The famous model in the photo was built by Watson and Crick...

The famous model in the photo was built by Watson and Crick in 1953 to demonstrate the secondary structure of DNA. This was not the first attempt at modeling DNA's structure, however. Before 1953 there were several failed attempts. The accumulation of data from different research groups was key to arriving at the correct model. What were the incorrect versions of DNA structure, and how did Watson and Crick get it right? In 1951 , Brwin Chargaff was accumulating data on the molar ratios of nucleotides using DNA obtained from a variety of sources. Some of these data are provided in the following table: Compare the molar ratios presented from each organism tested and between different organisms. Explain how these data could be used to show that Levene's tetranucleotide model is incorrect. What do they imply about the primary structure of DNA in different organisms?

The famous model in the photo was built by Watson and Crick in 1953 to demonstrate the secondary structure of DNA. This was not the first attempt at modeling DNA's structure, however. Before 1953 there were several failed attempts. The accumulation of data from different research groups was key to arriving at the correct model. What were the incorrect versions of DNA structure, and how did Watson and Crick get it right?
In 1951 , Brwin Chargaff was accumulating data on the molar ratios of nucleotides using DNA obtained from a variety of sources. Some of these data are provided in the following table:
Compare the molar ratios presented from each organism tested and between different organisms. Explain how these data could be used to show that Levene's tetranucleotide model is incorrect.
What do they imply about the primary structure of DNA in different organisms?

Key Concepts

Accumulation and Integration of Diverse Data

The development of the correct DNA model was achieved through synthesizing information from multiple research efforts, demonstrating the importance of interdisciplinary data accumulation. Evidence from chemical analyses, structural imaging, and comparative studies across organisms combined to overturn flawed hypotheses and establish a model that accurately reflected the molecular basis of heredity.

Watson-Crick Double Helix Model

Watson and Crick integrated data from various sources, including X-ray diffraction and chemical composition analyses such as Chargaff’s rules, to propose the double helix model. Their model accurately depicted DNA as two strands winding around each other with specific base pairing, offering a structural framework that explained both the uniformity in the basic structure and the variations in the nucleotide sequence that underlie genetic diversity.

Early DNA Models

Prior to Watson and Crick's breakthrough, models such as Levene's tetranucleotide hypothesis were proposed, which suggested that DNA was composed of repetitive nucleotide units that did not vary between organisms. These incorrect models failed to account for the observed variation in nucleotide composition across different species and could not explain how DNA could store genetic information.

Chargaff's Rules

Chargaff's findings, which revealed that the amounts of adenine and thymine were equal and similarly for guanine and cytosine (A=T and G=C), challenged earlier models and provided a quantitative basis for the structure of DNA. These observations were key to disproving the idea of a repetitive tetranucleotide structure and supported the idea of complementary base pairing, crucial for the structural stability and replication fidelity of DNA.

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