GCSE OCR Gateway Organic Chemistry C6.2h The Structure of DNA

C6.2h  To be able to recall that DNA is a polymer made from four different monomers called nucleotides and that other important naturally-occurring polymers are based on sugars and amino-acids.

The structure of DNA

You’ll probably have heard that the basic structure of DNA is a double helix.

There is an account of the determination of the DNA structure in the book of the same title by James D Watson (see image below)

Here is the "Think Different" Apple poster celebrating Watson and Crick's achievement.

This poster is a signed copy by Watson.  

But both Watson and Crick would not have got very far with their analysis of the DNA structure had it not been for the work of crystallography Rosalind Franklin who produced and analysed the crucial Xray image of the double helix seen here:

This image indicates that dna has a double helix structure.  There are two strands of polymer and each twists round the other.

But what does DNA stand for?

DNA stands for De-oxyriboNucleic Acid.  The molecule is a condensation polymer.  It has several component parts.

Each strand is formed from a sugar—phosphate—nucleotide monomer. 

The sugar is always de-oxyribose, the phosphate is always PO₄^3– but the nucloetides can be one of four types:

Guanine (G)  Cytosine (C)  Adenine (A) or Thiamine (T)

The nucleotides are all bases.  Note the basic amino —NH₂ groups in each one.

The bases also fit together in pairs like this:

This means that the double helix is like a circular or spiral staircase the base pairs forming the steps up the inside of each DNA strand.

For those of us who want more information here is a picture of the double helic with the four bases and the structure flattened out showing how the bases pair up.