Regulation of Transcription and Translation

• What are transcription factors?

  • Proteins which regulate transcription of specific target genes in eukaryotes
  • By binding to a specific DNA base sequence on a promoter region
• Describe how transcription can be regulated using transcription factors:

  1. Transcription factors move from cytoplasm to nucleus
  2. Bind to DNA at a specific DNA base sequence on a promoter region (upstream of a target gene)
  3. This stimulates or inhibits transcription of target gene by helping or preventing RNA polymerase from binding
• Explain how Oestrogen affects transcription:

  1. Oestrogen a is lipid-soluble hormone so diffuses into cell across phospholipid bilayer
  2. In cytoplasm oestrogen binds to a receptor molecule on a transcriptional factor (forms oestrogen-receptor complex)
  3. The binding alters the tertiary structure of the DNA binding site on the transcription factor making it able to bind to the DNA (activated)
  4. The activated transcription factor enters nucleus via nuclear pore
  5. Binds to a DNA promoter region
  6. Allows RNA polymerase to bind stimulating the transcription of gene
• Explain why oestrogen only affects target cells:

  Other cells do not have oestrogen receptors
• 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
• Describe the regulation of translation:

  1)Enzymes break up dsRNA to form siRNA

  2)Small interfering RNA (siRNA) binds to a protein, forming an RNA-induced silencing complex (RISC)

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

  3) Single-stranded siRNA within RISC binds to target mRNA with a

  complementary base sequence

  4) This leads to hydrolysis of mRNA into fragments which are then

  degraded OR prevents ribosomes binding

  5) Reducing / preventing translation of target mRNA into protein
• Describe what is meant by epigenetics:

  ● Heritable changes in gene function / expression without changes to the base sequence of DNA

  ● Caused by changes in the environment (eg. diet, stress, toxins)
• Describe what is meant by epigenome:

  All chemical modification of DNA and histone proteins - methyl groups on DNA and acetyl groups on histones
• Describe how Methylated CpG repeats silence genes:

  • CpG islands are common in gene promoter regions
  • They are likely to become methylated
  • The presence of repeated CpG sequences near a gene promoter can inhibit activity of RNA polymerase
  • As transcription factor can't bind to promoter region as repressor has binded RNA polymerase does not bind to DNA and start transcription
• Explain how methylation can inhibit transcription:?

  1)Increased methylation of DNA - methyl

  groups added to cytosine bases in DNA

  2)So nucleosomes (DNA wrapped around

  histone) pack more tightly together

  3)Preventing transcription factors and RNA polymerase binding to promoter
• Explain how acetylation can inhibit transcription:

  1)Decreased acetylation of histones increases positive charge of histones

  2)So histones bind DNA (negatively charged) more tightly

  3)Preventing transcription factors and RNA polymerase binding to promoter
• Explain the relevance of epigenetics on disease development and treatment:

  ● Environmental factors (eg. diet, stress, toxins) lead to epigenetic changes

  ● These can stimulate / inhibit expression of certain genes that can lead to disease development

  ○ Increased methylation of DNA OR decreased acetylation of histones inhibits transcription

  ○ Decreased methylation of DNA OR increased acetylation of histones stimulates transcription

  ● Diagnostic tests can be developed that detect these epigenetic changes before symptoms present

  ● Drugs can be developed to reverse these epigenetic changes
• In DNA methylation what type of chromatin is produced?

  Heterochromatin
• In histone acetylation what type of chromatin is produced?

  Euchromatin