A DNA molecule contains FIFTY base pairs of nucleotides. Thirty of the nucleotides are paired with adenine. How many of the nucleotides are paired with guanine? *

A. 20
B. 50
C. 40
D. 30

A. 20

Explanation:

A DNA molecule consists of two long polynucleotide chains made up of four types of nucleotides viz: Adenine, Guanine, Cytosine and Thymine. Each chain is called STRAND. The complementary base pairing rule proposed by Chargaff states that, in a DNA molecule, Adenine nucleotide will always hydrogen bond to Thymine nucleotide i.e. A-T while Cytosine nucleotide will always hydrogen bond to Guanine nucleotide i.e. G-C.

According to Chargaff, the amount of each base is equal to the amount of its complementary pair i.e. Adenine=Thymine and Guanine=Cytosine. Based on this, if 30 of the 50 nucleotides are paired with adenine, this means that Thymine nucleotides are 30 in the DNA molecule.

However, the remaining 20 (50-30) nucleotide pairs will contain a pairing of Guanine to Cytosine. This means 20 of the Cytosine nucleotides will be paired with Guanine.

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