6.1.2

• Regulation of gene expression at the transcriptional level

  In prokaryotic cells

  enzymes that catalyse the metabolic reactions involved in basic cellular functions are synthesised at a fairly constant rate

  → enzymes that may only be needed under specific conditions are synthesised varying rates, according to the needs of the cell
• The lac operon

  the bacterium E. coli normally metabolises glucose as respiratory substrate

  However, if glucose is absent + the disaccharide lactose is present, lactose induces the production of two enzymes:

  • lactose permease, which allows lactose to enter the bacterial cell
  • beta-galactosidase, which hydrolyses lactose to glucose + galactose

  although the enzymes to metabolise lactose are only induced if there is no glucose available, it is the lactose + not the absence of glucose that induces the enzymes
• The lac operon 2

  lac open consists of a length of DNA, about 6000 base pairs long, containing an operator region lacO next to the structural genes lacZ + lacY that code for the enzymes beta-galactosidase + lactose permease, respectively

  • next to the operator region, lacO, is the promoter gene, P, to which the enzyme RNA polymerase binds to begin transcription of the structural genes lacZ + lacY

  → the operator region + promoter region are the control sites

  
• The lac operon 3

  a small distance away from the operon is the regulatory gene, I, that codes for a repressor protein (LacI)

  • when this regulatory gene is expressed, the repressor protein produced binds to the operator, preventing RNA polymerase from binding to the promoter region

  → the repressor protein therefore prevents the genes lacZ + lacY from being transcribed (expressed), so the enzymes for lactose metabolism are not made - the genes are ‘off’
• The lac operon 4

  • when lactose is added to the culture medium, once all the glucose has been used, molecules of lactose bind to the LacI repressor or protein molecules; alters the shape of the LacI repressor protein, preventing it from binding to the operator

  →the RNA polymerase enzyme can then bind to the promoter region + begin transcribing the structural genes into mRNA that will then be translated into two enzymes; thus lactose induces the enzymes needed to break it down

  
• In eukaryotic cells

  every cell in an eukaryotic organism has the same genome but because different cells use it differently, they function differently

  • in neurones, the genes being expressed differ to some extent from those being expressed in the liver or kidney cell, although all cells express the basic ‘housekeeping’ genes
  • transcription factors are proteins, or short non-coding pieces of RNA, that act within the cell’s nucleus to control which genes in a cell are turned on or off
• In eukaryotic cells 2

  • they slide along a part of the DNA molecule, seeking + binding to their specific promoter regions

  → they may then aid or inhibit the attachment of RNA polymerase to the DNA, + activate or suppress transcription of the gene

  • they are essential for the the regulation of gene expression in eukaryotes, ensuring that different genes in different types of cells are activated or suppressed
• In eukaryotic cells 3

  • some transcription factors involved in regulating the cell cycle
  • tumour suppressor genes + proto-oncogenes help regulate cell division via transcription factors

  → mutations to those genes can lead to uncontrolled division or cancer

  about 8% of genes in the human genome encode transcription factors

  • many genes have their promoter regions some distance away, along the unwound length of DNA but because of how DNA can bend the promoter region may not be too far away spatially
• Post-transcriptional gene regulation

  introns and exons

  within a gene there are non-coding regions of DNA called introns, which are not expressed

  • they separate the coding or expressed regions, which are called exons
  • all DNA of a gene, both introns + exons, is transcribed

  → the resulting mRNA is called primary mRNA

  • primary mRNA is then edited + the RNA introns - lengths corresponding to the DNA introns - are removed
  • the remaining mRNA exons, corresponding to the DNA exons, are joined together

  → endonuclease enzymes may be involved in the editing + splicing processes
• Post-transcriptional gene regulation 2

  • some introns may themselves encode proteins + some may become short non-coding lengths of RNA involved in gene regulation
  • some genes can be spliced in different ways

  → a length of DNA with its introns + exons can, according to how it is spliced, encode more than one protein

  
• Post-translation level of gene regulation

  involves action of proteins

  many enzymes are activated by being phosphorylated

  • Cyclic AMP, cAMP, is an important second messenger involved in this activation

  
• Post-translation level of gene regulation 2

  • Glucagon binds to a receptor on the target cell's plasma membrane.
  • The receptor activates a transmembrane protein which then activates a G protein.
  • The G protein activates adenyl cyclase, which converts ATP to cAMP.
  • cAMP activates protein kinase A (PKA).
  • PKA catalyses the phosphorylation of various proteins, hydrolysing ATP in the process,activating enzymes in the cytoplasm (e.g., glycogen to glucose conversion).
  • PKA may phosphorylate CREB, which acts as a transcription factor in the nucleus, to regulate transcription