Structure of DNA

Click here or the image below to download free resources from alevelbiology.co.uk!

Click to download!

This is a legacy topic. View the most up to date content by clicking an exam board tab above or visiting the home page.

DNA (deoxyribonucleic acid) is a large molecule which carries the genetic information, or blueprint, of all life on Earth. Mutations arising in the DNA code account for the diversity upon which evolution by natural selection can work. Therefore, it is not far-fetched to say that DNA is one of the central, most important molecules in living organisms.

For such an important molecule, it sure looks beautiful:


...actually, in reality it looks more like this:


Pink candy floss anyone? The above image is a scanning electron micrograph, as you can see the 3D shape of DNA. The pink colour is likely due to a stain used.

DNA is a double helix i.e. two individual strands running along each other in an anti-parallel way, connected to one another by relatively weak hydrogen bonds. DNA's structure can be learned easily by thinking about the strands and the "stuff in-between" separately.


What are the strands made of?

The strands are made of repeating units consisting of a deoxyribose (sugar) molecule with a phosphate molecule attached to it; hence, it is called a sugar-phosphate backbone.


What is the centre made of?

Attached to the sugar molecules in the backbone are a different type of molecule called nitrogenous base. There are 4 bases in DNA: adenine, thymine, cytosine and guanine. These are abbreviated by their initials: A, T, C and G.

The hydrogen bonds are formed between these bases. Due to their complementary shapes, A always pairs with T, and C always pairs with G. A-T is linked by 2 H bonds, while C-G is linked by 3.

Here is a diagram of this arrangement:



And another:




DNA is a very stable molecule, as its purpose of carrying genetic information is very important. Features of this are:

1. DNA is very temperature-resistant, and the H bonds only break at temperatures of about 92 degrees Celsius
2. The sugar-phosphate backbone acts as a shield to the bases, preventing interference from outside chemical reactions
3. The double helix gives stability
4. Many H bonds contribute to the stability
5. The structure of the sugar-phosphate backbone itself confers strength.
Comments