How many hydrogen bonds are there between adenine and thymine?

2

The correct answer is option 2. 2.

In the structure of DNA, the bases form specific pairs held together by hydrogen bonds. The double helix structure of DNA is stabilized by these hydrogen bonds between complementary bases:

Adenine (A) and Thymine (T): Adenine and thymine pair with each other through the formation of two hydrogen bonds. These hydrogen bonds occur between specific atoms in the adenine and thymine molecules:

One hydrogen bond forms between the amino group at the 6th position of adenine (a hydrogen donor) and the carbonyl oxygen at the 4th position of thymine (a hydrogen acceptor).

The second hydrogen bond forms between the hydrogen at the 1st position of adenine's imine group (a hydrogen donor) and the nitrogen at the 3rd position of thymine (a hydrogen acceptor).

Cytosine (C) and Guanine (G): In contrast, cytosine and guanine form three hydrogen bonds, which makes the C-G pair slightly stronger than the A-T pair.

Here is a more detailed explanation of the hydrogen bonds between adenine and thymine:

First Hydrogen Bond: The amino group \((–NH_2)\) of adenine has a hydrogen atom that forms a hydrogen bond with the carbonyl oxygen \((=O)\) of thymine. The nitrogen atom in the amino group of adenine acts as the hydrogen bond donor, while the oxygen atom in the carbonyl group of thymine acts as the hydrogen bond acceptor.

Second Hydrogen Bond: The nitrogen atom in the imine group of adenine (located at the 1st position of the ring) forms a hydrogen bond with the hydrogen atom attached to the nitrogen at the 3rd position of thymine. In this case, the nitrogen in adenine is the hydrogen bond donor, and the nitrogen in thymine is the hydrogen bond acceptor.

These hydrogen bonds are crucial for the stability of the DNA double helix and ensure the accurate pairing of the bases during DNA replication and transcription processes.

Therefore, the correct answer is 2.