Origins of cells

It is the **smallest self-sustaining unit of life** — it can take in materials, release energy, grow and reproduce on its own. All known life is **cellular**.

About **3.5–4 billion years ago**, by **abiogenesis** (life from non-living chemistry) on the early Earth — a **unique, one-off** series of events.

Living cells arise **only from pre-existing living cells** — the rule we observe **today**.

Living matter arising from **non-living chemistry** — required for the **first** cell, under early-Earth conditions.

**No free oxygen** (reducing atmosphere), **no ozone** (so **intense UV**), **volcanic gases** (CH₄, NH₃, H₂O, CO₂), **lightning**, **high temperatures**, and **liquid water**.

With **no oxygen**, the molecules were **not destroyed by oxidation**, so they could **build up** instead of breaking down.

1) organic **monomers**, 2) **polymers**, 3) **self-replicating** molecules, 4) **membrane-bound protocells**.

Organic molecules forming **without any living organisms** — by chemistry from **inorganic** precursors plus energy.

A small **building-block** molecule; many monomers join to make a **polymer** (e.g. amino acids → proteins).

A **reducing** mix of **methane (CH₄), ammonia (NH₃), hydrogen (H₂) and water vapour** — modelling the early atmosphere.

It produced **amino acids** and other organic monomers, showing that organic molecules can form **abiotically**, without life.

**Deep-sea hydrothermal vents** (mineral-catalysed synthesis) and **extraterrestrial delivery** on meteorites (e.g. the **Murchison** meteorite).

Polymers are **chains of monomers**, so the building blocks must exist **first** — this is **step 1** of the origin of cells.

Free **oxygen** would destroy organic molecules, so the lack of oxygen let the new monomers **survive and accumulate**.

A simple **membrane-bound droplet** that forms by itself in water — a key **step between organic chemistry and true cells**, but **not yet alive**.

**Phospholipids (and fatty acids) self-assemble** into a **bilayer vesicle** in water **spontaneously** — **no enzymes** are needed.

The **hydrophobic effect** — the water-hating **tails** are pushed together away from water while the **heads** face the water, so a bilayer forms and seals into a vesicle.

Using a **membrane** to separate an **internal space** — with its own contents and chemistry — from the surroundings.

It **concentrates reactants** (faster reactions), **retains products**, gives a **different internal chemistry**, and **protects self-replicating molecules**.

Droplets that concentrated reactants and protected their replicators **reacted faster and reproduced more**, so their type became **more common** — natural selection before true life.

It has a membrane and an inside, but it **lacks reliable heredity** — it cannot copy its contents **accurately**, so it is only a stepping stone to true cells.

Because evolution needs **inherited variation** — information can only be passed on if the molecule carrying it can be **copied**.

**RNA** — the early stage when it did both jobs is called the **RNA world**.

**Store genetic information** (its base sequence is a code) AND **act as a catalyst** (**ribozymes**).

An **RNA molecule that acts as a catalyst** — it folds up and speeds up reactions, the way a protein enzyme does.

DNA is **double-stranded** → **more stable** and easier to repair → a **lower mutation rate**, so information is kept more reliably.

Proteins are built from **20 amino acids** (vs RNA's **4 bases**) → far more chemical variety → far more **versatile catalysts**.

The **ribosome's catalytic core is a ribozyme (rRNA)** — the protein-building machine is itself run by RNA. (Also: translation uses mRNA/tRNA/rRNA; ATP & NAD are ribonucleotides.)

**RNA world** → **DNA** takes over **information storage** → **proteins** take over **catalysis**.

The **Last Universal Common Ancestor** — the single ancestral population from which **all life alive today** descends.

No — it is the **LAST** (most recent) common ancestor of all surviving life. Earlier cells almost certainly existed.

A near-universal **genetic code**, **DNA/RNA**, **ATP** as energy currency, **ribosomes**, and **common metabolic pathways** — all best explained by **common ancestry**.

By comparing DNA/protein **sequences** and using the **molecular clock**, giving roughly **4 billion years ago** (likely at hydrothermal vents).

A host cell **engulfed a free-living aerobic bacterium** that **survived inside** as an endosymbiont and became the **mitochondrion**.

Mitochondria/chloroplasts have their own **circular DNA**, **70S ribosomes**, a **double membrane**, and divide by **binary fission** — all bacterial features.

A photosynthetic **cyanobacterium** — engulfed and kept as an endosymbiont. (An aerobic bacterium gave the mitochondrion.)