Question

The "melting point" of a DNA molecule is the temperature at which the double- helical strand breaks apart. Suppose you are given two DNA samples. One sample contains 45 percent \(\mathrm{C}-\mathrm{G}\) base pairs while the other contains 64 percent \(\mathrm{C}\) -G base pairs. The total number of bases is the same in each sample. Which of the two samples has a higher melting point? Why?

Short answer

The DNA sample which contains 64 percent CG base pairs would have a higher melting point. This is because CG base pairs are tied by three hydrogen bonds, requiring more energy to break and thus a higher temperature to melt.

Step-by-step solution

Understand the given information

There are two DNA samples. One has 45 percent C-G base pairs, the other has 64 percent C-G base pairs. The total number of bases is the same in each sample.

Applying knowledge about DNA structure

In DNA, A-T base pairs are linked by two hydrogen bonds while C-G base pairs are linked by three hydrogen bonds. The greater the hydrogen bonds the more energy is required to break them, indicating a higher melting point.

Compare the C-G content of the two samples

Since the DNA sample with 64 percent C-G base pairs has a higher percentage of C-G pairs than the one with 45 percent C-G base pairs, there are more hydrogen bonds to break in the former. This means it will require a higher temperature to 'melt'.

Key concepts

C-G Base Pairs

Understanding the significance of C-G base pairs is crucial in comprehending the stability of DNA molecules. C-G, or cytosine-guanine, base pairs are known for their robustness within the DNA helix. This is primarily because these pairs are linked by not one, but three hydrogen bonds, unlike adenine-thymine (A-T) pairs, which are connected by two.

When looking at a DNA sample, the percentage of C-G content plays a pivotal role in determining the melting point of the DNA. So, a DNA sample with 64% C-G base pairs would have a higher melting point than one with 45% C-G base pairs. This is due to the larger number of more robust tri-bonded C-G pairs which require more energy to separate during the denaturation process. These attributes mark C-G base pairs as a fundamental factor in the stability and integrity of the DNA structure.

DNA Structure

DNA, or deoxyribonucleic acid, serves as the blueprint for life, carrying the genetic instructions for the development of all living organisms. Its structure is often described as a double helix, resembling a twisted ladder.

The sides of the ladder are made of alternating sugar and phosphate groups, and the rungs consist of paired nitrogenous bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G). These pairings are strictly governed by the principles of complementary base pairing. In the context of melting points, a DNA structure with more C-G base pairs is inherently more stable under thermal stress, because the three hydrogen bonds in each C-G pairing make the molecule more resistant to heat-induced separation, elevating its melting point compared to a structure with fewer C-G pairs.

Hydrogen Bonds

Hydrogen bonds are a type of weak chemical bond that occur when a hydrogen atom is attracted to a highly electronegative atom, such as nitrogen or oxygen. Within the DNA molecule, hydrogen bonds are responsible for the pairing of the nitrogenous bases between the two strands of the double helix.

The number and strength of these hydrogen bonds determine how tightly the two strands are held together, and by extension, affect the melting point of DNA. The more hydrogen bonds present, the higher the temperature needed to 'melt' or separate the strands. For instance, C-G base pairs, with their three hydrogen bonds, contribute significantly to the overall stability of DNA under thermal conditions, resulting in a higher melting point for DNA strands rich in C-G pairings. This concept underscores the importance of hydrogen bonding in maintaining the structure and integrity of the DNA under various physical stresses.