Human Biology · MU Plovdiv

17. Epigenetic control of gene expression. X – chromosome inactivation. Genomic imprinting.

Inside: 17 sections · 10 labelled pictures · 27 flashcards · 7 min

Topic 17 of 66 in the MU Plovdiv syllabus for Human Biology (Medicine, Year 1). The free AceMed note covers it in 16 sections, about 994 words (7 min of reading), with 27 flashcards.

In this note

  1. Regulation of Gene Activity. Epigenetics
  2. I. Introduction
  3. II. Levels of Gene Regulation
  4. III. Epigenetics
  5. IV. Epigenetics in Development and Disease
  6. V. Key Takeaways
  7. Non-Traditional (Non-Mendelian) Inheritance
  8. II. Genomic Imprinting
  9. III. Uniparental Disomy (UPD)
  10. IV. Dynamic Mutations (Repeat Expansion Disorders)
  11. V. Gonadal Mosaicism
  12. VI. Incomplete Penetrance
  13. VII. Variable Expressivity
  14. VIII. Pleiotropy
  15. IX. De Novo Mutations
  16. X. Clinical Importance

I. Introduction

Gene activity is not constant. Even though every cell in the body carries the same DNA, only a subset of genes are expressed in each cell type. Regulation ensures that genes are switched on or off at the right place, time, and quantity. This regulation happens at multiple levels: transcriptional, post-transcriptional, translational, and post-translational.

II. Levels of Gene Regulation

1. Chromatin Structure

  • DNA is wrapped around histones to form nucleosomes.
  • Euchromatin → loose, transcriptionally active.
  • Heterochromatin → tightly packed, transcriptionally silent.
  • Chromatin remodeling complexes can reposition nucleosomes, exposing promoters to RNA polymerase.

2. Transcriptional Regulation

  • Promoters: sequences (e.g., TATA box, CAAT box) that recruit RNA polymerase II.
  • Enhancers: DNA elements that increase transcription when bound by activator proteins.
  • Silencers: sequences that repress transcription when bound by repressors.
  • Transcription factors: proteins that control initiation. They can be general (needed for all genes) or specific (cell/tissue-dependent).

3. RNA Processing (Post-Transcriptional Control)

  • Alternative splicing → different mRNA isoforms from the same gene.
  • RNA editing → nucleotide changes post-transcription (e.g., ApoB mRNA).
  • mRNA stability → regulated by elements in the 3′ UTR and by RNA-binding proteins.

4. Translational Regulation

  • Initiation is the most controlled step.
  • Regulatory proteins or microRNAs can block ribosome binding.

5. Post-Translational Regulation

  • Protein modifications (phosphorylation, acetylation, ubiquitination).
  • Protein degradation via ubiquitin–proteasome pathway.

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