The big idea: For life to evolve, the very first living things needed heredity — a way to pass information to the next generation.
That is only possible if there is a self-replicating molecule: a molecule that can be copied, so the information it carries can be passed on.
RNA is the strongest candidate for that first molecule — and the early stage of life when RNA did this job is called the RNA world.
- Heredity
- Passing genetic information from one generation to the next.
- Self-replicating molecule
- A molecule that can be copied, so its sequence (its information) is passed on.
- RNA
- Ribonucleic acid — a nucleic acid whose base sequence can store information AND which can act as a catalyst.
- Ribozyme
- An RNA molecule that acts as a catalyst (speeds up a chemical reaction), the way a protein enzyme does.
- RNA world
- A proposed early stage of life in which RNA both stored genetic information and catalysed reactions, before DNA and proteins took over those roles.
Why no copying means no evolution: Evolution by natural selection needs variation that can be inherited.
If a molecule cannot be copied, any useful change it carries is lost when it breaks down — nothing is passed on.
So the search for the origin of life is really a search for the first self-replicating molecule.
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RNA stands out because it can do two jobs at once that, in cells today, are split between two different molecules.
Read this as cause and effect: because RNA can do both jobs, one molecule could carry information and catalyse its own copying — which is exactly what a first self-replicating system needs.
RNA's two abilities
- Store information — like DNA, RNA's base sequence is a code, so it can carry genetic information.
- Catalyse reactions — unlike DNA, some RNAs (ribozymes) can fold up and speed up reactions, the way enzymes do.
- Put together: an RNA could catalyse the copying of RNA — including itself. That gives copies, copying errors (variation) and therefore selection: an RNA world that can evolve.
| Molecule | Can store genetic information? | Can act as a catalyst? |
|---|---|---|
| DNA | Yes — its base sequence is a code | No — it does not catalyse reactions |
| Protein | No — it is not copied from itself | Yes — enzymes are proteins |
| RNA | Yes — its base sequence is a code | Yes — ribozymes are catalytic RNAs |
The hand-over: each job went to a better specialist: RNA is a jack of both trades but master of neither. Once life had DNA and proteins, each job moved to a molecule that does it better:
Information storage → DNA. DNA is double-stranded, so it is more stable and easier to repair. That means a lower mutation rate, so genetic information is kept more reliably than in single-stranded RNA.
Catalysis → proteins. Proteins are built from 20 amino acids, versus RNA's 4 bases. Far more chemical variety means proteins can fold into far more shapes and act as far more versatile catalysts (enzymes) than ribozymes.
| Job | Did it in the RNA world | Took it over later | Why the swap was an upgrade |
|---|---|---|---|
| Store information | RNA (single-stranded, easily damaged) | DNA | DNA is double-stranded → more stable and easier to repair → a LOWER mutation rate, so information is kept more reliably |
| Catalyse reactions | RNA ribozymes (built from only 4 bases) | Proteins (enzymes) | Proteins are built from 20 amino acids → far more chemical variety than 4 bases → far more versatile catalysts |
The order of events: So the proposed sequence is:
RNA world (RNA does both jobs) → DNA takes over information storage → proteins take over catalysis.
RNA did not disappear — it became the go-between (mRNA, tRNA, rRNA) that still links DNA's information to the proteins that get built.
The footprints RNA left behind: If RNA really came first, we would expect to still find RNA doing central jobs in modern cells — and we do:
• The ribosome (the protein-building machine) has a catalytic core made of RNA (rRNA) — it is a ribozyme. The machine that builds proteins is itself run by RNA.
• Translation depends on RNA at every step: mRNA, tRNA and rRNA.
• Key cofactors such as ATP and NAD are ribonucleotides — RNA-like building blocks — hinting at a time when RNA chemistry ran the cell.
These are treated as evidence that RNA came first.
| Clue we see in cells today | Why it points back to an RNA world |
|---|---|
| The ribosome's catalytic core is RNA (rRNA), not protein — it is a ribozyme | The machine that builds every protein is itself run by RNA → catalytic RNA is older than the proteins it makes |
| Translation depends entirely on RNA: mRNA, tRNA and rRNA | The flow of genetic information into proteins is still carried by RNA at every step → a leftover of an RNA-run world |
| Key cofactors are ribonucleotides — e.g. ATP and NAD | The cell's energy and electron carriers are RNA-like building blocks → relics from when RNA chemistry ran the cell |
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How this is tested: On Paper 1A (multiple choice) you may be asked to identify the property of RNA that makes it a good candidate for the first molecule (it can both store information and catalyse reactions), or to pick a piece of evidence for an RNA world (the ribosome's catalytic core is a ribozyme).
On Paper 2 an Explain / Outline question can ask why RNA, rather than DNA or protein, is proposed as the first self-replicating molecule — so be ready to give RNA's dual role AND the reason it matters (it could catalyse its own copying).
IB-style question — why RNA is proposed as the first self-replicating molecule
Explain why RNA, rather than DNA or protein, is proposed as the first self-replicating molecule in the origin of life. [4]
How to score all four marks
- RNA can store information. Like DNA, RNA's base sequence acts as a code, so it can carry genetic information that can be passed on.
- RNA can act as a catalyst. Some RNAs are ribozymes — they fold up and speed up reactions, the way enzymes do. DNA cannot do this, and a protein cannot store copyable information about itself.
- The two together allow self-replication. Because one RNA molecule can both carry information and catalyse the copying of RNA (including itself), it could replicate — something neither DNA nor protein can do alone.
- That gives variation and selection. Copying produces errors (variation) that are inherited, so the population of RNAs can undergo natural selection — i.e. it can evolve. (Award 1 mark per distinct point, up to 4.)
Final answer
RNA can both store genetic information (its base sequence is a code) and act as a catalyst (ribozymes), so a single RNA could catalyse its own copying. That self-replication, with copying errors, gives heritable variation and selection — something DNA (no catalysis) or protein (no copyable information) cannot do alone.
Why this scores full marks: It names both of RNA's jobs (store information and catalyse), explains why that combination matters (self-replication of RNA, including itself), and links it to variation and selection — and it says clearly why DNA and protein each fall short on their own.
A common way to lose marks is to mention only that RNA stores information (true of DNA too) and forget the catalytic (ribozyme) half — that half is the whole reason RNA is special.