Question

Explain why increasing the NaCl concentration increases the temperature at which the two strands of DNA “melt” apart.

Short answer

The positively charged backbones of DNA are neutralized by a solution rich in Na⁺ ions at high NaCl concentrations. Thus, more energy is required to separate them and hence the melting temperature of the two strands is high.

Step-by-step solution

Introduction

A helical strand formed of complementary nucleotide base pairs makes up a deoxyribonucleic acid (DNA)molecule. A DNA molecule is made up of two strands that are wrapped around each other in the shape of a twisted ladder. Sugar and phosphate molecules are alternately used to make the ladder's rails. Two bases are joined together with two or three weak hydrogen bonds to form the steps.

Increasing NaCl concentration increases the temperature at which the two strands of DNA are melted.

When the concentration of NaCl rises, the temperature at which two strands of DNA "melt"apart rises. More sodium ions protect the negatively charged DNA backbones at greater NaCl concentrations, lowering electrostatic repulsions and necessitating more energy to separate the strands.

The negative charge and the concentration of sodium chloride (NaCl) are proportional, meaning that the higher the NaCl concentration, the higher the negative charge. The solution contains many Na⁺ ions, which neutralizethe negatively charged backbones of DNA at high NaCl concentrations.