7.22 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

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