Structure of Purines and Pyrimidines:

Purines and pyrimidines are the two types of nitrogenous bases found in DNA and RNA. They are the building blocks of nucleotides, which are the monomers that make up the polynucleotide chains in DNA and RNA. The structure of purines and pyrimidines are as follows:

Purines:
- Purines are double-ring structures consisting of a six-membered ring fused to a five-membered ring.
- The six-membered ring is a pyrimidine ring, while the five-membered ring is an imidazole ring.
- Adenine and guanine are the two purines found in DNA and RNA.

Pyrimidines:
- Pyrimidines are single-ring structures consisting of a six-membered ring.
- Cytosine, thymine, and uracil are the three pyrimidines found in DNA and RNA.
- Thymine is found only in DNA, while uracil is found only in RNA.

Why the distance between two nucleotide chains in DNA remains almost constant?

The distance between two nucleotide chains in DNA remains almost constant due to several factors, including:

Hydrogen Bonds:
- Hydrogen bonds form between the nitrogenous bases of complementary nucleotides on opposite strands of DNA.
- Adenine always pairs with thymine via two hydrogen bonds, while guanine always pairs with cytosine via three hydrogen bonds.
- The hydrogen bonds hold the two strands of DNA together, maintaining a constant distance between them.

Base Stacking:
- Base stacking occurs due to the hydrophobic interactions between the nitrogenous bases of adjacent nucleotides on the same strand of DNA.
- The bases stack on top of each other, forming a stable, rigid structure.
- This stacking also helps maintain the distance between the two nucleotide chains by preventing them from getting too close together.

Sugar-Phosphate Backbone:
- The sugar-phosphate backbone of DNA is negatively charged due to the phosphate groups.
- The negative charges repel each other, keeping the two nucleotide chains from getting too close together.
- The distance between the two nucleotide chains is maintained by the length of the sugar-phosphate backbone, which is constant.

In conclusion, the distance between two nucleotide chains in DNA remains almost constant due to the hydrogen bonds between complementary nitrogenous bases, base stacking, and the repulsion between the negatively charged sugar-phosphate backbones. These factors work together to maintain the structural integrity of DNA and ensure that it can function properly.

most welcome btw dis is given in ncert in dna chapter u can read it from dere also