Scientist's hand holding DNA sequencing autoradiogram against a light box for inspection. Nucleotides are the structural & functional units of DNA. Each nucleotide possesses one of four nitrogenous bases - thymine (T), adenine (A), cytosine (C) and guanine (G). In the DNA double- helix, two strands of nucleotides are coiled and joined through base pairing; adenine always pairs with thymine and guanine with cytosine. The sequence in which these 4 types of nucleotides are arranged along the molecule determines genetic information. Here, nucleotides labelled with a radioactive marker have been separated by electrophoresis and blotted against an X-ray film.

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