Question

What kind of intermolecular attraction holds the DNA double helix together?

Short answer

The DNA double helix is held together by hydrogen bonds and van der Waals forces.

Step-by-step solution

Identifying the Intermolecular Forces

The DNA double helix is held together by two main types of intermolecular attractions: hydrogen bonds and van der Waals forces. Hydrogen bonds occur between the nitrogenous base pairs, while van der Waals forces exist between the stacked base pairs.

Explaining Hydrogen Bonds in DNA

Hydrogen bonds form between specific base pairs: adenine (A) pairs with thymine (T) via two hydrogen bonds, and cytosine (C) pairs with guanine (G) via three hydrogen bonds. These hydrogen bonds are crucial for the specific pairing and stability of the DNA structure.

Understanding Van der Waals Forces

Van der Waals forces, also known as stacking interactions, occur between the adjacent base pairs along the same DNA strand. These weak attractive forces provide additional stability to the DNA helix by helping to maintain the helical shape and bringing the stacked bases closer together.

Key concepts

Hydrogen Bonds

Hydrogen bonds play a critical role in the structure of the DNA double helix. These bonds form between nitrogenous base pairs, specifically between adenine and thymine, and cytosine and guanine.

In the DNA double helix, adenine (A) pairs with thymine (T) through two hydrogen bonds, while cytosine (C) pairs with guanine (G) through three hydrogen bonds. This specific pairing is known as Watson-Crick base pairing, named after the scientists who discovered the double helical structure of DNA. The hydrogen bonds, despite being weak individually, collectively provide significant stability to the DNA molecule.

These bonds help ensure that the base pairs stick together, allowing DNA to maintain its distinctive double-helix shape. This pair-specific bonding is also essential for various biological processes, such as DNA replication and transcription, where the accuracy of base pairing is critical.

Van der Waals Forces

Van der Waals forces, also known as stacking interactions, are another type of intermolecular attraction that provides stability to the DNA double helix. These weak forces occur between the stacked nitrogenous base pairs along the same strand of DNA.

While individual van der Waals forces are not very strong, their collective contribution helps stabilize the overall three-dimensional structure of DNA. They play a vital role in preserving the helical shape by keeping the stacked base pairs tightly packed together. This close packing is crucial because it prevents the DNA helix from unwinding or misaligning.

These forces originate from transient induced electrical interactions between closely spaced molecules, with neighboring bases attracting each other just enough to contribute to the integrity of the DNA structure. This subtle but essential force ensures that the DNA maintains its compactness and stability throughout various biological processes.

Nitrogenous Base Pairs

Nitrogenous base pairs are the building blocks that form the rungs of the DNA ladder. The DNA double helix is composed of two strands that run in opposite directions, and the nitrogenous bases are located on the inside of the helix, protected by the sugar-phosphate backbone.

There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair specifically due to hydrogen bonding: adenine pairs with thymine, and cytosine pairs with guanine. This specific base pairing is critical for maintaining the genetic code and ensuring accurate replication and transcription.

Understanding the interactions between these base pairs is essential because they determine the genetic information carried within an organism. Each sequence of bases encodes the necessary instructions for building proteins and performing various cellular functions. The correct pairing and sequence of base pairs are vital to the proper functioning of cells and the organism as a whole.