Problem 15 - Chemistry Principles and Practice

Question

Describe the Watson and Crick model of DNA. How does this model account for the experimental fact that the \(\mathrm{A} / \mathrm{T}\) and \(\mathrm{G} / \mathrm{C}\) ratios are \(1 ?\)

Step-by-Step Solution

Answer

The Watson and Crick model describes DNA as a double helix with complementary base pairing, explaining the \(\frac{A}{T}\) and \(\frac{G}{C}\) ratios as 1 due to specific pair matching.

Step 1: Introduction to the Model

The Watson and Crick model describes DNA as a double helix, which is composed of two strands that spiral around each other. This structure is made up of nucleotides, consisting of a phosphate group, a sugar called deoxyribose, and four types of nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The model highlights how the two strands run in opposite directions, known as antiparallel orientation.

Step 2: Base Pairing Explained

According to the Watson and Crick model, the two strands of DNA are held together by hydrogen bonds between the nitrogenous bases. Specifically, Adenine (A) pairs with Thymine (T) through two hydrogen bonds, and Guanine (G) pairs with Cytosine (C) through three hydrogen bonds. This specific pairing is referred to as complementary base pairing.

Step 3: Ratio Explanation

The experimental observation that \(\frac{A}{T}\) and \(\frac{G}{C}\) ratios are always 1 is explained by the model through complementary base pairing. Since A always pairs with T and G always pairs with C, the amount of A will always equal the amount of T, and the amount of G will always equal the amount of C, leading to these ratios being equal to 1.

Key Concepts

Watson and Crick model

DNA, the molecule of life, is elegantly structured according to the Watson and Crick model. This model describes DNA as a double helix, similar to a twisted ladder. Two strands of DNA spiral around each other, forming a stable structure. Each strand is made up of building blocks called nucleotides. These nucleotides contain three components: a phosphate group, a sugar called deoxyribose, and one of four nitrogenous bases - Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

The two strands of the DNA helix run in opposite directions, which is referred to as an antiparallel orientation. This antiparallel setup is crucial for the stability and functionality of DNA in carrying genetic information. Understanding this model provides insights into how genetic information is stored and transferred from one generation to the next.

nucleotide pairing

In DNA, the sequence of nucleotides determines the genetic instructions. Nucleotide pairing is key to understanding DNA's structure and function. In the Watson and Crick model, each base on one strand pairs with a specific base on the opposite strand. This is known as complementary base pairing.

During nucleotide pairing:

• Adenine (A) pairs with Thymine (T)
• Guanine (G) pairs with Cytosine (C)

These pairings occur due to hydrogen bonds that form between the bases. A and T are linked by two hydrogen bonds, while G and C form stronger connections with three hydrogen bonds. It is these specific pairings that ensure the DNA structure is stable and can duplicate accurately during cell division.

double helix

The iconic double helix structure of DNA resembles a spiral staircase or twisted ladder. Imagine two long strands of nucleotides twisted around each other. This formation is what gives DNA its characteristic shape. The sugar-phosphate backbone forms the sides of the ladder, while the paired nitrogenous bases form the rungs.

The double helix not only provides structural stability but also plays a critical role in DNA replication. When a cell divides, the DNA helix unzips, allowing each strand to serve as a template for a new partner strand. This capacity for separation and reformation is a fundamental property that supports life's continuity.

complementary base pairing

Complementary base pairing is a fundamental concept that explains the consistent ratios of bases discovered in DNA through experiments by Chargaff. In the Watson and Crick model, bases pair specifically: Adenine with Thymine, and Guanine with Cytosine. This specificity leads to equal ratios of A to T and G to C in any given DNA molecule.

Without this complementary nature, the DNA's ability to replicate and transmit genetic information accurately would be compromised. Because the bases adhere strictly to this pairing rule, when the double helix unwinds during replication, new strands can be formed with precision, ensuring genetic accuracy and stability.