3.8.2.2 Regulation of Transcription and Translation

• Describe the role of transcription factors in controlling gene expression. (6 marks)

  • Transcription factors are proteins that bind to specific DNA sequences. (1)
  • They interact with the promoter region of a gene. (1)
  • Activator transcription factors help RNA polymerase bind to the DNA, increasing transcription. (1)
  • Repressor transcription factors block RNA polymerase, preventing transcription. (1)
  • The binding of transcription factors is influenced by hormones and environmental factors. (1)
  • This regulation ensures genes are expressed only when needed. (1)
• Explain how oestrogen can activate the transcription of a gene. (5 marks)

  • Oestrogen is a lipid-soluble hormone that diffuses through the cell membrane. (1)
  • It binds to an oestrogen receptor, forming an oestrogen-receptor complex. (1)
  • This complex acts as a transcription factor. (1)
  • It binds to the promoter region of a target gene. (1)
  • This increases transcription by allowing RNA polymerase to bind and initiate gene expression. (1)
• Describe how siRNA can inhibit gene expression. (4 marks)

  • Small interfering RNA (siRNA) binds to a complementary mRNA sequence. (1)
  • It forms a siRNA-mRNA complex. (1)
  • This complex is recognized and broken down by an enzyme called RISC (RNA-induced silencing complex). (1)
  • As a result, the mRNA cannot be translated, preventing protein synthesis. (1)
• Describe how epigenetic changes can lead to the inactivation of a gene. (5 marks)

  • Epigenetic changes involve modifications that do not alter the DNA sequence but affect gene expression. (1)
  • DNA methylation adds methyl groups to cytosine bases. (1)
  • This prevents transcription factors from binding to the promoter region. (1)
  • Histone modification (e.g., deacetylation) makes DNA more tightly packed. (1)
  • Both mechanisms reduce transcription, silencing the gene. (1)
• Explain how increased DNA methylation can affect gene expression. (4 marks)

  • Methyl groups are added to cytosine bases in DNA. (1)
  • This prevents transcription factors from binding to the gene. (1)
  • As a result, RNA polymerase cannot initiate transcription. (1)
  • The gene is silenced and cannot produce its protein. (1)
• Describe how histone modification affects the accessibility of DNA to transcription factors. (4 marks)

  • Acetylation of histones reduces their positive charge. (1)
  • This weakens the attraction between histones and DNA, making it more accessible. (1)
  • This allows transcription factors to bind, increasing gene expression. (1)
  • Deacetylation has the opposite effect, making DNA less accessible and reducing transcription. (1)
• Transcription Factors

  • These are proteins that bind to DNA and control transcription.
  • They either:
  • Activate transcription → Help RNA polymerase bind to the gene.
  • Repress transcription → Block RNA polymerase from binding.
• The hormone oestrogen can alter transcription by altering molecules called transcription factors.

  • Oestrogen is a lipid-soluble hormone that can diffuse through the cell membrane.
• The action of oestrogen in controlling transcription:

  • Oestrogen binds to a receptor on a transcription factor in the cytoplasm.
  • This changes the shape of the transcription factor, activating it.
  • The transcription factor moves into the nucleus via nuclear pore and binds to DNA.
  • RNA polymerase is recruited → Gene is transcribed.
• RNA interference (RNAi) prevents translation by degrading mRNA. This is done by small RNA molecules.
• Small interfering RNA (siRNA) are small double-stranded RNA molecules that can bind to mRNA as their base sequence is complementary.
• Role of siRNA (small interfering RNA):

  • The siRNA molecule binds to a protein, forming a complex.
  • This complex binds to complementary mRNA, causing it to be broken down into smaller fragments.
  • Since mRNA is degraded, the gene is not translated into protein.
• Role of miRNA (microRNA)

  • In animals, miRNA binds to mRNA but doesn’t fully degrade it. Instead, it blocks translation.
  • In plants, miRNA works like siRNA, cutting up mRNA completely.
• Epigenetics involves heritable changes in gene expression without altering the DNA sequence. These changes are influenced by environmental factors that inhibit transcription by:

  • increasing methylation of DNA
  • decreased acetylation of associated histones
• DNA Methylation (Gene OFF)

  • Methyl groups (-CH₃) are added to cytosine bases in DNA.
  • This prevents transcription factors from binding to DNA.
  • Gene is switched off (silenced).
• Decreased acetylation of histones (Gene ON)

  • Histones are positively charged proteins closely associated with DNA, which is negatively charged
  • Decreased acetylation (-COCH₃) increases their positive charge, making DNA more tightly wound
  • Transcription factors can no longer access DNA, so the gene is switched off
• If tumor suppressor genes are silenced by DNA methylation, cancer can develop.