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.