Chapter 26, Problem 26.43AP

Interpretation Introduction

Interpretation:

Intermolecular attraction that holds the DNA double helix together has to be interpreted.

Concept Introduction:

Composition of nucleic acid: Nucleic acid is a polymer of nucleotides. Each nucleotide has three parts: a sugar, a nitrogenous base, and a phosphate group. Two nucleotides are joined by phosphate diester linkage where a free phosphate on 5’ carbon of one nucleotide and a free –OH group on 3’ carbon of another nucleotide.

Nitrogenous bases: Five types of nitrogenous bases (has unique one-letter code A, G, T, U, and C) are derived from two parent compounds called purine and pyrimidine. The purine derivatives are Adenine and Guanine, which are two fused nitrogen containing rings. The pyrimidine derivatives are Thymine, Cytosine, and Uracil, which are nitrogen-containing six-membered ring.

Base pairing in DNA: The two strands of the DNA double helix run in the opposite directions- one in 5’ to 3’ direction and other from 3’ to 5’ direction. The hydrogen bonding between two strands enhances the stability of the DNA; where the alignment of hydrophobic nitrogenous bases in the interior and hydrophilic phosphate and sugar groups on the exterior also enhance the stability. Adenine and thymine gives a pair forming two hydrogen bonds and cytosine and guanine gives rise to another pair forming three hydrogen bonds.

Hydrogen bonding: The electrostatic attraction exists between high electronegative atom and hydrogen atom.