Explain how the RNA world hypothesis accounts for the origin of self-replicating molecules before the first cells arose on Earth.

IB DP Biology Standard Level (2023 syllabus) — A2.1 Origins of cells · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

RNA can act as both a catalyst (ribozyme) and carry genetic information
RNA could therefore replicate itself without the need for protein enzymes
RNA molecules that replicated more accurately or efficiently would be selected, allowing evolution of RNA sequences
RNA world precedes / sets the stage for DNA and protein‑based life, with RNA eventually being replaced by DNA for information storage and proteins for catalysis

Examiner tips

Use the phrase "RNA world" early to show you know the concept
Show the dual role of RNA (catalysis + information) to hit the first two points
Explain selection of better replicators to cover the third point
Mention the transition to DNA/proteins to satisfy the final point

Common mistakes

Confusing RNA with DNA or proteins in the explanation
Omitting the catalytic role of ribozymes
Failing to mention the evolutionary selection of better replicators

Mark scheme (4 marks)

RNA can act as both a catalyst (ribozyme) and carry genetic information
RNA could therefore replicate itself without the need for protein enzymes
RNA molecules that replicated more accurately or efficiently would be selected, allowing evolution of RNA sequences
RNA world precedes / sets the stage for DNA and protein-based life, with RNA eventually being replaced by DNA for information storage and proteins for catalysis

Key terms in this question

RNA world hypothesis