A scientist analyzed a segment of DNA from a human chromosome and found that the percentage of thymine molecular bases (T) was 35%. Which row in the chart below contains the correct percentages of the other molecular bases in the DNA segment?
Row
Guanine (G) Cytosine (C) Adenine (A)
15%
25%
25%
A.
25%
25%
15%
B.
15%
15%
35%
C.
35%
15%
15%
D

C.  15%  15%  35%

Explanation:

This follows the rules of base pairing.  The amount of thymine (T) equals the amount of adenine (A), and the amount of guanine (G) equals the amount of cytosine (C).

Since thymine is equal to adenine and 35% of the bases is thymine, 35% of the bases is also adenine.  This gives you a total of 70%.

35% + 35% = 70%

Now you need to figure out how much cytosine and guanine are in the bases.  Since 70% of the bases are adenine and thymine, 30% will be cytosine and guanine.

100% - 70% = 30%

Since the amount of cytosine and guanine are equal, divide 30% by 2 to figure out how much each base takes of the total.

30/2 = 15%

Cytosine and guanine each take up 15% of the molecular bases.

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