Transcription Factors

• Gene expression can be controlled at the transcriptional level by altering the rate of transcription of genes. e.g. increased transcription produces more mRNA, which can be used to make more protein.
• Gene expression is controlled by transcription factors - proteins that bind to DNA and switch genes on or off by increasing or decreasing the rate of transcription.
• Factors that increase the rate of transcription are called activators and those that decrease the rate are called repressors.
• The shape of a transcription factor determines whether it can bind to DNA, or not, and can sometimes be altered by the binding of some molecules.
• Since some molecules can change the shape of a transcription factor this means the amount of certain molecules in an environment or a cell can control the synthesis of some proteins by affectnin g transcription factor binding.
• In eukaryotes, transcription factors bind to specific DNA sites near the start of their target genes - the genes they control the expression of.
• In prokaryotes, control of gene expression often involves transcription factors binding to operons.
• An operon is a section of DNA that contains a cluster of structural genes, that are transcribed together, as well as control elements and sometimes a regulatory gene.
• The Structural genes code for useful proteins, such as enzymes.
• The control elements include;

  • A Promotor - A DNA sequence located before the structural genes that RNA polymerase binds to.
  • An Operator - A DNA sequence that transcription factors bind to.
• The regulatory gene codes for an activator or repressor.