# The Origin of Life and the RNA World

Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/the-origin-of-life-and-the-rna-world

## The Chicken-and-Egg Problem

Modern life stores information in DNA and does chemistry with protein. But DNA cannot copy itself without protein enzymes, and proteins cannot be made without the information in DNA. Either one alone is useless, so which came first?

The **RNA world** hypothesis resolves this by proposing a molecule that does both. RNA carries a sequence, so it stores information; and **ribozymes**, catalytic RNAs, were discovered in the 1980s doing genuine chemistry. A molecule that is both gene and enzyme needs no partner to get started.

The evidence is not merely convenient. The ribosome's catalytic core is RNA, not protein: the peptide bond of every protein you contain is made by a ribozyme. Central cofactors such as ATP, NAD and coenzyme A all carry nucleotide components, looking very much like relics.

The proposed sequence runs: RNA world → RNA plus protein, once RNA learned to direct peptide synthesis → DNA takes over storage, being chemically more stable, leaving RNA as the intermediary it still is.

The **genetic code** is essentially universal across all life, which is powerful evidence for a single common ancestry. Its structure is arithmetic: with 4 bases, a codon of length $n$ gives $4^n$ combinations. One base gives 4, two give 16, both short of the 20 amino acids needed, while three give $4^3 = 64$, comfortably enough. The code is therefore triplet, and **degenerate**: several codons specify the same amino acid.

**LUCA**, the last universal common ancestor, is the inferred organism from which all present life descends. It is not the first cell, merely the most recent one whose descendants are all still here.

> **Common pitfall:** reading LUCA as "the first living thing". It is the last *common* ancestor. Life almost certainly existed for a long time before it, in lineages that left no surviving descendants.

## Practice questions

7 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Why does the RNA world hypothesis resolve the chicken-and-egg problem of DNA and protein?

A. RNA is far more chemically stable than DNA, allowing it to endure early conditions
B. RNA can both carry information and catalyse reactions, so it needs no partner
C. RNA spontaneously forms triplet codons that specify amino acids without any code
D. RNA can self-replicate indefinitely without requiring any nucleotide precursors

**Answer:** B. RNA can both carry information and catalyse reactions, so it needs no partner

**Why:** The deadlock exists only if information and catalysis must live in different molecules. RNA does both, ribozymes are real catalysts, so a self-sufficient starting point becomes possible. RNA is in fact less stable than DNA, which is why DNA later took over storage.

Page: https://tryals.app/practice/biology-i/the-origin-of-life-and-the-rna-world/why-does-the-rna-world-hypothesis-resolve-the-chicken-and-egg-problem

### 2. Place these stages of early molecular evolution in the order the RNA world hypothesis proposes.

**Answer:**

1. Simple organic molecules form and accumulate
2. RNA molecules appear that both store information and catalyse reactions
3. RNA begins directing the synthesis of proteins
4. DNA takes over information storage from RNA

**Why:** The sequence runs from prebiotic chemistry to a self-sufficient RNA world, then to RNA-directed protein synthesis, and finally to DNA taking over storage. Each step is a division of labour that improves on the one before.

Page: https://tryals.app/practice/biology-i/the-origin-of-life-and-the-rna-world/place-these-stages-of-early-molecular-evolution-in-the-order-the-rna

### 3. A hypothetical genetic code uses 4 bases and codons of length 2. How many distinct codons does it have?

**Answer:** 16

**Why:** $4^2 = 16$ distinct codons, not enough for 20 amino acids, which is precisely why life uses triplets. Doublets would have forced a smaller amino-acid alphabet.

Page: https://tryals.app/practice/biology-i/the-origin-of-life-and-the-rna-world/a-hypothetical-genetic-code-uses-4-bases-and-codons-of-length-2-how

### 4. The Last Universal Common Ancestor (LUCA) marks a genealogical bottleneck rather than the inception of biology. What follows from this distinction for our understanding of early evolution?

A. The universal genetic code was formulated only after LUCA diversified
B. LUCA represents the abiotic transition from chemistry into genuine life
C. Only organisms that postdate LUCA ever possessed functional ribozymes
D. Independent lineages existed earlier but left no modern descendants

**Answer:** D. Independent lineages existed earlier but left no modern descendants

**Why:** Confusing LUCA with the origin of life ignores extinct pre-LUCA lineages. Molecular relics like ribozymes and the shared genetic code were already established by the time LUCA lived, rather than developing after divergence.

Page: https://tryals.app/practice/biology-i/the-origin-of-life-and-the-rna-world/the-last-universal-common-ancestor-luca-marks-a-genealogical

### 5. Which observations support the idea that RNA preceded DNA and protein?

A. DNA is chemically less stable than RNA
B. Ribozymes catalyse real chemical reactions
C. Several central cofactors contain nucleotide components
D. The catalytic core of the ribosome is made of RNA

**Answer:** B. Ribozymes catalyse real chemical reactions; C. Several central cofactors contain nucleotide components; D. The catalytic core of the ribosome is made of RNA

**Why:** The RNA ribosome core, nucleotide-containing cofactors and working ribozymes all look like relics of an earlier RNA-based world. DNA is *more* stable than RNA, not less, that stability is exactly why it inherited the storage role.

Page: https://tryals.app/practice/biology-i/the-origin-of-life-and-the-rna-world/which-observations-support-the-idea-that-rna-preceded-dna-and-protein

### 6. The genetic code is essentially the same in bacteria, plants and animals.

**Answer:** True

**Why:** True, the code is near-universal, with only minor variants in mitochondria and a few lineages. That universality is one of the strongest pieces of evidence for common descent, since an arbitrary code would not be shared by chance.

Page: https://tryals.app/practice/biology-i/the-origin-of-life-and-the-rna-world/the-genetic-code-is-essentially-the-same-in-bacteria-plants-and

### 7. Sort each molecule by the role the RNA world hypothesis assigns it in modern cells.

**Answer:**

- Stores information long-term: DNA
- Carries out catalysis: Protein enzymes
- Does both in modern cells: Ribosomal RNA, Ribozymes generally

**Why:** Modern life divides the labour — DNA stores, protein catalyses — but RNA still does both, which is exactly the relic the hypothesis predicts. The ribosome is the clearest case: an RNA enzyme building every protein you have.

Page: https://tryals.app/practice/biology-i/the-origin-of-life-and-the-rna-world/sort-each-molecule-by-the-role-the-rna-world-hypothesis-assigns-it-in