3.8.2.2 Regulation of transcription and translation

• • In eukaryotes transcription of target genes can be stimulated or inhibited when specific transcriptional factors move from the cytoplasm into the nucleus
• • Transcription occurs when a transcriptional factor - which is a protein - binds to DNA in the nucleus
• The role of the steroid hormone, oestrogen, in initiating transcription. - Oestrogen is NOT the transcriptional factor

  • Oestrogen diffuses through the phospholipid bilayer (because it is lipid-soluble)
  • It binds to receptor site on the transcriptional factor
  • Causes transcriptional factor to change shape, makes it complementary and able to bind to DNA
  • Therefore transcription is initiated
• In eukaryotes,

  • transcription of target genes can be stimulated or inhibited
  • when specific transcriptional factors move from the cytoplasm into the nucleus.
• • Transcriptional factors bind to promoter sequences if binding site is complementary
• • The role of the steroid hormone, oestrogen in initiating transcription
  • Oestrogen is lipid-soluble / non-polar so diffuses across the phospholipid bilayer
  • Binds to receptor of transcriptional factor
  • Shape of DNA binding site is changed so transcriptional factor binds to promoter region
  • WHICH STIMULATES RNA POLYMERASE!
• Epigenetic control of gene expression in eukaryotes.

  • Epigenetics involves heritable changes in gene function, without changes to the base sequence of DNA.
  • These changes are caused by changes in the environment that inhibit transcription by:
  • increased methylation of the DNA or
  • decreased acetylation of associated histones.
• Suggest how transcription factors can REPROGRAMME cells to form iPS cells (basically how transcription factors work) - 2 marks

  1. Attach to gene / DNA / promoter region
  2. STIMULATE / INHIBIT TRANSCRIPTIOn / RNA POLYMERASE - I missed out on!
  3. Note: Genes being expressed / inhibited or switched on / off is not enough on its own.
• Transcription factors are important in the synthesis of particular proteins. Describe how [3 marks, June 2011]

  1. Bind to DNA/gene
  2. 2. At specific region/base sequence/promoter sequence;
  3. Stimulate transcription / PREVENTS transcription
• Describe and explain how expression of the target gene is affected by siRNA [2 marks, June 11]

  1. No longer able to make specific protein / cannot make whole protein / mRNA cannot be translated
  2. Because mRNA has been cut into pieces

  Do NOT accept mRNA destroyed / do not accept gene not expressed - I made this mistake
• Suggest why - scientists have suggested siRNA may be useful in treating diseases [2]

  1. Some diseases are GENETIC / caused by MUTATIONS (I missed out this mark)
  2. siRNA will stop product of this gene / the protein being produced / stops translation;
• Suggest how transcription factors can reprogramme cells to form iPS cells [2 marks]

  1. Attach to gene / DNA / promoter region
  2. Stimulate / inhibit transcription / RNA polymerase - THIS MARK I LOST
• siRNA would only affect gene expression in cells infected with HIV. Suggest two reasons why [2 marks] - June 2013 Unit 5

  • Only infected cells have HIV protein on surface
  • So carrier only attaches to/specific to these cells/siRNA can only enter these cells
  • OR siRNA (base sequence) complementary/specific to one mRNA
  • Only infected cells contain mRNA of HIV/this gene/ stops translation of this gene/only binds to this mRNA /destroys this mRNA;
• What is RNA interference (RNAi)?

  ● Inhibition of translation of mRNA produced from target genes, by RNA molecules eg. siRNA, miRNA

  ● This inhibits expression of (silencing) a target gene

  • This happens in eukaryotes and some prokaryotes.
• Describe the regulation of translation by RNA interference PART ONE

  1. Small interfering RNA (siRNA) or micro-RNA (miRNA) is incorporated into / binds to a protein, forming an RNA-induced silencing complex (RISC)

  ○ siRNA synthesised as double-stranded RNA → 1 strand incorporated into mRNA

  ○ miRNA synthesised as a double-stranded hairpin bend of RNA → both strands incorporated into mRNA you want to inhibit

  2. Single-stranded miRNA / siRNA within RISC binds to target mRNA with a complementary base sequence
• Describe the regulation of translation by RNA interference

  • 3. This leads to hydrolysis of mRNA into fragments which are then degraded OR prevents ribosomes binding
  • 4. Reducing / preventing translation of target mRNA into protein
• • Students should be able to interpret data provided from investigations into gene expression AND
  • evaluate appropriate data for the relative influences of genetic and environmental factors on phenotype.
• MISTAKE : “Oestrogen is a transcription

  factor.”

  EXPLANATION

  • Oestrogen is a steroid hormone that binds to and activates oestrogen receptors, which are the actual transcription factors.