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DNA codes for proteins by the sequence of nucleotides in the DNA. Each set of 3 nucleotides codes for a specific amino acid. Sometimes, a change in a nucleotide does NOT change the amino acid (and therefore, the protein) because some amino acids have more than one set of 3 nucleotides that code for them. Leucine, for example, is coded for by CTT, CTA, CTC, and CTG. A change in the last nucleotide wouldn't matter. But for others, it matters very much. CAC codes for histidine, and CAA for glucagon. Also, if a nucleotide is added or lost from a strand of DNA it alters the entire sequence, as the "frame" shifts; the "reading" of the DNA is off, and all the sets of 3 are altered.
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