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DNA sequencing. A scientist marks reference points on an autoradiogram. These bands correspond to the base-pair (nucleotide) sequence of a section of DNA following its separation into fragments by the process of gel electrophoresis. Each group of four banded lines represents the nucleotide sequence of A-G-C-T (Adenine-Guanine-Cytosine-Thymine). Nucleotides are the structural and functional units of DNA. From this map the sequence of nucleotides can be determined in a strand of DNA, in order to find the structure of genes. (KEYSTONE/SCIENCE PHOTO LIBRARY/ROBERT LONGUEHAYE, NIBSC)