Cell Biology - The Origin of Cells

The principle of Biogenesis states that cells can only be formed by the division of pre-existing cells. This was famously supported by Louis Pasteur’s experiments using swan-neck flasks, which disproved spontaneous generation.

The first cells must have arisen from non-living material through a four-step process: 1. Non-living synthesis of simple organic molecules (e.g., amino acids and sugars); 2. Assembly of these molecules into polymers; 3. Formation of polymers that can self-replicate (allowing for inheritance); 4. Packaging of these molecules into membranes with an internal chemistry different from their surroundings ($protocells$).

The Miller-Urey experiment provided evidence for the non-living synthesis of organic molecules. They simulated early Earth's reducing atmosphere ($CH_{4}$, $NH_{3}$, $H_{2}$, and $H_{2}O$) and used electrical discharges to represent lightning, resulting in the formation of amino acids.

The Endosymbiotic Theory explains the origin of eukaryotic cells. It proposes that $mitochondria$ and $chloroplasts$ were once free-living prokaryotes that were engulfed by a larger anaerobic prokaryote via endocytosis. Instead of being digested, they evolved a symbiotic relationship.

Evidence for Endosymbiosis (Mnemonic: MADS): 1. Membranes (double membrane bound); 2. Antibiotics (susceptibility suggests bacterial origins); 3. DNA (have their own naked circular $DNA$); 4. Size/Division (reproduce via a fission-like process and have $70S$ ribosomes).

The $RNA$ world hypothesis suggests that $RNA$ was the first genetic material because it can both store information and catalyze chemical reactions (acting as a $ribozyme$).