29. The hypochromic effect is used to estimate the melting temperature for double- stranded DNA. It
arises because:
(a) Double stranded DNA absorbs more UV light than single stranded DNA,
(b) Double stranded DNA is less colourful than single stranded DNA
(c) Double stranded DNA is more colourful than single stranded DNA
(d) Stacked bases in double stranded DNA absorb less UV light than unstacked base in single stranded
DNA

Introduction

The hypochromic effect is a well-known phenomenon in molecular biology, particularly when studying the melting temperature (Tm) of double-stranded DNA (dsDNA). This effect plays a crucial role in the field of DNA analysis, as it allows researchers to estimate the stability of DNA structures based on their UV absorption properties. In this article, we will explore the hypochromic effect, how it relates to DNA melting, and why it is used to estimate the Tm of double-stranded DNA.

What is the Hypochromic Effect?

The hypochromic effect refers to the decrease in UV absorbance observed when DNA transitions from a single-stranded state to a double-stranded form. This phenomenon occurs due to the stacking interactions between the nitrogenous bases in the double-stranded form, which reduce the absorption of UV light.

• UV Absorption Difference: Single-stranded DNA (ssDNA) absorbs more UV light than double-stranded DNA. The stacking of bases in the double helix causes the bases to be less exposed to the surrounding environment, leading to lower UV absorbance compared to their single-stranded counterparts.

• Melting Temperature: The melting temperature (Tm) of DNA is the temperature at which half of the DNA molecules become denatured, transitioning from double-stranded to single-stranded. The hypochromic effect is useful in estimating Tm because the absorbance of UV light changes as the DNA strands separate.

Why Does the Hypochromic Effect Occur?

The hypochromic effect arises because:

• Base Stacking: In double-stranded DNA, the bases are stacked in a regular structure, which restricts their ability to absorb UV light. When DNA is denatured (separated into single strands), these stacked bases become unstacked, exposing more of the aromatic rings of the bases to the environment, thereby increasing the absorption of UV light.

• UV Absorption: Single-stranded DNA has a more open structure, and its bases are more exposed to UV light. This leads to higher UV absorbance when compared to the double-stranded form.

Thus, the hypochromic effect is due to the fact that stacked bases in double-stranded DNA absorb less UV light than the unstacked bases in single-stranded DNA.

Answer to the Question

Let’s break down the options in the question:

The hypochromic effect is used to estimate the melting temperature for double-stranded DNA. It arises because:

• (a) Double stranded DNA absorbs more UV light than single stranded DNA
  This is incorrect. The opposite is true. Double-stranded DNA absorbs less UV light than single-stranded DNA due to the stacking interactions between the bases.

• (b) Double stranded DNA is less colourful than single stranded DNA
  This is not accurate. DNA’s color is not the basis for the hypochromic effect, which is related to UV absorption.

• (c) Double stranded DNA is more colourful than single stranded DNA
  This is also incorrect. The color of DNA is not relevant in the context of the hypochromic effect, which focuses on UV absorption differences.

• (d) Stacked bases in double stranded DNA absorb less UV light than unstacked bases in single stranded DNA
  This is the correct answer. The stacking interactions in double-stranded DNA reduce the exposure of the bases, causing less UV light absorption compared to single-stranded DNA.

Thus, the correct answer is:

(d) Stacked bases in double stranded DNA absorb less UV light than unstacked bases in single stranded DNA.

Conclusion

The hypochromic effect is a key concept in DNA analysis, particularly for estimating the melting temperature of DNA. It arises from the difference in UV absorbance between single-stranded and double-stranded DNA. The stacking of bases in the double-helix structure of DNA reduces its ability to absorb UV light, which is why double-stranded DNA absorbs less UV light compared to single-stranded DNA. This effect is essential for studying DNA denaturation and understanding the stability of DNA molecules.

Answer:

The correct answer is:
(d) Stacked bases in double stranded DNA absorb less UV light than unstacked bases in single stranded DNA