How did Cells Evolve? Theory of Origin

Theory Cells – dynamic change over time The appearance organelles more organized cell structure. How? Evolutionary concept - natural progression

Timeline

Prokaryote vs. Eukaryote

Origin of Eukaryotes Two Theories Endosymbiosis Autogeny Prokaryotic cells engulfed other prokaryotic cells by endocytosis. Autogeny the invagination (in-folding) of the plasma membrane to form the endomembrane system

The EndosymbioticTheory The hypothesis suggests that chloroplasts and mitochondria are the result of years of adaptation initiated by the endocytosis of bacteria and blue-green algae. According to this theory, blue green algae and bacteria were not digested after being engulfed; they became symbiotic instead. This did not occur once, but most likely many times

There are two. One cell "engulfs". A double membrane prokaryotic cells There are two One cell "engulfs" A double membrane prokaryotic cells the other cell can be found inside

Endosymbiotic Hypothesis Margulis L (1970) Origin of Eukaryotic Cell, Vol. Yale University, New Haven, CT.

What is the Evidence? Mitochondria resemble bacteria in size and morphology. They are bounded by a double membrane Some enzymes and inner membrane transport systems resemble prokaryotic plasma membrane systems. Mitochondrial division resembles bacterial binary fission They contain a small circular loop of genetic material (DNA). Bacterial DNA is also a circular loop. They produce a small number of proteins using their own ribosomes which look like bacterial ribosomes. Their ribosomal RNA resembles bacterial rRNA.

But... If all cells arise from other cells, where did the first cell come from?

Abiogenesis Revisited Could early cells have formed from nonliving matter? Alexander Oparin and J.B.S. Haldane in the 1920’s thought they could, but not in today’s oxygen-rich atmosphere. Could this hypothesis be tested?

Miller-Urey Experiment Stanley Miller and Harold Urey set up an apparatus to test Oparin’s hypothesis. They recreated the best understanding of early Earth’s atmosphere They got organic molecules.

Surface or deep water? Miller’s studies showed that organic molecules could form on a watery surface, given energy. Other evidence shows that deep ocean thermal vents are also a source of organic molecules.

Repeated Experiments Stanley Miller repeated the experiments in multiple ways throughout his life, using different energy sources, and refining the gas mixtures as scientists better understood the early earth’s atmosphere. The results still show that organic molecules can form under early Earth’s conditions.

Cell membranes in all organisms today are made up largely of lipids. What about cells? Experiments have shown that lipids spontaneously form cell-sized bubbles when agitated in water. Cell membranes in all organisms today are made up largely of lipids.

Metabolism First? Lipid bubbles can collect other organic molecules within. If these formed compartments where chemical reactions took place, could this be the origin of some of the chemical reactions that led to metabolism.

Genetic Material First? The “RNA World” hypothesis suggests that RNA was the first self- replicating molecule. Unlike DNA, RNA does not need enzymes to replicate itself. It can also act as an enzyme to catalyze some chemical reactions.

Of the two theories... ... the RNA World Hypothesis has the most support at present. However, both processes could contribute to the formation of cells.

? The question is open. ? ? ? ? We have tantalizing clues that show that certain cell parts can form under the conditions present on early Earth. At present we do not know exactly which events actually occurred, and in what order. ? ?

Evidence in support of the endosymbiotic theory Similarities between mitochondria, chloroplasts, & prokaryotes: Circular DNA Ribosomes Binary fission

Theory of Cell Evolution