Question

What might Watson and Crick have concluded had Chargaffs data from a single source indicated the following? $$\begin{array}{cccc}\mathbf{A} & \mathbf{T} & \mathbf{G} & \mathbf{C} \\\\\% & 29 & 19 & 21 & 31\end{array}$$ Why would this conclusion be contradictory to Wilkins's and Franklin's data?

Short answer

Answer: The contradiction Watson and Crick might have faced is that the given Chargaff's data did not support the base pairing rule where the amounts of A equal the amounts of T and G equal C, which is critical for the double helix structure of DNA. This was in opposition to the data from Wilkins and Franklin, which strongly supported the double helix structure with base pairs in a 1:1 proportion.

Step-by-step solution

Analyze the given Chargaff's data

Observe the given percentages of the four nucleotides: Adenine (A): 29% Thymine (T): 19% Guanine (G): 21% Cytosine (C): 31%

Apply Chargaff's base pairing rule

According to Chargaff's base pairing rule, the amounts of Adenine (A) should equal the amount of Thymine (T); likewise, the amounts of Guanine (G) should equal the amount of Cytosine (C). However, in the given data, A is not equal to T, and G is not equal to C: A \% = 29\% \neq 19\% = T \% G \% = 21\% \neq 31\% = C \%

Understand the contradiction with Wilkins's and Franklin's data

Wilkins's and Franklin's data showed X-ray diffraction patterns, which strongly supported the idea of a double helix structure of DNA with base pairs (A-T and G-C) in a 1:1 proportion. However, with the given Chargaff's data where A is not equal to T and G is not equal to C, the double helix structure would not be supported.

Conclusion

Watson and Crick might have concluded that Chargaff's base pairing rules were not valid for the given data, as the percentages of A and T, and G and C are unequal. This would have created a contradiction with Wilkins's and Franklin's data, which strongly supported the double helix structure of DNA with base pairs in a 1:1 proportion.

Key concepts

Base Pairing

Base pairing is a fundamental concept in DNA structure, which involves the specific pairing of nitrogenous bases. Each base on one strand of the DNA molecule has a complementary partner on the opposite strand, forming base pairs. The base adenine (A) pairs exclusively with thymine (T), and guanine (G) pairs with cytosine (C). This specific pairing is known as Chargaff's rules or complementary base pairing. Chargaff discovered that in any given DNA molecule, the amount of adenine is equal to the amount of thymine, and the amount of guanine is equal to the amount of cytosine. These equalities (A=T and G=C) are essential for the stable structure of DNA. The evidence of base pairing was crucial for Watson and Crick to deduce the double helix model because it explained how DNA could replicate itself and carry genetic information.

Double Helix Structure

The double helix structure of DNA is one of the most iconic images in biology. Discovered by James Watson and Francis Crick, this structure describes the shape of the DNA molecule as a twisted ladder. In this model, two long strands of nucleotides run in opposite directions, coiling around a common axis. This twisting forms the double helix. Each strand is composed of a sugar-phosphate backbone and attached nitrogenous bases.
The bases on one strand form hydrogen bonds with complementary bases on the opposite strand, creating "rungs" of the ladder. The specific pairing (A with T, and G with C) is what maintains the uniform distance between the strands and ensures the molecule's stability. The discovery of the double helix was greatly aided by the X-ray diffraction patterns, which supported the consistent intermolecular spacing and helical symmetry.

X-ray Diffraction Patterns

X-ray diffraction is a technique used to determine the three-dimensional structure of molecules. It works by passing X-rays through a crystallized form of the molecule and capturing the diffraction pattern on a detector. Rosalind Franklin and Maurice Wilkins famously used this technique to study DNA, producing critical evidence for its helical structure. The clear pattern showed the spacing between base pairs and the regularity of the DNA's shape.

• The photograph, known as "Photo 51," was vital in revealing the helical structure of DNA.
• The diffraction pattern implied a repeating structure, with features indicating the bases paired centrally within the helix.
• This data was instrumental in allowing Watson and Crick to finally formulate the double helix model with complementary base pairs.

The X-ray diffraction patterns directly contradicted any data that would show non-equal base pairing, as the helical model accommodates equal amounts of A-T and G-C pairs.