Q.Study the given molecular structure of double-stranded polynucleotide chain of DNA and answer the questions that follow. [Figure of double-stranded DNA polynucleotide chain with base pairs A-T, T-A, G-C, C-G, showing 5' and 3' ends]

The key idea here is Biochemical Bonds and the structural characteristics of DNA.

(a) How many phosphodiester bonds are present in the given double-stranded polynucleotide chain?

In a polynucleotide chain with N nucleotides, there are N-1 phosphodiester bonds. The given figure shows 4 base pairs (A-T, T-A, G-C, C-G), meaning each individual strand contains 4 nucleotides. Therefore, each strand has 4-1=3 phosphodiester bonds. For the double-stranded chain, the total number of phosphodiester bonds is 3+3=6. …

Concept and Intuition: Biochemical Bonds in DNA

DNA's structure is a marvel of molecular engineering, held together by a precise arrangement of different types of chemical bonds. Understanding these bonds is fundamental to comprehending its stability and function.

1. Phosphodiester Bonds: These are the covalent bonds that form the backbone of each individual DNA strand. They link the 5'-phosphate group of one nucleotide to the 3'-hydroxyl group of the next nucleotide, creating a strong, stable sugar-phosphate chain. These bonds are crucial for maintaining the integrity of the genetic information along a single strand.

2. Hydrogen Bonds: These are weaker, non-covalent bonds that form between complementary nitrogenous bases (Adenine with Thymine, Guanine with Cytosine) across the two DNA strands. A-T pairs form two hydrogen bonds, while G-C pairs form three. These bonds are responsible for holding the two strands together, forming the double helix. Their relative weakness allows for strand separation during replication and transcription.

3. N-glycosidic Bonds: These are covalent bonds linking the nitrogenous base to the 1'-carbon of the deoxyribose sugar within a single nucleotide.

4. Base Stacking Interactions: These are non-covalent, hydrophobic interactions between the planar rings of adjacent bases within the same strand. They contribute significantly to the overall stability of the double helix by minimizing contact between the hydrophobic bases and the aqueous environment.

Now, let's address each part of the question based on these principles.

1. Phosphodiester Bonds in the Given Double-Stranded Polynucleotide Chain

The question asks about the number of phosphodiester bonds in the given double-stranded polynucleotide chain. While the figure is not provided here, a typical representation for such a question shows a short segment of DNA. Let's assume the figure depicts a segment with 4 base pairs. This means there are 4 nucleotides on one strand and 4 nucleotides on the complementary strand, making a total of 8 nucleotides.

1. Understanding Phosphodiester Bonds: A phosphodiester bond connects the 5'-phosphate of one nucleotide to the 3'-hydroxyl of the next nucleotide in a strand.

   • If a strand has N nucleotides, it will have N-1 phosphodiester bonds. This is because the first nucleotide has a free 5'-phosphate, and the last nucleotide has a free 3'-hydroxyl, but all internal nucleotides are linked by these bonds.

2. Counting Bonds per Strand:

   • For the first strand, with 4 nucleotides, the number of phosphodiester bonds is 4 - 1 = 3.
   • For the second (complementary) strand, also with 4 nucleotides, the number of phosphodiester bonds is 4 - 1 = 3.

3. Total Phosphodiester Bonds: Since there are two strands, the total number of phosphodiester bonds in the double-stranded segment is the sum of bonds in each strand.

   • Total bonds = 3 (strand 1) + 3 (strand 2) = 6.

2. Base Pairs per Helical Turn and Distance Between Base Pairs

The structure of DNA is a double helix, and its dimensions are well-established for the most common form, B-DNA, which is found under physiological conditions.

1. Base Pairs per Helical Turn: In the standard B-DNA double helix, one complete turn of the helix spans approximately 10 base pairs. This is a characteristic feature of its helical geometry.

2. Distance Between Adjacent Base Pairs: The distance between the planes of two consecutive base pairs stacked one above the other in the helix is approximately 0.34 nm (or 3.4 Å).

3. Calculating Length of One Helical Turn (Optional but good for understanding):

   • Since there are 10 base pairs per turn and each base pair is 0.34 nm apart, the total length of one complete helical turn is: 10 base pairs/turn × 0.34 nm/base pair = 3.4 nm/turn.

3. Additional Stability to the Helical Structure of DNA …