Nucleic acids are the chief information-bearing molecules in cells. Recall that they consist of polymers of repeating units called nucleotides that contain a sugar, a phosphate group, and a nitrogenous structure called a base. The base is the part that is variable and thus carries information. In deoxyribonucleic acid (DNA) there are four types of bases: adenine (A), cytosine (C), guanine (G), and thymine (T). The sequence of a nucleic acid polymer is defined by the order of these bases, which we can represent with a string of A's, C's, G's, and T's.
Large strands of DNA, such as are found in bacterial chromosomes, can have millions of nucleotide units. Though you might expect that the four bases would occur in roughly equal numbers in such sequences, this is often not the case. The percentage of nucleotides that are G or C, called the GC content, varies considerably among organisms and can be used to categorize and compare them. For example, Salmonella enterica typhi, the pathogenic species of Salmonella that causes typhoid fever, has a GC content of approximately 52%. The GC content of other bacteria ranges from about 25% to 75%.
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