action of hormones

• overview

  • hormones can alter events inside a cell by effecting gene expressions
  • gene expression is controlled by transcription factors
  • TFs control transcription of gene by binding to region of DNA
  • hormones can act on these TFs
• effect of hormones inside the cell

  • steroid and thyroid hormones are lipid soluble so pass through phospholipid bilayer
  • thus can cross cell surface membrane- enter the nucleus and bind to TFs
• example: thyroxine hormone


  at normal body temp

  • thyroid hormone receptor(TF) binds to a section of DNA at the start of gene
  • gene codes for protein which increases metabolic rate
  • as long as TF bound to thay region of DNA, RNA polymerase cannot bind and gene cannot be transcribed so it is 'switched off'
• example: thyroxine hormone


  at cold temps

  • thyroxin released which binds to thyroid hormone receptor(tf)
  • so now cannot bind to region of DNA on gene, thus RNA polymerase can bind and transcribes gene- gene is 'switched on'
  • protein is produced which increases metabolic rate= releasing heat via exothermic reactions=increases body temp
• effect of hormones from outside cell

  • protein and peptide hormones cannot cross cell membrane eg insulin, adrenaline, glucagon
  • they bind to receptors in membrane on cells- activates messenger molecules inside cytoplasm(second messenger)
  • this activated second messager activate enzymes which triggers a chain of reactions inside cell(a cascade)
  • may result in changes to activity of TFs which could then effect gene expression in cell