DNA structure and making proteins

The structure of DNA

James Watson and Francis Crick worked out the structure of DNA in 1953. By using data from other scientists (Rosalind Franklin and Maurice Wilkins) they were able to build a model of DNA. The X-ray crystallography data they used showed that DNA consists of two strands coiled into a double helix.

A photograph of an X-ray diffraction experiment on DNA

The famous X-ray diffraction photograph of DNA taken by Rosalind Franklin, known as photograph 51

DNA is a polymer made from four different nucleotides. These are arranged in a repeating fashion. Each nucleotide consists of alternating sugar and phosphate sections with one of the four different bases attached to the sugar.

Diagram illustrating the structure of DNA pairs

Base pairs

Each strand of DNA is made of chemicals called bases. Note that these are different to bases in relation to acids and alkalis in chemistry. There are four different bases in DNA:

  • thymine, T
  • adenine, A
  • guanine, G
  • cytosine, C

There are chemical bonds between the two strands in DNA, formed by pairs of bases. They always pair up in a particular way, called complementary base pairing:

  • thymine pairs with adenine (T–A)
  • guanine pairs with cytosine (G–C).
DNA strand showing bases pairs: TA, GC, AT repeated randomly.Base pairs on a section of DNA

Genes are sections of the DNA. Each gene has the code for creating a specific protein. The sequence of bases in the gene controls which amino acids are joined in order to make a specific new protein (or enzyme) molecule. The proteins are then folded into their correct shape to make them functional.

Making proteins from amino acids

Each gene acts as a code, or set of instructions, for making a particular protein. Some of these proteins control the cell's internal chemistry. They tell the cell what to do, give the organism its characteristics, and determine the way its body works.

How a protein is producedA triplet of DNA bases codes for one amino acid. A protein is a chain of amino acids, folded into the correct shape.

To enable genes to code for proteins, the bases A, T, G and C get together - not in pairs - but in triplets. This is how it works:

  1. Each triplet of bases codes for one particular amino acid.
  2. Amino acids are made in the number and order dictated by the number and order of base triplets.
  3. The amino acid molecules join together in a long chain to make a protein molecule. The number and sequence of amino acids determines which protein is produced.

Each protein is made up of large numbers of amino acid molecules. Since many of the proteins made are enzymes, genes control the cell's internal chemistry.