Question

The Watson-Crick model is based on certain experimental observations of base composition and molecular dimensions. Describe these observations and show how the Watson-Crick model accounts for each.

Short answer

Answer: The key experimental observations behind the Watson-Crick model of DNA are Chargaff's rules and Rosalind Franklin's X-ray diffraction data. The model accounts for Chargaff's rules by proposing complementary base pairing (A with T and C with G), which explains the equal amounts of A and T, and C and G in DNA and maintains a constant purine-pyrimidine ratio. The model also accounts for the X-ray diffraction data by confirming the helical structure of DNA and providing dimensions that align with Franklin's measurements, including a 2 nm diameter and 3.4 nm helical repeat.

Step-by-step solution

Describe Chargaff's rules

Erwin Chargaff discovered two main rules regarding the base composition of DNA: 1. The quantity of adenine (A) is equal to the quantity of thymine (T) and the quantity of cytosine (C) is equal to the quantity of guanine (G) in a DNA molecule. Mathematically, this can be represented as: \[A = T, \quad C = G\] 2. The ratio of purines (adenine and guanine) to pyrimidines (thymine and cytosine) is constant across species, known as the Chargaff's second parity rule: \[\frac{A + G}{T + C} = \text{constant}\]

Describe X-ray diffraction data

Rosalind Franklin's X-ray diffraction studies provided crucial information about the structure of DNA. Key observations from her work include: 1. DNA has a helical structure, as indicated by the X-shaped pattern in the X-ray diffraction images. 2. The pattern suggests that DNA has a repeating structure, known as the periodicity. 3. The dimensions obtained from the diffraction data are consistent with a B-form DNA structure, with a diameter of 2 nm and a helical repeat (distance between repeating units) of 3.4 nm.

Explain the Watson-Crick model's accounting for Chargaff's rules

The Watson-Crick model proposes a double helical structure for DNA, with two antiparallel strands held together by hydrogen bonds between complementary base pairs. The model accounts for Chargaff's rules in the following way: 1. Adenine (A) always pairs with thymine (T) through two hydrogen bonds, while cytosine (C) pairs with guanine (G) through three hydrogen bonds. This complementary base pairing explains Chargaff's first rule (A = T, C = G). 2. The purine-pyrimidine pairing (A with T, and G with C) maintains a constant width of the DNA double helix and accounts for Chargaff's second rule with a constant ratio. These base pairing rules and the backbone hydrogen-bonding consistency maintain the double helical structure of DNA.

Explain the Watson-Crick model's accounting for X-ray diffraction data

The double helical structure proposed by the Watson-Crick model accounts for Rosalind Franklin's X-ray diffraction data: 1. The model confirms the helical structure of DNA, with hydrogen-bonded bases forming the core of the helix and the sugar-phosphate backbone on the exterior. 2. The Watson-Crick model suggests that a full rotation of the DNA helix occurs every 10 base pairs (or every 3.4 nm), consistent with the periodicity observed in Franklin's data. 3. The diameter of the Watson-Crick DNA double helix model is 2 nm, which aligns with the dimensions obtained from Rosalind Franklin's measurements for B-form DNA. In conclusion, the Watson-Crick model successfully accounts for the experimental observations of base composition (Chargaff's rules) and molecular dimensions (X-ray diffraction data) which led to a deeper understanding of DNA structure and function.