AP BIOLOGY

THE MOLECULAR BASIS OF INHERITANCE

STRUCTURE OF NUCLEIC ACIDS

Question [CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
If a cell has 20% adenine in it, what % of cytosine must it have?
A
10%
B
20%
C
30%
D
80%
Explanation: 

Detailed explanation-1: -Since adenine is 20%, then thymine is 20% as well. The total of both is 40%. From 100 remains 60% which is divided equally between guanine and cytosine, so each is 30%.

Detailed explanation-2: -Answer: There would be 20% Cytosine. Adenine (30%) = Thymine (30%) Guanine (20%) = Cytosine (20%)

Detailed explanation-3: -If you have 20% of adenine than you have 20% of thymine, because the amount of adenine and thymine is equal. 20% plus 20% is 40% of adennine and thymine. From 100% DNA bases subtract 40% and you will get 60%.

Detailed explanation-4: -Since cytosine and guanine are always present in equal numbers, the percentage of guanine molecule is 30%.

Detailed explanation-5: -Hence, the sum of the purine residues equals the sum of the pyrimidine residues; i.e. A + G = T + C. Here, [A] = 30%, therefore, % of [T] is also 30%. Thus, [G]+[C] = 100-60 = 40%.

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