Hydrogen Bonding At first, Watson and Crick could not explain what forces held the two strands of DNA's double helix together. They then discovered that hydrogen bonds could form between certain nitrogenous bases, providing just enough force to hold the two strands together. As you may recall, hydrogen bonds are relatively weak chemical forces.

Does it make sense that a molecule as important as DNA should be held together by weak bonds? Indeed, it does. If the two strands of the helix were held together by strong bonds, it might well be impossible to separate them. As we will see, the ability of the two strands to separate is critical to DNA's functions.

FIGURE 12–7 Base Pairing The two strands of DNA are held together by hydrogen bonds between the nitrogenous bases adenine and thymine, and between guanine and cytosine.

Base Pairing Watson and Crick's model showed that hydrogen bonds could create a nearly perfect fit between nitrogenous bases along the center of the molecule. However, these bonds would form only between certain base pairs—adenine with thymine, and guanine with cytosine. This nearly perfect fit between A–T and G–C nucleotides is known as base pairing, and is illustrated in Figure 12–7.

Once they observed this process, Watson and Crick realized that base pairing explained Chargaff's rule. It gave a reason why [A] = [T] and [G] = [C]. For every adenine in a double-stranded DNA molecule, there has to be exactly one thymine. For each cytosine, there is one guanine. The ability of their model to explain Chargaff's observations increased Watson and Crick's confidence that they had come to the right conclusion, with the help of Rosalind Franklin.

12.2 Assessment

Review Key Concepts

1. 1. Review List the chemical components of DNA.

   2. Relate Cause and Effect Why are hydrogen bonds so essential to the structure of DNA?

2. 1. Review Describe the discoveries that led to the modeling of DNA.

   2. Infer Why did scientists have to use tools other than microscopes to solve the structure of DNA?

3. 1. Review Describe Watson and Crick's model of the DNA molecule.

   2. Apply Concepts Did Watson and Crick's model account for the equal amounts of thymine and adenine in DNA? Explain.

VISUAL THINKING

4. Make a three-dimensional model showing the structure of a DNA molecule. Your model should include the four base pairs that help form the double helix.

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