cell differentiation

• differential gene expression

  only certain genes in the DNA of the stem cell are activated and get expressed
• every nucleus within the stem cells of a multicellular organism contains the same genes, that is, all stem cells within an organism have an identical genome
• despite the stem cells having the same genome, they are able to specialise into a diverse range of cell types because during differentiation certain genes are expressed ('switched' on)
• • under certain conditions, some genes in a stem cell are activated, whilst others are inactivated
  • mRNA is transcribed from active genes only
  • this mRNA is then translated to form proteins
• these proteins are responsible for modifying the cell (e.g they help to determine the structure of the cell and the processes that occur within the cell)
• as these proteins continue to modify the cell, the cell becomes increasingly specialised
• the process of specialisation is irreversible (once differentiation has occurred, the cell remains in its specialised form)
• eukaryotes use transcription factors to control gene expression 


  transcription factor: a protein that controls the transcription of genes by binding to a specific region of DNA

  they 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 via transcription factors
• activators: transcription factors that increase the rate of transcription

  they work by helping RNA polymerase to bind to the DNA at the start of a gene and to begin transcription of that gene
• repressors: transcription factors that decrease the rate of transcription they work by stopping RNA polymerase from binding to the DNA at the start of a gene, inhibiting transcription of that gene
• some transcription factors bind to the promoter region of a gene 


  this binding can either allow or prevent the transcription of the gene from taking place
• in prokaryotes, control of gene expression often requires the binding of transcription factors to operons
• an operon is a section of DNA that includes


  cluster of structural genes that are transcribed together (codes for useful proteins e.g enzymes)

  control elements, including a promoter region (DNA sequence that RNA polymerase initially binds to) and operator region (where transcription factors bind)

  some operons may include regulatory genes that code for activators or repressors
• operon: cluster of genes that are controlled by the same promoter
• lac operon is found in some bacteria and is one of the most well known operons
• • lac operon controls the production of the enzyme lactase and two other structural proteins
  • lactase breaks down the substrate lactose so that it can be used as an energy source in the bacterial cell
• lac operon is known as an inducible enzyme (it's only synthesised when lactose is present)

  this helps the bacteria avoid wasting energy and materials
• the components of the lac operon are found in the following order

  • promoter for structural genes
  • operator
  • structural gene lacZ that codes for enzyme beta galactosidase (catalyses hydrolysis of lactose)
  • structural gene lacY that codes for permease (allows lactose into the cell)
  • structural gene lacA that codes for transacetylase
• located to the left (upstream) of the lac operon on the bacterium's DNA there is also the

  • promoter for regulatory gene
  • regulatory gene lacI that codes for the lac repressor protein
• the lac repressor protein has 2 binding sites that allow it to bind to the operator in the lac operon and also to lactose (the effector molecule)
• 1st binding site

  when the lac repressor binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter
• 2nd binding site

  when the lac repressor binds to lactose the shape of the repressor protein distorts and the repressor protein can no longer bind to the operator
• when lactose is absent 


  regulatory gene is transcribed and translated to produce lac repressor protein

  lac repressor protein binds to the operator region upstream of lacZ

  due to presence of the repressor protein RNA polymerase is unable to bind to the promoter region

  transcription of the structural genes does not take place

  no lactase enzyme is synthesised
• when lactose is present

  • uptake of lactose by the bacterium
  • lactose binds to second binding site on repressor protein, distorting its shape so that the repressor protein cannot bind to the operator region
  • RNA polymerase is then able to bind to the promoter region and transcription takes place
  • mRNA from all 3 structural genes is translated
  • enzyme lactase is produced, lactose can be broken down and used for energy