The Molecular Basis of Inheritance

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Additional Evidence That DNA Is the Genetic Materia

Prior to the 1950s, it was already known that DNA

Is a polymer of nucleotides, each consisting of three components: a nitrogenous base, a sugar, and a phosphate group

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Erwin Chargaff analyzed the base composition of DNA

From a number of different organisms

In 1947, Chargaff reported

That DNA composition varies from one species to the next

This evidence of molecular diversity among species

Made DNA a more credible candidate for the genetic material

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Building a Structural Model of DNA: Scientific Inquiry

Once most biologists were convinced that DNA was the genetic material

The challenge was to determine how the structure of DNA could account for its role in inheritance

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Maurice Wilkins and Rosalind Franklin

Were using a technique called X-ray crystallography to study molecular structure

Rosalind Franklin

Produced a picture of the DNA molecule using this technique

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Watson and Crick deduced that DNA was a double helix

Through observations of the X-ray crystallographic images of DNA

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Franklin had concluded that DNA

Was composed of two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior

The nitrogenous bases

Are paired in specific combinations: adenine with thymine, and cytosine with guanine

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Watson and Crick reasoned that there must be additional specificity of pairing

Dictated by the structure of the bases

Each base pair forms a different number of hydrogen bonds

Adenine and thymine form two bonds, cytosine and guanine form three bonds

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Concept 16.2: Many proteins work together in DNA replication and repair

The relationship between structure and function

Is manifest in the double helix

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The Basic Principle: Base Pairing to a Template Strand

Since the two strands of DNA are complementary