Control of gene expression

• Housekeeping gene= genes that are constantly required, eg: enzymes which are necessary for reactions in metabolic pathways like respiration.
• Protein based hormones are only required by certain cells at certain times to carry out a short lived response. They are coded for by tissue specific genes.
• Enzymes which are necessary for reactions present in metabolic pathways like respiration are constantly required and the genes that code for these are called housekeeping genes.
• The entire genome of an organism is present in every cell that contains a nucleus. This includes genes not required by that cell so the expression of genes and the rate of synthesis of protein products like enzymes and hormones has to be regulated.

  • Genes can be turned on or off and the rate of product synthesis increased or decreased depending on demand.
• Gene regulation is the same in prokaryotes and eukaryotes however the stimuli that cause the changes in gene expression and the responses are more complex in eukaryotes.

  • Multicellular organisms have to respond to changes in the internal and external environment.
• Ways genes are regulated:

  • Transcriptional
  • Post-transcriptional
  • Translational
  • Post-translational
• Transcriptional= genes can be turned off or on.
• Post-transcriptional= mRNA can be modified which regulated translation and the types of proteins produced.
• Translation= translation can be stopped or started.
• post-translational= proteins can be modified after synthesis which changes their functions.
• Chromatin remodelling= DNA is a very long molecule and has to be wound around proteins called histones in order to be packed into the nucleus of a cell.

  The resulting DNA/protein complex is called a chromatin.
• heterochromatin= is tightly wound DNA causing chromosomes to be visible during cell division.

  Transcription of genes is not possible when DNA is tightly wound because RNA polymerase cannot access the genes.
• Euchromatin= is loosely wound DNA present during interphase.

  Genes can be freely transcribed.

  • Protein synthesis does not occur during cell-division but during interphase between cell divisions.
  • This regulation ensures the proteins necessary for cell division are synthesised in time.
  • Also prevents the complex and energy consuming process of protein synthesis from occuring when cells are actually dividing.
• Histone modification= DNA coils around histones because they are positively charged and DNA is negatively charged.

  • Histones can be modified to increase or decreases the degree of packing.
  • Addition of acetyl groups or phosphate groups reduces the positive charge on the histones, this causes DNA to coil less tightly, allowing certain genes to be transcribed.
  • Addition of methyl groups makes histones more hydrophobic so they bind more tightly to each other causing DNA to coil more tightly and preventing transcription of genes.
• Epigenetics= describes control of gene expression by the modification of DNA.
• Operon= group of genes that are under the control of the same regulatory mechanism and are expressed at the same time.

  • Efficient way of saving resources because if certain gene products are not needed, then all of the genes involved in their production can be switched off.
• Lac operon 


  • If glucose is in short supply, lactose can be used as a respiratory substrate. Different enzymes are needed to metabolise lactose.
  • The lac operon is a group of three genes, involved in the metabolism of lactose.
• The lac operon are structural genes as they code for three enzymes and they are transcribed onto a single long molecule of mRNA.

  • A regulatory gene is located near to the operon and codes for a  repressor protein that prevents the transcription of the structural genes in the absence of lactose.
  • The repressor protein is constantly produced and binds to an area called a operator, which is close to structural genes.
  • The binding of this protein prevents RNA polymerase binding to DNA and beginning transcription.
  • Section of DNA that is the binding site for RNA polymerase is called the promotor.
• When lactose is present, it binds to the repressor protein causing it to change shape so it can no longer bind to the operator.

  As a result RNA polymerase can bind to the promotor, the three structural genes are transcribed and the enzymes are synthesised.
• Role of cAMP


  The binding of RNA polymerase only results in a slow rate of transcription that needs to be increased to produce the required quantity of enzymes to metabolise lactose efficiently.

  • This is achieved by the binding of another protein, cAMP repressor protein.
• RNA processing 


  The product of transcription is a precursor molecule, pre-mRNA.

  This is modified forming mature mRNA before it can bind to a ribosome and code for the synthesis of the required protein.

  • A cap and tail are added and they both help to stabilise mRNA and delay degradation in the cytoplasm
  • Cap aids binding of mRNA to ribosomes.
• RNA editing 


  Nucleotide sequence of some mRNA molecules can also be changed through base addition, deletion or substitution.

  • Result in the synthesis of different proteins which may have different functions.
  • This increases the range of proteins that can be produced from a single mRNA molecule or gene.
• Translational control


  • Degradion of mRNA- the more resistant the molecule the longer it will last in the cytoplasm, that is, a greater quantity of protein synthesised.
  • Binding of inhibitory proteins to mRNA prevents it binding to ribosomes and the synthesis of proteins.
  • Activation of initiation factors which aid the binding of mRNA to ribosomes.
• Protein kinases 


  = Enzymes that catalyse the addition of phosphate groups to proteins.

  • The addition of a phosphate group changes the tertiary structure and so the function of a protein.
  • Important regulators of cell activity.
  • Activated by cAMP
• Post-translational control 


  = Modification to the proteins that have been synthesised

  • Addition of non-protein groups such as carbohydrate chains, lipids or phosphates.
  • Modifying amino acids and the formation of bonds such as disulfide bridges.
  • Folding or shortening of proteins.
  • Modification by cAMP