BINDING DOMAINS OF BACTERIOPHAGE LAMBDA REPRESSOR COMPLEXED WITH DNA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ All cells of a given individual inherit the same (unique) DNA from the parents. Discrete stretches of the DNA, called genes, carry the genetic code which, in the process of transcription, controls the synthesis of protein molecules in the cell. The genetic code in the gene specifies the sequence of amino acids required to build the new protein. As an organism develops, a large variety of different cell types emerge, as different cell types 'choose' to use some genes and not others, to synthesise some proteins and not others. This process of regulation is accomplished by regulatory protein molecules which bind to particular sites on the DNA, either enhancing transcription of the DNA (promotors) or preventing transcription (repressors). This image shows the DNA-binding domains of a repressor from phage lambda, which infects bacteria. The DNA is shown in a space-filling representation, where atoms are depicted as solid spheres of appropriate van der Waals radius. It is colour coded in a very simple fashion, which distinguishes the sugar-phosphate backbone of the double helical structure (red) from the stacked pairs of nucleotide bases (cyan). The repressor protein binding domains are shown in a secondary structure representation, showing stretches of alpha-helix as solid cylinders with randomly coiled polypeptide chain shown as virtual alpha-carbon bonds. The two symmetry related domains are shown in yellow and green: the helix-turn-helix motif which actually binds to the DNA major groove is highlighted in white. This helix-turn-helix motif occurs in many repressor proteins, suggesting a common strategy for protein binding to DNA.