How does one gene differ from another? DNA is composed of four kinds of units called nucleotides—adenine, cystosine, guanine, and thymine (A,C,G, and T)—that are strung together like beads on a necklace in different combinations. The sequence of these nucleotides functions like a code using four different letters that are arranged in an infinite variety of spellings. The cell uses part of the DNA code to help determine what sequence of amino acids to build into a protein and another part to help determine when and where to turn on or off a protein. Each person’s genome—the complete set of
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