because a purine always bind to pyrimidine if purine purine get attached it will not maintain the uniform diameter of dna and form zig zag position or z dna

Introduction:
In DNA, adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). This pairing is known as base pairing, and it plays a critical role in maintaining the structure and function of DNA. This question seeks to understand why A=T and Cytosine triple bond to Guanine, and why it is not possible for A=G or A=C.

Explanation:

1. Base Pairing:
- Base pairing refers to the specific hydrogen bonding between complementary nitrogenous bases in DNA.
- Adenine and thymine form two hydrogen bonds between them, while cytosine and guanine form three hydrogen bonds.
- These hydrogen bonds stabilize the DNA double helix structure and ensure accurate replication and transcription.

2. Hydrogen Bonding:
- Adenine and thymine can form two hydrogen bonds due to their molecular structure.
- The nitrogenous base adenine contains an amino group (NH2) that can form hydrogen bonds, while thymine contains a carbonyl group (C=O) that can accept hydrogen bonds.
- These hydrogen bonds provide stability to the DNA structure.

3. Complementary Base Pairing:
- Adenine and thymine are complementary bases because they can form hydrogen bonds with each other.
- Similarly, cytosine and guanine are complementary bases due to the formation of three hydrogen bonds between them.
- These specific base pairings ensure that the DNA strands are held together in a stable manner.

4. A=T and Cytosine Triple Bond to Guanine:
- The molecular structure of adenine is complementary to thymine, allowing them to form two hydrogen bonds.
- Cytosine and guanine have molecular structures that enable the formation of three hydrogen bonds between them.
- These specific hydrogen bonding patterns between A and T, and between C and G, provide stability to the DNA double helix structure.

5. A=G or A=C:
- The molecular structures of adenine and guanine do not allow them to form stable hydrogen bonds with each other.
- Adenine lacks the necessary chemical groups to form three hydrogen bonds with guanine, while guanine cannot form two hydrogen bonds with adenine.
- Similarly, adenine and cytosine lack the required molecular structures to form stable hydrogen bonds.

Conclusion:
The base pairing in DNA follows specific rules to maintain the stability and structure of the DNA double helix. Adenine always pairs with thymine, forming two hydrogen bonds, while cytosine always pairs with guanine, forming three hydrogen bonds. The molecular structures of these bases determine their ability to form stable hydrogen bonds with each other. Therefore, A=G or A=C pairing is not possible due to the lack of complementary molecular structures required for stable hydrogen bonding.