The Structure of DNA

(The Double Helix)

DNA is made up of six smaller molecules -- a five carbon sugar called deoxyribose, a phosphate molecule and four different nitrogenous bases (adenine, thymine, cytosine and guanine). Using research from many sources, including chemically accurate models, Watson and Crick discovered how these six subunits were arranged to make the the structure of DNA. The model is called a double helix because two long strands twist around each other like a twisted ladder. The rails of the ladder are made of alternating sugar and phosphate molecules. The steps of the ladder are made of two bases joined together with either two or three weak hydrogen bonds.


The basic building block of DNA is called a NUCLEOTIDE. A nucleotide is made up of one sugar molecule, one phosphate molecule and one of the four bases. Here is the structural formula for the four nucleotides of DNA. Note that the purine bases (adenine and guanine) have a double ring structure while the pyrimidine bases (thymine and cytosine) have only a single ring. This was important to Watson and Crick because it helped them figure out how the double helix was formed.

These pictures show a ball and stick model of two DNA nucleotides. Gray balls are carbon atoms, blue balls are nitrogen, red balls are oxygen and the pink ball is phosphorous. The hydrogen atoms are not shown.

Adenine Nucleotide (purine) Cytosine Nucleotide (pyrimidine)

Base Pairs

The nucleotides of DNA line up so that the sugar and phosphate molecules make two long backbones like the handrails of a ladder. To make the rungs of the ladder, two bases join together, between the sugar molecules on the two handrails. The phosphate molecules do not have any "rungs" between them. THERE IS ONLY ONE WAY THE BASES CAN PAIR UP ON THE RUNGS OF THE DNA LADDER. An adenine molecule only pairs with a thymine. A cytosine only pairs with a guanine. They can pair in either order on a rung, giving four possible combinations of bases --

A-T or T-A and C-G or G-C

Believe it or not, it is this chain of base pairs that makes up the code that controls what everything looks like. (See How DNA Works to learn how.) Below is a picture showing how the bases pair. You will see that a purine with two rings always pairs with a pyrimidine with one ring. In this way the width of the DNA molecule stays the same. The dotted lines represent weak hydrogen bonds. These form between parts of the molecules that have weak positive and negative charges. Because the hydrogen bonds are weak, they are able to break apart more easily than the rest of the DNA molecule. This is important when DNA reproduces itself and when it does its main work of controlling traits that determine what an organism looks like.

Adenine and Thymine pairing ***** Guanine and Cytosine pairing

The Double Helix Model

In this model of a very short section of DNA you can see how the A-T and C-G base pairs make up the rungs of the ladder and the sugars and phosphates make up the two long strands. In this picture the DNA is not twisted. The DNA in one chromosome would actually be hundreds of thousands of bases long

These two models shows how all the atoms of the sugars, phosphates and nitrogenous bases fit together to make the "spiral staircase" or "twisted ladder" shape first suggested by the x-ray diffraction pictures of DNA taken by Rosalind Franklin and Maurice Wilkins.


Back to the Index

Or try these Internet siites to see how colleges teach about the structure of DNA