Preview
IB DP Biology — Unit D: Continuity and Change | HL | D2.3 / D1.2 Gene Expression & Regulation
📋 What You Will Learn
- Identify the levels at which gene expression can be regulated — from chromatin remodelling to post-translational modification
- Explain how transcription factors, promoters, and enhancers control whether a gene is switched on or off
- Describe how RNA processing (5' capping, polyadenylation, splicing) shapes the final mRNA before translation
- Explain translational control mechanisms including miRNA silencing, ribosome stalling, and codon usage
- Analyse experimental data on gene expression patterns and evaluate what they reveal about regulatory mechanisms
- Apply understanding of gene regulation to contexts including development, cell differentiation, and disease
🔍 Guiding Question How do cells with identical DNA produce such radically different proteins — and what molecular machinery makes this selective expression possible?
🌍 Why This Matters Every cell in your body carries the same 3.2 billion base pairs of DNA — yet a neuron and a pancreatic beta cell are as different as a piano and a submarine. The difference is not in the genome. It is in which genes are read, when, and how much. Understanding this is the key to understanding cancer, development, and the future of personalised medicine.
The Problem the Cell Had to Solve
Imagine inheriting a library of 20,000 books but only ever needing to read a few hundred at any given moment — and needing different books in different rooms. That is the challenge facing every eukaryotic cell. The genome is fixed. The proteome must be dynamic. Between these two realities lies the entire machinery of gene regulation.
Gene expression is not a binary switch. It is a dimmer — adjustable at multiple points, in multiple ways, by multiple molecular actors. Regulation can occur before transcription even begins, during RNA processing, during translation, or even after the protein is made. Each checkpoint is a layer of control, and each layer is an opportunity for the cell to respond to its environment.
📖 Key Definition — Gene Expression Gene expression is the process by which the information encoded in a gene is used to synthesise a functional gene product — typically a protein, but also functional RNA molecules such as rRNA, tRNA, or miRNA. Expression involves transcription and, for protein-coding genes, translation.
📖 Key Definition — Transcriptional Regulation Transcriptional regulation refers to any mechanism that controls whether, when, or how much a gene is transcribed into RNA. It operates primarily at the level of DNA accessibility and the binding of regulatory proteins to specific DNA sequences near the gene.
📖 Key Definition — Translational Regulation Translational regulation refers to mechanisms that control whether, when, or how efficiently an mRNA molecule is translated into protein. It operates after transcription and includes control by small RNA molecules, RNA-binding proteins, and codon usage.
Level 1 — Chromatin Remodelling: Regulation Before Transcription Begins
Before RNA polymerase can transcribe a gene, it must physically reach the DNA. In eukaryotes, DNA is not naked — it is wound tightly around histone proteins to form nucleosomes, which compact further into chromatin. This packaging is not merely structural. It is regulatory. Tightly packed chromatin (heterochromatin) is transcriptionally silent. Loosely packed chromatin (euchromatin) is accessible and potentially active.
Histone Modification
The tails of histone proteins extend outward from the nucleosome core and can be chemically modified by enzymes. Acetylation of histone tails by histone acetyltransferases (HATs) neutralises the positive charge on lysine residues, weakening the interaction between histones and the negatively charged DNA backbone. This loosens chromatin structure and promotes transcription. Deacetylation by HDACs (histone deacetylases) has the opposite effect — it