control of gene expression

• Why is gene regulation necessary?

  • Prevents vital resources being wasted
  • can respond to changes in the environment at certain times
• what three points are gene regulation controlled at?

  • transcriptional- genes can be turned on or off
  • post transcriptional/pre-translational- mRNA can be modified which regulates translation and the types of protein produced
  • post translational- proteins can be modified after synthesis which changes their functions
• what are transcription factors?

  • promote transcription by binding to promotor on DNA
  • proteins that move in the cytoplasm
  • Attach RNA polymerase to the DNA
  • activators are examples of T factors
• what is the difference between activators and repressors?

  • activators- start transcription
  • repressors- repress transcription
• describe the actions of an activator and repressor

  • activator- transcription factor binds to promoter- RNA polymerase binds to DNA and gene transcribed using mRNA
  • Repressor-binds to operator (responsible for switching gene on/off, in this case off) gene switched off, inhibitor molecule binds to transcription factor, prevents transcription factor binding to promotor/RNA polymerase from binding, stops transcription
• what is the term given to the genes which code for enzymes that are required in metabolic processes all the time?

  housekeeping genes
• outline how chromatin remodelling controls gene expression

  • DNA is wound around proteins called histones in a eukaryotic cell to be packed into the nucleus of a cell
  • resulting complex is called a chromatin
  • heterochromatin is tightly wound around DNA causing chromosomes to appear condensed during protein synthesis
  • euchromatin is the loosely bound DNA present during interphase
  • protein synthesis can't happen with heterochromatin because RNA polymerase can't access the genes, but it can happen with euchromatin
  • this ensures proteins required for cell division are synthesised in time
• what is an advantage of not having protein synthesis occurring at the same time as cell division?

  protein synthesis is an energy consuming process- cells need energy to divide and most of that energy will be consumed by protein synthesis
• outline the process of how histone modification controls gene expression

  • the addition of acetyl groups (acetylation) or phosphate groups (phosphorylation) reduces positive charge on histones (making them more negative)
  • causes DNA to coil less tightly (as DNA is negatively charged and histones are positively charged)
  • this allows certain genes to be transcribed
  • but the addition of methyl groups (methylation) makes histones more hydrophobic so they bind more tightly together
  • causing DNA to coil more tightly preventing transcription
• what is meant by the term epigenetics?

  control of gene expression by the modification of DNA (sometimes used to describe all the different ways in which gene expression is regulated)
• what is an operon?

  a section of DNA that contains a cluster of genes
• where are operons more common? prokaryotes or eukaryotes?

  prokaryotes- as they have smaller and simpler genomes
• why are operons an efficient way of saving resources?

  if certain gene products aren't needed, then all genes involved in their production can be switched off
• what is the lac operon?

  • group of three genes (lacZ, lacY, and lacA) which code for enzymes that metabolise lactose (which can be used as an alternative respiratory substrate to glucose by bacteria if glucose is in short supply)
  • these genes are structural genes (genes that code for proteins with specific functions) as they code for three enzymes, transcribed onto single mRNA molecule
• what is the regulatory gene?

  • located near the operon and codes for a repressor protein
  • code for proteins that control the expression of structural genes
• how is transcription (the binding of RNA polymerase to DNA) prevented in prokaryotes?

  • In absence of glucose, E coli can use lactose as its respiratory substrate
  • When lactose isn't present, repressor protein binds to an area called the operator, also close to the structural genes. operator switches the lac operon gene off as the repressor is bound to it, preventing RNA polymerase from binding to the promoter
  • prevention of transcription= down regulation
• what is the section of DNA which is the binding site for RNA polymerase known as?

  promotor
• what is an activator? state an example

  • begin the process of transcription
  • eg- lactose
• What is a repressor protein?

  prevents the transcription of structural genes in the absence of lactose

  
• what is the prevention of transcription called?

  down-regulation
• what is the role of cAMP in transcriptional control (in transcribing mRNA that codes for enzymes which metabolise lactose)

  • binding of RNA polymerase needs to happen at a faster rate than it does for required quantity of enzymes
  • achieved by the binding of cAMP
  • this binding is only possible when CRP binds to cAMP
  • transport of glucose into an E coli cell decreases cAMP levels(as conc of glucose increases)
  • this reduces the transcription of the genes responsible for the metabolism of lactose
  • when glucose is present it's the preferred respiratory substrate so it gets metabolised over lactose
• what is RNA editing?

  removal of introns to produce mature RNA. occurs in the nucleus
• what is an intron and an exon?

  intron= section of gene that doesn't code for an AA (junk DNA)

  exon= does code for AAs

  when a gene is transcribed from pre-mRNA to mature mRNA, only the exons are left and the introns are removed (splicing- introns removed- exons joined together)
• where does pre-mRNA come from?

  it is a product of transcription
• how does RNA editing have a role in pre translational control?

  • nucleotide sequence of some RNA molecules can be changed through addition deletion or insertion
  • same effect as point mutations and produce different proteins with different functions
  • increases range of proteins produced from mRNA molecules/genes
• what is a splisosome?

  helps carry out the splicing process where introns form a loop shape and are removed
• what is post translational control?

  involved modifications to the proteins that have been synthesised
• state examples of post translational control

  • addition of non protein groups
  • modifying AAs
  • forming/shortening proteins
  • modification by cAMP
• explain how the presence/absence of lactose affects the expression of the lac operon genes

  • When lactose isn't present, operator switches the lac operon gene off as the repressor is bound to the operator, preventing RNA polymerase from binding to the promoter
  • When lactose is present, it needs to be metabolised so it acts as an activator. Binds to repressor protein and changes its shape so it can't attach to the DNA and block the promoter. Therefore RNA polymerase can bind to promotor and begin transcription
• two aspects of post-transcriptional control?

  • RNA processing
  • RNA editing