Table of Contents
Biomolecules Gateway Page
Jmol Tutorial

In this module:

Polymerization of Nucleotides
Base Pairing 1
Base Pairing 2
Complementary Sequences
Polymerase Chain Reactions

Complementary Nucleotide Sequences

Because of the nature of complementary base pairing, if you know the sequence of one strand of DNA Click here for a ChemEd DL Featured Molecule , you can predict the sequence of the strand that will pair with, or "complement" it. Click here for ChemEd DL resources 

Remember that the polymer forms in a unidirectional manner from the 5' carbon that is bound to a phosphate group to the 3' carbon that is bound to the hydroxyl group. DNA sequences are often written in a shorthand notation using the one-letter name for each nucleotide, and listing the nucleotides in the 5' to 3' direction. This usually involves reversing the sequence after writing it complementary to the one you are given.

Click on the step numbers below to see the steps in writing a complementary sequence. Click on the mouse at left to clear the images and text.


The original sequence of nucleotides is written in the 5' to 3' direction.


The complementary sequence is written, matching A with T and C with G. The resulting sequence is in the 3' to 5' orientation.


The new sequence is reversed, resulting in the complementary sequence in the correct orientation.

Give the DNA sequence that will pair with the following stretches of DNA. Question 1 simulates the first step, finding the complementary sequence. Question 2 adds the second step, reversing the sequence to give the proper 5'-3' orientation. Question 3 combines these two steps without any hints on the orientation, i.e., it just gives and expects the sequences without explicitly giving the 5' and 3' ends.

1. 5'-TGC-3'

2. 5'-ATCCG-3'


3'-TGC-5' 5'-GCCAT-3'
3'-ACG-5' 5'-GCCTA-3'
3'-GCA-5' 5'-TAGGC-3'
3'-CGT-5' 5'-CGGAT-3'
Correct! You have paired A with T and G with C and given the correct 5' to 3' orientation.
Remember, A pairs with T and G pairs with C.
Remember to give the correct 5' to 3' orientation.

Complementary Sequences