Table of Contents
Biomolecules Gateway Page
Jmol Tutorial

In this module:

Polymerization of Nucleotides
Base Pairing 1
Base Pairing 2
Complementary Sequences


After completing this module you should be able to:

  • Recognize a nucleotide, the monomer of DNA, and its parts
  • Describe how nucleotides are polymerized by the formation of phosphodiester bonds
  • Recognize hydrogen bond donors and acceptors
  • Determine the nucleotide sequence complementary to a given DNA sequence

The Form of Genetic Information in the Cell

Ever since the rediscovery of the genetics work of Gregor Mendel in the early 20th century, biologists had sought the chemical identity of the genetic material, the molecule that passes traits such as height, eye color, and blood type, from parents to offspring. Early on, it was assumed that proteins must be the carriers of genetic information. With 20 different side chains (See the Proteins 1 module), they were thought to be information-rich molecules that could pass on traits from generation to generation.

Experiments conducted in the 1940s and 1950s clearly showed that the genetic material was deoxyribonucleic acid (DNA) , however. DNA is a polymer of nucleotides : a backbone of alternating sugars and phosphate groups decorated with one of four aromatic nitrogen-containing bases . The question remained, though: how could DNA, with only four bases, carry the enormous amount of information needed to describe complicated organisms?

Part of the answer came in 1953, when James Watson and Francis Crick discovered the three-dimensional structure of DNA using X-ray diffraction images of DNA fibers taken by Rosalind Franklin and Maurice Wilkins. As the structure at right shows, the nucleotides are arranged in two antiparallel helical strands with complementary hydrogen bonding patterns. This structure immediately suggested how DNA could make copies of itself, passing the information it carries on to other cells. Further work has also shown how DNA stores the information needed to make proteins, the energy-generating, maintenance, defense, and communcation engines of the cell. In this and the following modules you will learn about DNA and how it stores the information that makes you you.