USA Biology Olympiad (USABO) Practice Exam

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When the base composition of DNA from Mycobacterium tuberculosis was determined, if 18% of the bases were adenine, what is the G + C content?

The correct answer is: 64%

In DNA, the base pairing rules dictate that adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G). This pairing means that the amount of adenine should equal the amount of thymine, while the amount of cytosine should equal the amount of guanine. If 18% of the bases are adenine, then 18% of the bases will also be thymine. This accounts for 36% of the total base composition (18% A + 18% T). The remaining percentage of the bases must be made up of cytosine (C) and guanine (G). Since the total percentage must equal 100%, the sum of the percentages of cytosine and guanine would be: 100% - 36% = 64%. Since cytosine and guanine are equal in quantity, the G + C content will be 64%. This confirms that the correct answer is indeed 64%, indicating a significant proportion of the DNA that makes up these two bases, which are often associated with needs for DNA stability and function in organisms like Mycobacterium tuberculosis.