3.4 Nitrogen Base Pairing Rules

FIVE NITROGENOUS BASES

There are five nitrogen bases that exist in
living things; they are separated into two
groups based on the number of rings in their
structure.

● Purines have a double ring of carbon and

nitrogen atoms.
○ Adenine (A) and Guanine (G) are purines.

● Pyrimidines have a single ring of carbon and

nitrogen atoms.
○ Cytosine (C), Thymine (T) and Uracil (U) are

Pyrimidines.

LG #1: Explore & Interact:

BASE-PAIRING IN DNA

●A purine always bonds with a

pyrimadine

●Adenine (A) always pairs with

thymine (T)

○ A to T (apple tree)

●Cytosine (C) always pairs with

guanine (G)

○ C to G (chewing gum)

LG #1: Explore & Interact:

Chargaff’s Rule

○Erwin Chargaff was a biochemist who

studied the DNA of different organisms

○He found that for any DNA sample:

■% adenine = % thymine
■% cytosine = % guanine
■This is known as Chargaff’s Rule

○Example: If there is 20% adenine in a

DNA molecule, there will be 20%
thymine.

Draw me!

LG #1: Explore & Interact:

BECAUSE OF THIS PAIRING

DNA is complementary
●Complementary: bases on one strand

match up with the bases on the other
strand (A-T and G-C)
○Example: Side 1- ATG GGC CTA

Side 2- TAC CCG GAT

LG #1: Explore & Interact: