Question

Identify the correct complementary base pairing among the following.
A) Adenine – Thymine and Guanine – Cytosine
B) Adenine – Guanine and Thymine- Cytosine
C) Adenine – Cytosine and Thymine- Guanine
D) Guanine- Adenine and Cytosine - Adenine

Answer 1

Four nitrogenous bases are present inside the DNA nucleotide. These bases are divided into purines and pyrimidines. Purine is often paired with pyrimidine, and vice versa.

Complete answer:
First we should know about complementary base pairing to answer this question. Adenine, guanine, cytosine, and thymine are the four bases of DNA nucleotides. These bases are divided into purine and pyrimidines. Examples of purine bases are adenine and guanine. This suggests that their arrangement is a six-atom ring containing nitrogen linked with a five-atom ring containing nitrogen that combines two atoms to merge the two rings. Examples of pyrimidine bases are thymine and cytosine. A single nitrogen-containing six atom ring is made to form these bases. The rule of Chargaff, also known as the complementary base pairing rule, notes that pairs of DNA base pairs are like purine with pyrimidine and vice versa.
Now, let us find the solution from the options-
DNA base pairs are often adenine with thymine (A-T) and cytosine with guanine (C-G), according to the Chargaff rule.
For two complementary strands of DNA, there are about 20 Å. It will actually take too much space for two purines and two pyrimidines together to fit in the gap between two strands. That's why A can't bond to G and C can't bond to T.
While being purine and pyrimidine, A does not pair with C.
Hydrogen bonds help to stabilize the DNA molecule.
In that space, adenine with thymine and cytosine with guanine are the only pairs that can create hydrogen bonds. A and T create two hydrogen bonds while C and G create three hydrogen bonds. These bonds of hydrogen that bind the two strands and stable the molecule, causing the ladder-like double helix to form.

Thus, the correct answer is option (A) Adenine – Thymine and Guanine – Cytosine.

Note: This rule is named after scientist Erwin Chargaff, who found that among nearly all DNA molecules there are approximately equal amounts of adenine and thymine, and also guanine and cytosine. These ratios differ from one organism to another, but the exact concentrations of A are always basically the same as T and are always the same as G and C.