Question

Analysis of a double-stranded DNA molecule reveals that it contains a total of 120 purines and 120 pyrimidines. This DNA molecule could be composed of...

a. 120 thymine and 120 adenine molecules.
b. 120 adenine and 120 cytosine molecules.
c. 120 guanine and 120 thymine molecules.
d. 120 cytosine and 120 thymine molecules.
e. 120 adenine and 120 uracil molecules.
Please select the correct option.

Answer 1

This DNA molecule is composed of 120 thymine and 120 adenine molecules, as adenine pairs with thymine and both are present in equal amounts in a DNA molecule.

Step-by-step explanation:

Analysis of a double-stranded DNA molecule reveals that it contains a total of 120 purines and 120 pyrimidines. This DNA molecule could be composed of 120 thymine and 120 adenine molecules. This is because adenine (A) and guanine (G) are purines, and thymine (T) and cytosine (C) are pyrimidines. Adenine always pairs with thymine, and guanine always pairs with cytosine. Therefore, if there are 120 purines, there must be an equal number of adenine and guanine molecules. The same logic applies to pyrimidines. Given that adenine pairs with thymine, the correct composition of this DNA molecule would include an equal number of adenine and thymine molecules, making option a the correct answer.