Preview
IB DP Biology — D: Continuity and Change | SL | D1.2 Protein Synthesis
📋 What You Will Learn
- Identify the roles of RNA polymerase, the template strand, and the coding strand during transcription
- Explain the stages of transcription — initiation, elongation, and termination — in eukaryotic and prokaryotic cells
- Describe the three key mRNA processing steps that occur in eukaryotes before translation can begin
- Analyse how alternative splicing of pre-mRNA increases proteome diversity beyond what genome size alone would predict
- Apply understanding of transcription to interpret experimental data on gene expression and mRNA structure
🔍 Guiding Question How is the information encoded in DNA converted into a form that can be used to direct the synthesis of proteins, and what additional processing steps are required in eukaryotes before this information can be translated?
🌍 Why This Matters Every cancer cell that proliferates uncontrollably does so because a gene has been misread — transcription has gone wrong. Many of the most promising cancer therapies, including RNA-based drugs and CRISPR interventions, work precisely by targeting the transcription machinery or the mRNA molecules it produces. Understanding transcription is not abstract molecular biology — it is the foundation of modern medicine.
The Problem DNA Had to Solve
DNA is the master archive. It holds every instruction the cell will ever need — but it never leaves the nucleus. It is too valuable, too irreplaceable, to be sent out into the cytoplasm where ribosomes are waiting. The cell needed a messenger: a disposable, single-stranded copy of just the relevant section, small enough to pass through nuclear pores and robust enough to carry the code intact.
That messenger is mRNA. And the process of making it — reading the DNA and writing the RNA — is transcription. Before we follow the machinery step by step, we need the vocabulary that makes the mechanism precise.
📖 Key Definition — Transcription Transcription is the synthesis of RNA using a DNA template, catalysed by RNA polymerase. The base sequence of the RNA produced is complementary and antiparallel to the template strand of DNA.
📖 Key Definition — Template Strand The template strand (also called the antisense strand) is the strand of DNA that RNA polymerase reads in the 3′ to 5′ direction, using it as a template to synthesise the complementary RNA strand in the 5′ to 3′ direction.
📖 Key Definition — Coding Strand The coding strand (also called the sense strand or non-template strand) is the DNA strand that has the same base sequence as the mRNA produced, except that thymine (T) replaces uracil (U). It is not directly read by RNA polymerase.
📖 Key Definition — RNA Polymerase RNA polymerase is the enzyme that catalyses transcription. It unwinds the DNA double helix, reads the template strand, and assembles ribonucleotides into a complementary RNA strand by forming phosphodiester bonds between successive nucleotides.