DNA Structure
DNA is a double helix. The helix is constructed of repeating parts called nucleotides. Nucleotides
consist of three parts, a phosphate group, a sugar (deoxyribose), and a nitrogenous base. The base is
the only variable part of a nucleotide. The base may be adenine, guanine, cytosine, or thymine.
Adenine and guanine are purines. They are made of two rings. Cytosine and thymine are pyrimidines.
They have only one ring. The bases are commonly referred to as A, G, C, and T. Outline the parts of a
nucleotide below. Outline the phosphate red, the sugar blue, and the base black.
Next color the parts of the nucleotide. Color the phosphate red, the sugar blue, and the bases as
follows. A orange, G green, C yellow, and T gray.
The parts of a DNA double helix are made of specific parts of a nucleotide. The sides of the DNA ladder
are made of alternating phosphates and sugars. One side of the helix starts with a phosphate. This is
called the 5’ (five prime) side because the phosphate is attached to carbon number 5 of the sugar ring.
The other side of the helix starts with a sugar on top. This is called the 3’ side since the third carbon of
the sugar ring is facing up. Label the 5’ side and the 3’ side. Since the sides of the helix run in opposite
directions, they are called antiparallel. Color the phosphate groups red and the sugars blue.
The “rungs” of the DNA ladder are made of the bases of the nucleotides. Two bases join together is the
center of the helix. Base A always pairs with base T, and C pairs with G. This is called complementary
base pairing. Since a purine always pairs with a pyrimidine the helix remains an even width throughout
its length. Color the parts of a nucleotide the same as you did above and outline the box showing one
nucleotide.
On the double helix, make the sugar-phosphate backbones purple and color the bases so that the
complementary base pairing is correct.