6. What is a second messenger? Discuss how a cytokine might 
     act on a second messenger. 
Second messengers are molecules that relay signals received at the cell surface such as the arrival of protein hormones, growth factors, etc. at receptors there
to target molecules in the cytosol and/or nucleus. 

But in addition to their job as relay molecules, second messengers serve to greatly amplify the strength of the signal. Binding of a ligand to a single receptor at the cell surface may end up causing massive changes in the biochemical activities within the cell. 

There are 3 major classes of second messengers: 

  • cyclic nucleotides (e.g., cAMP and cGMP) 
  • inositol trisphosphate (IP3) and diacylglycerol (DAG) 
  • calcium ions (Ca2+) 
Cytokine uses a common cell signalling mechanism involving a cytosolic protein NF-kB (nuclear factor kappa B). NF-kB acts as the second messenger. NF-kB is released from its inhibitor (JKB) when the cell is stimulated by cytokine and enter the nucleus bound to the transcription factors p65/p50. This initiates a series of reaction including protein synthesis.
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