Question

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

Answer 1

A. 20

Step-by-step 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.

Answer 2

A. 20

Step-by-step explanation:

According to Chargaff's rule, the ratio of purine to pyrimidine bases in a DNA sequence must be 1:1. More specifically, the number of guanine should balance that of cytosine while the number of adenine should balance that of thymine.

Hence, if 30 nucleotides are paired with adenine and only thymine can paired with adenine, it means that the remaining base pairs which is guanine/cytosine is 20. Therefore, the remaining 20 nucleotides (out of 50) must be paired with guanine.

The correct option is A.