This can clearly distinguish the four DNA letters, A, C, G and T.

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The codons were written in a four-letter alphabet, A, C, G, T, that abbreviated the names of the chemicals of which DNA is made.

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Most important, they can't read the 3 billion letters--A, C, T and G, for the nucleotides adenine, cytosine, thymine and guanine--in the human genome from beginning to end, the way one reads a book.

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Each DNA molecule in the body contains two long strands of chemical letters, or bases--A, T, C and G--that come together like a twisted ladder (a.k.a. the double helix).

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Embedded in each set of 23 chromosomes are 3 billion base pairs of DNA, known by the letters A, T, C and G, for the chemical bases adenine, thymine, cytosine and guanine.

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This one gene is the longest of the 25, 000 that make up the human genome, and it consists of 2.2 million "letters" (A, T, G and C for four different nucleic acids), which spell out the recipe for making the muscle protein.

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Every time the enzyme connects two bases--an A to a T or a C to a G--an electrically charged ion is released and detected by sensors on the machine.

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The four bases - adenine (A), guanine (G), thymine (T), and cytosine (C) - spell out the genes.

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