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Lecture 1

BIOB10Y3 Lecture 1: BIOB10 LECTURE 1

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Biological Sciences
Aarti Ashok

The origin of Eukaryotic cells Prokaryotic cells arose before eukaryotic cells fossil record Fossil record shows that prokaryotic cells were present in rocks approx. 2.7B years old o Which was approx. 1B years before evidence of eukaryotes Eukaryotic cells arise from prokaryotes Similarities noted between them (genetic code, metabolism) The two types of cells related to one another bc they share many complex traits (e.g., very similar genetic codes, enzymes, metabolic pathways, and plasma membranes) that could not have evolved independently in different organisms It was believed that the process gradual evolution causing eukaryotes to evolve from prokaryotes in which the organelles of the eukaryotic cell became more complex Wrong Margulishypothesis certainorganellesofeukaryoticcell(mostnotablythemitochondriaandchloroplasts)hadevolved from smaller prokaryotic cells that had taken up residence in the cytoplasm of a larger host cell referred to as endosymbiont theory describes how a single composite cell of greater complexity could evolve from two or more separate, simpler cells living in a symbiotic relationship with one another Endosymbiont theory An endosymbiont is a combination of 2 cells living together in a symbiotic relationship; one cell lives inside the other cell. endo meaning inside or within o They are not just living in proximity (near) to each other but one actually lives inside of the other cell This type of relationship is what we think led to the primitive (ancientearliest) eukaryotic cells that tend to be our ancestor Thats where all of our internal complexity, in terms of the internal components such as mitochondria have got to come to arrive Our earliest prokaryotic ancestors were presumed to have been anaerobic heterotrophic cells: o Anaerobic meaning, they derive energy from food matter without using molecular oxygen (O2) o Heterotrophic meaning, they were unable to synthesize organic compounds from inorganic precursorsancestor (such as CO2 and water) Instead had to obtain preformed organic compounds from their environment These prokaryotic ancestors are thought to have acquired the ability to form internal membrane compartments, allowing formation of a nucleus by containing the DNA within an internal membrane This development of internal membranes produces the first organism that would be considered eukaryote like in terms of having a nucleus or other internal compartments Bc this is the first organism that gave rise to all eukaryotes First eukaryotic common ancestor (FECA) Once thought presence of internal membrane only eukaryotic trait BUT NOW also some bacteria can form extensive complex internal membrane systems Ex. The bacterium Gemmata obscurigkobus, which forma a variety of complex internal membranes o However, 3D reconstructions of this bacterium show these membranes do not form closed compartments like eukaryotic organelles o Hence, key step in producing the FECA was not formation of internal membranes itself BUT the further development of these membranes into close internal compartments, particularly a compartment surrounding the DNA to produce a nucleus
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