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IB DP Biology — A — Unity and Diversity | SL | A1.2 Non-Coding RNA and Chromatin Organisation
📋 What You Will Learn
- Identify the major classes of non-coding RNA (ncRNA) — including miRNA, siRNA, and lncRNA — and distinguish them from mRNA
- Explain how small interfering RNA (siRNA) and microRNA (miRNA) silence gene expression through the RNA-induced silencing complex (RISC)
- Describe how chromatin structure — euchromatin and heterochromatin — regulates access to DNA and therefore controls gene expression
- Explain how histone modification and DNA methylation alter chromatin packing and influence transcription
- Analyse experimental data on gene silencing and chromatin remodelling to evaluate how epigenetic changes affect gene expression
- Apply understanding of ncRNA and chromatin dynamics to unfamiliar biological contexts, including disease and development
🔍 Guiding Question How do non-coding RNAs and chromatin organisation regulate gene expression without altering the nucleotide sequence of DNA?
🌍 Why This Matters In 2006, Andrew Fire and Craig Mello won the Nobel Prize in Physiology or Medicine for discovering RNA interference — a mechanism in which small RNA molecules silence specific genes with extraordinary precision. This discovery revealed that the genome is far more regulated than anyone imagined: most of the human genome is transcribed, but only about 2% codes for protein. The rest produces RNA molecules that control, silence, and restructure the genome itself.
The Problem the Cell Had to Solve
Every cell in your body carries the same DNA. A neuron and a liver cell are genetically identical — yet they look nothing alike and perform completely different functions. The question is not what genes a cell has, but which genes it chooses to express. And the answer, it turns out, is written not just in the sequence of DNA, but in the molecules that wrap it, silence it, and read it.
For decades, biologists assumed that RNA was merely a messenger — a temporary copy of DNA on its way to becoming protein. Then came a series of discoveries that overturned this assumption entirely. RNA molecules that never become protein turned out to be among the most powerful regulators in the cell. They silence genes, remodel chromosomes, and coordinate development. They are not junk. They are the control layer.
📖 Key Definition — Non-Coding RNA (ncRNA) Non-coding RNA is any RNA molecule that is transcribed from DNA but is not translated into protein. Non-coding RNAs include transfer RNA (tRNA), ribosomal RNA (rRNA), microRNA (miRNA), small interfering RNA (siRNA), and long non-coding RNA (lncRNA), among others. They perform structural, catalytic, and regulatory roles in the cell.
Classes of Non-Coding RNA
Not all RNA is created equal. The cell produces a remarkable diversity of RNA molecules, each with a distinct size, structure, and function. Understanding the major classes is the foundation for understanding how gene expression is regulated at the RNA level.
Transfer RNA (tRNA) and Ribosomal RNA (rRNA) — The Structural Workhorses
These are the most abundant non-coding RNAs in the cell. Transfer RNA carries amino acids to the ribosome during translation, with each tRNA molecule bearing a specific anticodon that base-pairs with the complementary codon on mRNA. Ribosomal RNA forms the structural and catalytic core of the ribosome itself — the large and