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DNA usually exists in a double-stranded structure, with both strands coiled together to form the characteristic double-helix. Each single strand of DNA is a chain of four types of nucleotide: adenine, cytosine, guanine, and thymine. A nucleotide consists of a phosphate and a deoxyribose sugar — forming the backbone of the DNA double helix — plus a base that points inwards. Nucleotides are matched between strands through hydrogen bonds to form base pairs. Adenine pairs with thymine and cytosine pairs with guanine.
The physical pairing of bases in DNA means that the information contained within each strand is redundant — the nucleotides on a single strand can be used to reconstruct nucleotides on a newly synthesized partner strand.
DNA strands have a directionality, and the different ends of a single strand are called the “3′ end” and the “5′ end” (these refer to the carbon atom in ribose that the next phosphate in the chain attaches to). In addition to being complementary, the two strands of DNA are antiparallel — they are orientated in opposite directions. This directionality has consequences in DNA synthesis, because DNA polymerase can only synthesize DNA in one direction by adding nucleotides to the 3′ end of a DNA strand.
The chemical structure of DNA.
