DNA or deoxyribonucleic acid is a double helical molecule, with its two strands connected to each other through nucleotide pairing. There are 4 different nucleotides in a DNA molecule. These include the purines: adenine and guanine, and pyrimidines: thymine and cytosine. These 4 bases are designated by symbols A, G, T and C, respectively. As a rule, adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C). It is this pairing of bases that connects the two DNA strands together. These pairing rules are used to determine the base pair sequence of a complementary strand. For example, if one given DNA strand is denoted as GGCAGTTCA, then by using these rules, we know that the complementary strand would be CCGTCAAGT.
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