DNA Structure and Replication

ID #2069

Please explain the following: In our attempts to sequence the DNA of a magnolia, you find that 40% of the dNTP's are G's and C's. What is the percentage of A's present in this sequenced DNA? The anwer given was 30 %. I know that the amount of A has to equal the amount of T, and G with C, but the book also says the amount of A and G equals the amount of T and C. And that's where I'm confused.

If the amount of G + C = 40%, then the amount of A + T has to equal 60%. Since A and T are always base paired, this 60% will be divided equally. It is also true that the % of purines has to equal the % of pyrimidines, but that doesn't change the answer to this question. If there is 20% guanine (which you know from the question itself), and 30% adenine (which you calculated to answer this question), that's 50% purines, which means there's also 50% pyrimidines.

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