Question

The DNA double helix (Figure 24.30 ) at the atomic level looks like a twisted ladder, where the "rungs" of the ladder consist of molecules that are hydrogen-bonded together. Sugar and phosphate groups make up the sides of the ladder. Shown are the structures of the adenine-thymine (AT) "base pair" and the guanine-cytosine (GC) base pair: You can see that AT base pairs are held together by two hydrogen bonds and the GC base pairs are held together by three hydrogen bonds. Which base pair is more stableto heating? Why?

Short answer

GC base pairs are more stable to heating due to three hydrogen bonds compared to two in AT base pairs.

Step-by-step solution

Understanding Base Pairs

The base pairs in DNA are adenine-thymine (AT) and guanine-cytosine (GC). Each pair is connected through hydrogen bonds, a type of chemical bond that can vary in number, impacting the stability of the bond.

Counting Hydrogen Bonds

AT base pairs are connected by two hydrogen bonds, while GC base pairs are connected by three hydrogen bonds. The number of hydrogen bonds contributes to the stability of the DNA structure; more hydrogen bonds generally mean a stronger and more stable connection.

Assessing Stability

Since GC base pairs have three hydrogen bonds compared to the two in AT base pairs, GC base pairs are more stable to heating. The additional hydrogen bond in GC pairs provides additional stability, making them less susceptible to separation under heat.

Key concepts

Adenine-Thymine Bond

In the DNA structure, adenine (A) pairs with thymine (T), forming what is known as an adenine-thymine (AT) base pair. This pairing is an essential feature of the DNA double helix, ensuring genetic fidelity during replication. What's fascinating is that the AT bond relies on two specific hydrogen bonds, which is a bit like a pair of adhesive points holding the two molecules together.

Hydrogen bonds, while weaker than covalent bonds, play a critical role in stabilizing the structure of DNA. In the case of AT base pairs, these hydrogen bonds are enough to ensure stability under normal conditions but provide lesser resistance to external forces, such as heat, compared to other base pairs.

The two hydrogen bonds between adenine and thymine create a specific geometric alignment, contributing to the specific pairing and preventing thymine from pairing with other bases, keeping the genetic code accurate. This pairing is a prime example of the specificity in molecular biology where only adenine fits snugly against thymine, thanks to the complementary shape and hydrogen-bonding ability.

Guanine-Cytosine Stability

Guanine (G) and cytosine (C) form another base pair in the DNA structure, which is often abbreviated as the GC pair. Unlike the adenine-thymine pair, guanine and cytosine are connected through three hydrogen bonds. This additional bond significantly enhances the stability of the GC pair, especially under conditions involving increased temperature or physical stress.

The presence of three hydrogen bonds not only makes GC pairs more stable compared to AT pairs but also contributes to the overall robustness of the DNA double helix. This is why regions of DNA rich in GC pairs tend to have higher melting points, meaning they require more energy or heat to break apart.

• The extra hydrogen bond acts like an extra layer of glue, adding more security to the bond between guanine and cytosine.
• GC pairs are essential for the structural integrity of the DNA, especially when exposed to harsh conditions.

This increased stability renders GC-rich DNA regions more resistant to denaturation, a process where DNA loses its structure, and is crucial for DNA replication and cellular protection.

Hydrogen Bonding in DNA

Hydrogen bonding is the fundamental mechanism that holds the DNA double helix together. These bonds form between specific nitrogenous bases on opposing DNA strands, encoding the genetic information with precision. In DNA, adenine forms hydrogen bonds with thymine, and guanine does so with cytosine, dictated by the molecular geometry and electronic properties of these bases.

Hydrogen bonds are formed due to the attraction between a hydrogen atom partially positively charged, bonded to a highly electronegative atom (like nitrogen or oxygen), and another electronegative atom bearing a partial negative charge. This subtle yet strong interaction allows for the unique properties of DNA.

• The hydrogen bonds are flexible, allowing DNA to coil and uncoil as needed for biological processes, like transcription and replication.
• While flexible, they are still strong enough to hold the double helix structure together against mild disturbances.

Altogether, hydrogen bonds play a vital role, not only in creating the ladder-like structure of DNA but also in maintaining genetic stability and ensuring accurate replication during cell division.