Question

Study the given molecular structure of double-stranded polynucleotide chain of DNA and answer the questions that follow.

(a) How many phosphodiester bonds are present in the given double-stranded polynucleotide chain? 
(b) How many base pairs are there in each helical turn of double helix structure of DNA? Also write the distance between a base pair in a helix. 
(c) In addition to H-bonds, what confers additional stability to the helical structure of DNA?

Hint:

Phosphodiester bonds are between adjacent nucleotides. DNA has 10 base pairs per turn and 0.34 nm spacing. Stacking interactions stabilize DNA helices.

Answer 1

Step 1: Phosphodiester bonds link nucleotides within a single DNA strand. Each strand here has 6 nucleotides, so 5 bonds per strand. Therefore, in a double strand: \[ 5 + 5 = 10 \text{ phosphodiester bonds} \] Step 2: Each turn of the DNA double helix has approximately 10 base pairs. The distance between two adjacent base pairs is approximately 0.34 nm (nanometers). Thus, each helical turn spans: \[ 10 \times 0.34 = 3.4 \text{ nm} \] Step 3: Besides hydrogen bonds, base stacking interactions (hydrophobic interactions between bases) contribute additional stability to the helical structure.