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Activity 1: DNA Nucleotides and Tautomeric Form


Students will explore the specific base pairings that occur within the DNA molecule.

Background Information

DNA consists of two strands of phospate and sugar coiled around each other in a helical manner and held together by hydrogen bonding between pairs of nitrogenous bases. There are four bases: adenine (A) and guanine (G), which are purines, and thymine (T) and cytosine (C), which are pyrimidines. Guanine and thymine can have alternate molecular structures based on different locations of a particular hydrogen atom. A keto structure occurs when the hydrogen atom bonds to a nitrogen atom within the ring. An enol structure occurs when the hydrogen atom bonds to an nearby oxygen atom that sticks out from the ring. These two types of structures are known as tautomers. Both guanine and thymine can switch easily from one tautomer to another. The change in shape affects the three-dimensional shape of the molecule.

In the early 1950s, guanine and thymine were generally portrayed in the enol form, although there was little data to support the predominance of one form over the other. James Watson and Francis Crick discovered that by using the keto forms instead of the enol forms, they could "form" two base pairs, an adenine thymine pair and a guanine-cytosine pair, that had the same overall size and shape. These base pairs formed the basis for Watson and Crick's model of DNA.


For each group

Duplicate Handouts 1 and 2 to distribute to students.


  1. Have students cut out the bases. Ask students to identify the nitrogen rings, the free hydrogen atoms, and the phosphate groups for each base.
  2. Have students experiment to create different base pairs using the tautomeric forms of the bases. Point out that a stable pair is created when hydrogen atoms join with oxygen or nitrogen atoms to form hydrogen bonds between the two bases.

Discussion Questions

  1. What nucleotides can form a stable hydrogen bond with the adenine nucleotide? (Adenine and thymine.) With the cytosine nucleotide? (Cytosine and guanine.)

  2. How can the nucleotide pairs be arranged so the width of the pairs is always the same? (Adenine with the keto form of thymine and cytosine with the keto form of guanine.) Is this important? (Yes.) Why or why not? (It allows the width between phosphate strands to remain consistent.)
  3. Edwin Chargaff found that within human DNA the amounts of adenine and thymine were approximately equal and the amounts of guanine and cytosine were also approximately equal. If adenine paired with guanine, would Chargaff's findings hold? (Maybe.) What if adenine paired with thymine? (Yes.) If adenine paired with cytosine? (Maybe.)


Cytosine and adenine also form tautomers called imino and amino forms. Have students research the structure of these forms. Ask students to try to create models of base pairs using the alternate forms.


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