Activity 1: DNA Nucleotides and Tautomeric Form
Students will explore the specific base pairings that occur within the
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.
- Have students cut out the bases. Ask students to identify the nitrogen
rings, the free hydrogen atoms, and the phosphate groups for each base.
- 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.
- What nucleotides can form a stable hydrogen bond with the adenine nucleotide?
(Adenine and thymine.) With the cytosine nucleotide? (Cytosine and guanine.)
- 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.)
- 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.