Preview
IB DP Biology — A: Unity and Diversity | SL | A2.2 / A2.3
📋 What You Will Learn
- Identify the three types of RNA (mRNA, tRNA, rRNA) and describe their structural features
- Explain the process of transcription — how a DNA template is used to synthesise mRNA
- Explain the process of translation — how the mRNA sequence is decoded to produce a polypeptide
- Describe the role of codons, anticodons, and the genetic code in determining amino acid sequence
- Analyse the relationship between gene sequence, RNA, and protein structure in gene expression
- Apply understanding of the central dogma to interpret unfamiliar biological scenarios
🔍 Guiding Question How does the sequence of nucleotides in DNA determine the sequence of amino acids in a polypeptide, and what does this tell us about the unity of life on Earth?
🌍 Why This Matters Every protein your body makes — every enzyme, every hormone, every structural fibre — begins as a sequence of nucleotides in DNA. Understanding how that sequence becomes a functional protein is the foundation of modern medicine: from designing mRNA vaccines against COVID-19 to engineering insulin-producing bacteria. The mechanism is ancient, universal, and breathtakingly precise.
The Problem DNA Had to Solve
DNA is the master archive. It stores instructions for every protein a cell will ever need. But here is the problem: DNA is locked inside the nucleus, and protein synthesis happens in the cytoplasm. The cell cannot send its master copy out into the cytoplasm — that would risk damage to the irreplaceable blueprint. It needed a messenger.
That messenger is RNA. Ribonucleic acid acts as the working copy of genetic information — transcribed from DNA, exported from the nucleus, and read by ribosomes. The flow of information from DNA to RNA to protein is so fundamental that Francis Crick called it the central dogma of molecular biology . It is not a dogma in the religious sense — it is a description of the universal logic of life.
📖 Key Definition — Central Dogma The central dogma describes the directional flow of genetic information: DNA is transcribed into RNA, and RNA is translated into protein. This flow is universal across all cellular life, reflecting the deep unity of biology.
RNA Structure — The Messenger Molecule
Before exploring how RNA carries information, we need to understand what it is made of. RNA is structurally similar to DNA — but three critical differences make it suited to its role as a temporary, working copy.
📖 Key Definition — Ribonucleic Acid (RNA) RNA is a single-stranded polynucleotide composed of ribose nucleotides. Each nucleotide contains a ribose sugar (with a hydroxy group at the 2' carbon), a phosphate group, and one of four nitrogenous bases: adenine (A), uracil (U), cytosine (C), or guanine (G). RNA does not contain thymine.
RNA vs DNA — Three Key Structural Differences
First, the sugar. DNA contains deoxyribose — a pentose sugar lacking a hydroxy group at the 2' carbon. RNA contains ribose — with a hydroxy group at the 2' carbon. This single difference makes RNA less chemically stable than DNA, which is precisely why RNA is suited to being a temporary message rather than a permanent archive.
Second, the base. DNA