape. The battle became a metaphor for the entire war, and a central icon of courage on both sides. It was used by President Abraham Lincoln in his Gettysburg Address to mark the birth of a new nation dedicated to equality and democracy. [more...]