Question

What evidence did Watson and Crick have at their disposal in $1953?$ What was their approach in arriving at the structure of DNA?

Short answer

Briefly discuss the evidence that Watson and Crick had at their disposal in 1953 while proposing the structure of DNA and describe their approach in arriving at the structure. In 1953, Watson and Crick had various pieces of evidence at their disposal, including: 1. X-ray crystallography data of DNA fibers from Rosalind Franklin and Maurice Wilkins, which suggested a helical structure, 2. Chargaff's rules, which indicated specific base-pairing within DNA, and 3. The inspiration from Pauling's successful alpha-helix model for proteins. Their approach involved building physical models to test different configurations, determining the helical structure and its properties, and incorporating base pairing rules to propose the double-helix model of DNA structure. This model provided a possible mechanism for DNA replication and encoding genetic information and is now widely accepted.

Step-by-step solution

Background Information on Watson and Crick

In 1953, James Watson and Francis Crick were two scientists working at the Cavendish Laboratory in Cambridge, UK. Their primary goal was to understand the structure of DNA using evidence from various sources.

Key Evidence 1 - X-ray Crystallography Data

One critical piece of evidence available to Watson and Crick was the X-ray crystallography data of DNA fibers, particularly the images produced by Rosalind Franklin and Maurice Wilkins. The most famous of these images, Photo 51, showed a clear X-shape pattern. This pattern suggested that DNA had a helical structure.

Key Evidence 2 - Chargaff's rules

Erwin Chargaff's work on DNA base composition was another crucial piece of evidence that Watson and Crick used. Chargaff found that the amount of adenine (A) was equal to the amount of thymine (T), and the amount of guanine (G) was equal to cytosine (C). These findings became known as Chargaff's rules and hinted at the possible base-pairing within DNA.

Key Evidence 3 - Pauling's Alpha Helix Model

Although Linus Pauling's proposed model of DNA was incorrect, the success of Pauling's alpha-helix model for proteins inspired Watson and Crick in their search for the DNA structure. Pauling's work showed that applying the principles of chemistry and understanding molecular shapes could lead to accurate models of complex molecules.

Watson and Crick's Approach

Using these pieces of evidence, Watson and Crick began to explore possible DNA structures. Their approach can be distilled into three main activities: 1. Building physical models: Watson and Crick used models made of metal rods and wires to represent atoms and chemical bonds. This hands-on process allowed them to quickly test different configurations and evaluate structural compatibility with existing experimental data. 2. Determining the helical structure: Using X-ray crystallography data, Watson and Crick were able to establish that DNA had a helical structure. They deduced that the helix was a double helix with two intertwined strands running in opposite directions, also known as antiparallel strands. 3. Base pairing and pairing rules: Watson and Crick recognized that the Chargaff's rules suggested specific base-pairing within DNA. Eventually, they concluded that adenine (A) forms hydrogen bonds with thymine (T), and guanine (G) forms hydrogen bonds with cytosine (C). This complementary base pairing provided a possible mechanism for DNA replication and encoding genetic information. By synthesizing existing evidence from various sources and experimenting with physical models, Watson and Crick successfully proposed the double-helix model of DNA structure that is now widely accepted.

Key concepts

Watson and Crick

In 1953, two curious minds, James Watson and Francis Crick, worked tirelessly at the Cavendish Laboratory in Cambridge, UK, to solve one of biology's greatest mysteries: the structure of DNA. Their ambition was to piece together the scattered bits of information available to unlock the secrets of our genetic material.

They were known for their hands-on approach, not just relying on theoretical calculations but also on building physical models. These models were made from simple metal rods and wires, allowing them to visualize the arrangement of atoms and chemical bonds.

By doing this, they were able to test various configurations quickly and efficiently, trying to fit the pieces of the puzzle into a coherent whole. This dedication to understanding and visualizing the structure physically played a significant role in their eventual success. Their approach laid the groundwork for their groundbreaking model of DNA.

X-ray Crystallography

A pivotal piece of the DNA puzzle came from X-ray crystallography, a technique that gives insight into the molecular structure of crystalline substances. Thanks to Rosalind Franklin and Maurice Wilkins, Watson and Crick had access to crucial X-ray diffraction images of DNA.

Photo 51, one of the most famous images taken by Rosalind Franklin, revealed an unmistakable X-shaped pattern. This pattern was the first solid clue pointing towards a helical structure of the DNA molecule.

X-ray crystallography works by firing X-rays at a crystal and capturing the pattern made by the scattered beams. The resulting diffraction pattern provides information about the spatial arrangement of atoms in the crystal, and in this case, suggested a twisted coil structure or helix.

Without this technique, and Franklin's expert interpretation of the images, Watson and Crick would not have had the concrete evidence they needed to propose a double helix model.

Chargaff's Rules

Erwin Chargaff, an Austrian biochemist, made a significant discovery that played a critical role in understanding DNA's structure. His observations revealed that within any DNA molecule, the quantities of adenine (A) and thymine (T) were always equal, as were the quantities of guanine (G) and cytosine (C).

These observations came to be known as Chargaff's rules. They hinted at possible pairing relationships between these bases, suggesting a complementary nature within the DNA structure.

Chargaff's rules provided Watson and Crick with crucial insight into the DNA molecule's pairing mechanism, eventually helping them to deduce that A pairs with T, and G pairs with C. This pairing concept was foundational in demonstrating how genetic information could be coded and replicated, as each strand could serve as a template for creating a new partner strand.

Without Chargaff's keen insights, the concept of base pairing, essential to the functioning of DNA, might not have been uncovered so swiftly.

Double Helix Model

The culmination of Watson and Crick's dedicated research and synthesis of the available evidence led to the proposal of the double helix model of DNA.

This model suggested that DNA comprises two strands that wind around each other like a twisted ladder, forming a structure known as a double helix. Key to their model was the idea that these strands run in opposite directions, or are 'antiparallel.'

Watson and Crick's double helix not only explained how DNA can store genetic information but also provided a mechanism for its replication.

The complementary base pairing they proposed—adenine pairing with thymine, and guanine pairing with cytosine—not only confirmed Chargaff’s findings but also showed how DNA could replicate itself: by each strand serving as a template for a new, complementary strand.

The simplicity and elegance of the double helix model captured the scientific community’s attention and revolutionized molecular biology by revealing the physical and functional nature of genetic material.