Introduction:
The DNA molecule consists of two polynucleotide chains that are twisted around each other to form a double helix structure. The distance between these two chains remains almost constant due to several factors that contribute to the stability and structure of DNA.

Hydrogen Bonding:
One of the key factors that maintain the constant distance between the two polynucleotide chains in DNA is the hydrogen bonding between the nitrogenous bases. Adenine (A) forms two hydrogen bonds with thymine (T), while guanine (G) forms three hydrogen bonds with cytosine (C). These hydrogen bonds occur between the complementary base pairs and help to hold the two strands together.

Base Pairing:
The specific base pairing in DNA, where A always pairs with T and G always pairs with C, is essential for maintaining the constant distance between the chains. This complementary base pairing ensures that the distance between the two strands remains constant as each base pair has a specific size and shape.

Phosphodiester Bond:
The backbone of each polynucleotide chain in DNA is formed by a series of phosphodiester bonds. These bonds link the sugar-phosphate groups of adjacent nucleotides. The phosphodiester bonds contribute to the stability of the DNA molecule and help to maintain the constant distance between the chains.

Twisting of the DNA:
The double helix structure of DNA involves the twisting of the two polynucleotide chains around each other. This twisting, known as supercoiling, helps to maintain the constant distance between the chains. The twisting of the DNA molecule also aids in compacting the genetic information within the cell.

Overall Stability:
The combination of hydrogen bonding, base pairing, phosphodiester bonds, and twisting of the DNA molecule ensures its overall stability. This stability is crucial for the accurate replication and transmission of genetic information during cell division and DNA synthesis.

Conclusion:
The constant distance between the two polynucleotide chains in DNA is maintained by a combination of hydrogen bonding, base pairing, phosphodiester bonds, and the twisting of the DNA molecule. These factors contribute to the stability and structure of DNA, allowing it to function as the carrier of genetic information.