Gene expression

• Cell structures and functions vary

  • not all genes are expressed
  • genes are selectively switched on or off
  • proteins produced determine cell structure and processes
• During transcription


  • transcription factors
  • operons
• During mRNA modification (pre-mRNA -> mature mRNA)

  • RNA splicing
  • RNA editing
  • caps and tails
• Protein modification and activation

  • addition of non-protein groups
  • modifications of amino acids
  • modifications of protein folding
  • removal of a part of a polypeptide chain
  • activation with cAMP
  • cofactors
  • coenzymes
  • inhibitors
• Transcription rate 


  • determines how much of a protein is made
  • transcription rate is determined by transcription factors
• Transcription factors


  • bind to DNA or operons with its specific shape, this shape can be altered
  • in DNA they bind to specific DNA sites neat the start of a target gene
  • on an operand they bind to the operator
• Activators


  • transcription factor
  • turns a gene on to increase rate of transcription
• Repressor


  • transcription factor
  • turns a gene off to decrease rate of transcription
• Operons


  a group or a cluster of genes that are controlled by the same promoter
• Structural genes


  code for the protein itself
• Regulatory genes


  Code for transcription factors
• Control elements of operons

  • promoter - where RNA polymerase binds
  • operator - where transcription factors bind
• the lac operon


  • controls production of LACTASE
  • allows the breakdown of lactose so it can be used as an energy source
  • lactase is an inducible protein - only made when lactose is present
• The lac operon


  • P - promoter region
  • Lacl - codes for the lac repressor
  • P - promoter region
  • O - operator region
  • lacZ - LACTASE
  • lacY - permease
  • lacA - transacetylase
• The lac repressor


  • two binding sites
  • one binds to the operator region
  • other site binds to lactose (the effector molecule)
  • when lactose binds to the repressor, shape of other binding site changes so it cannot bind to the operator region of the operon
• splicing


  • removal of introns, exons remain, ends reconnect
  • pre-mRNA becomes mature mRNA
  • occurs in the nucleus
• editing


  • pre-mRNA can have changes made to the base sequence
• caps 


  • added to 5' end of DNA
  • to regulate nuclear export, increases stability in cytoplasm, promotes translation
• poly-A tail


  • lots of adenine nucleotides on 3' end
  • less mRNA is hydrolysed in the cytoplasm, so it lasts longer
  • allows for more protein to be synthesised
• caps and tails


  • improves stability
  • lengthens half-life of mRNA
  • allows for higher quantities of protein to be produced
• Translation


  • caps aid binding of mRNA to ribosomes
  • inhibitory or initiation factors can bind to mRNA, affecting binding of mRNA to ribosomes
• Addition of non-protein groups

  • components that make it functional (e.g. haem group)
  • tags that indicate what is is for (e.g. carbohydrate tags)
  • occurs in the golgi body
• Modifications of amino acids

  • adapting their chemical properties
  • changes shape and therefore function of protein
  • e.g. methyl and acetyl groups of histones
• Modification of protein folding

  • CHAPERONE PROTEINS control how proteins are folded
  • there are multiple ways a protein can be folded, influencing its function
• Removal of part of a poly-peptide chain

  • MASKING SEQUENCE can be removed to activate it
  • e.g. pepsinogen is the inactive form of pepsin
• cAMP activation


  • a molecule binds to cell membrane to stimulate the production of cAMP
  • cAMP alters the 3D shape of proteins to change the activity of them
  • cAMP acts as a secondary messenger
  • e.g. PKA is 4 units bound together which must be released by changing the 3D structure
• cofactors


  • loosely binds to proteins to hold their shape and perform their function
  • regulating their availability/concentration therefore regulates a proteins activity
• coenzymes


  • organic co-factors
  • deliver ions, atoms or chemical groups to an enzyme
  • e.g. NAD, FAD, NADP delivering H+
• Inhibitors


  • a way to regulate protein action post translation