Control of Gene Expression (Edexcel International A Level Biology)

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Control of Gene Expression

  • Hormones can alter the events inside a cell by influencing gene expression
  • Eukaryotes use transcription factors to control gene expression
    • A transcription factor is a protein that controls the transcription of genes by binding to a specific region of DNA
    • It is estimated that ~10 % of human genes code for transcription factors
      • There are several types of transcription factors that have varying effects on gene expression
        • E.g. transcription factors that increase the rate at which a gene is expressed are known as activators, while those that decrease gene expression are known as repressors
      • Transcription factors ensure that genes are being expressed in the correct cells, at the correct time and to the right level
      • Transcription factors allow organisms to respond to their environment
  • Some hormones achieve their effect by acting upon transcription factors

Effect of hormones inside cells

  • Hormones that can cross the cell surface membrane, e.g. steroid hormones and thyroid hormones, are able to enter the nucleus and bind to transcription factors that are present there
    • Steroid hormones are lipid soluble, allowing them to pass between the phospholipids of the cell surface membrane
  • An example of this is the hormonal regulation of body temperature
    • At normal body temperature a transcription factor known as the thyroid hormone receptor binds to a section of DNA at the start of a gene 
      • This gene codes for a protein that increases the metabolic rate, generating more heat and therefore increasing body temperature
    • As long as the thyroid hormone receptor is bound to the region of DNA at the start of the gene, the gene will not be expressed; it can be said to be switched off
      • In reality it is more likely that expression of the gene will be reduced rather than switched off entirely
    • However, in cold temperatures the body will release the hormone thyroxine which binds to the thyroid hormone receptor
    • Once the hormone and the receptor are bound together the thyroid hormone receptor allows RNA polymerase to bind to the start of the gene; the gene is said to be switched on and its rate of expression will increase
    • The protein which increases the metabolic rate is produced in larger quantities, leading to an increase in body temperature


The hormone thyroxine acts as a transcription factor by binding to the thyroid hormone receptor; this switches on the gene, allowing it to be transcribed by RNA polymerase

Effect of hormones from outside cells

  • Hormones that cannot cross the cell membrane, e.g. protein and peptide hormones, bind to receptors in the cell surface membrane
    • Examples of such hormones include
      • Adrenaline
      • Insulin
      • Glucagon
      • ADH
  • The binding of these hormones to cell surface membrane receptors initiates a process that activates messenger molecules in the cytoplasm of the cell known as second messengers
    • A common second messenger molecule is cyclic AMP (cAMP), formed from ATP
  • The activated second messenger molecules activate enzymes called protein kinases
  • Active protein kinase enzymes trigger a chain of reactions, known as a cascade, inside the cell
  • The cascade may result in changes to the activity of transcription factors which may then affect gene expression in the cell

Effect of adrenaline

Adrenaline acts by binding to a receptor on cell surface membranes; this activates the second messenger cAMP, leading to a cascade of reactions that affects the activity of the cell, e.g. by influencing transcription factors

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.