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Download Option D Evolution - A - Origin of Life
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Option D Evolution SL and HL A. Origin of Life on Earth Four Processes needed for life to begin: i. The non-living synthesis of simple organic molecules ii. The assembly of simple organic molecules into polymers. iii. The origin of self-replication molecules that made inheritance possible iv. The packaging of these molecules into membranes with an internal chemistry differ from their surroundings. I. Organic Molecule Formation i. The non-living synthesis of simple organic molecules 3 theories: 1. Formation organic molecules by energy sources (UV or electrical) Miller and Urey Experiment: *One issue is the mix of gases might not have reflected the atmosphere of early Earth Table 1: Organic molecules that have been produced from inorganic molecules in various experiments. Precursor molecule Formaldehyde CH2O Macromolecule of Life Ribose, glycerol Carbon monoxide + hydrogen CO + H2 Hydrogen cyanide HCN Cyanamide H2NCN Fatty acids Hydrogen cyanide Purines (adenine, guanine) Cyanamide Peptides and Phospholipids 2. Delivery by extraterrestrial objects (comets and/or asteroids) -Comets and asteroids have organic molecules -UV light can help create organic molecules -Organic molecules have been detected in interstellar clouds. 3. Organic synthesis driven by impact shocks (asteroids). -The impact of comets and/or asteroids on the Earth could have caused organic molecule formation. - However, if the first genetic material was RNA then this is not an issue. RNA is self-replicating and can also act as a catalyst. Some reactions in ribosomes are still catalyzed by rRNA. Additionally, RNA is used to synthesize proteins. This offers a way around the genes require proteins and enzymes require genes. - RNA has also been shown to catalyze the formation of more RNA. It can also bind amino acids together. RNA can also be transcribed into DNA (using reverse transcriptase) - Recent studies have shown that certain types of clay might have helped the first cells to be formed. Clay seems to speed along formation of proto-cell membranes and to keep RNA stable (RNA is generally unstable for long periods of time). - Remember that a prokaryotic cell does not have membrane bound organelles. So the above scenario would provide the basics for a prokaryotic cell to form. II. Organic molecules to Protobionts Protobionts - aggregate of abiotically produced organic molecules surrounded by a membrane or a membrane-like structure. ii. The assembly of simple organic molecules into polymers iii. The origin of self-replication molecules that made inheritance possible i. “Genes First Model” This model suggests that genes came first, then the metabolism (chemical reactions need for life). RNA is preferred over DNA as the first genetic material for a number of reasons: 1. RNA can store genetic information in its sequence of bases. 2. Single stranded RNA exhibits catalytic activity. RNA in the ribosome acts to catalyze protein synthesis in present day cells. 3. Laboratory studies have demonstrated that RNAs with no catalytic activity can evolve to structures with various catalytic functions. (see Ribozymes) 4. RNA has also been shown to catalyze the formation of more RNA. It can also bind amino acids together. RNA can also be transcribed into DNA (using reverse transcriptase) Ribozymes Messenger RNA in eukaryotic organisms contains sequences that are transcribed but are not part of a gene. The initial mRNA transcript needs to be processed to remove these intervening sequences, called introns, before the mRNA leaves the nucleus. The splicing together of the functional pieces of mRNA (the exons) is usually accomplished with protein enzymes. In the 1980's special sequences of RNA were found to be "autocatalytic." (self-catalyzing) ii. “Metabolism First Model” This model suggests that a primitive metabolism formed first which could provide an environment for the later start of RNA replication. Ideas: -hydrothermal vents could have acted like a cell membrane, trapping organic molecules inside, which could have reacted making complex molecules. -single amino acids can catalyze the creation of sugars from simple starting materials. iii. Possible Membranes or Membrane-like structures iv. The packaging of these molecules into membranes with an internal chemistry differ from their surroundings. a. Coacervates - spherical arrangements of lipid molecules making up a which is held together by hydrophobic forces. b. Microspheres- protein protocells that are small spherical units that form membrane like structures c. Clay Crystals – clay are a collection of crstyls which could have formed “cages” around organic molecules. Some types of clay can catalyze chemical reactions needed for life. d. Ice – Ice can have small packets of liquid water which could isolate organic molecules. iv. Possible Locations: Deep-sea hydrothermal vents - life may have began at the interface where chemically rich fluids, heated by some mechanisms like tidal forces of surrounding moons or planets, emerge from below the sea floor. Chemical energy is derived from the reduced gases by the redox reactions, such as hydrogen-sulfide and hydrogen coming out from the vent in contact with a suitable oxidant, such as carbon dioxide Volcanoes – Similar to hydrothermal vents Deep Hot Biosphere Model- states that life originated deep underground. Supports state that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Methane and iron would have been the sources of nutrients. Exogenesis - primitive life may have originally formed extraterrestrially, either in space or on a nearby planet (Mars). Panspermia - is the hypothesis that "seeds" of life exist already all over the Universe, that life on Earth may have originated through these "seeds", and that they may deliver or have delivered life to other habitable bodies. Clay theory - states that complex organic molecules arose gradually on a pre-existing, non-organic replication platform — silicate crystals in solution. B. Prokaryotic to Eukaryotic Organisms - Early prokaryotic bacteria had a major effect on the Earth’s environment. Cyanobacteria (also called blue-green algae) release large amounts of oxygen into the atmosphere. This killed off a lot of other organisms; oxygen was a poison at that time. Other organisms moved away from the oxygen. It also forced organisms to adapt The origin of Eukaryotic Cells -Evolved between 1.5-2.5 billion years ago, about the same time oxygen levels started to increase -Remember that Eukaryotic cells have membrane bound organelles. The idea is that eukaryotic cells evolved from prokaryotic cells. But then where did the membrane-bound organelles come from? Remember that prokaryotic cells do not have membrane bound organelles. Endosymbiotic Theory -States that mitochondria and chloroplasts evolved as independent prokaryotic cells, which were taken into a heterotrophic cell by endocytosis. Instead of being digested the cells were kept alive. In general, mitochondria and bacteria are basically the same size. This cannot be said about the other eukaryotic organelles. Mitochondria have a set of double membranes, as do most bacteria; and the lipid composition of mitochondrial membranes shows no similarities with the eukaryotic cell cytoplasm. If mitochondria evolved inside of proto-eukaryote, their membrane composition would be expected to be of the same material. Instead, it appears that mitochondrial membranes more closely resemble bacterial membranes in terms of lipid composition (Margulis, 1981, p. 217). The inner-membrane infoldings in the mitochondria lend more credence to the symbiosis theory. According to Margulis, the infoldings (cristae) "are adaptations that increase the surface area of oxidative enzymes, evolutionary analogues to the mesosomal membranes of many prokaryotes" (Margulis, 1981, p. 208). In addition, the cristae keep the various enzymes segregated according to use, as do bacteria. Mitochondrial ribosomal RNA sequences bear much more in common with bacteria than with ribosomes in the eukaryotic cytoplasm. For example, certain types of transfer RNA has been found to exist only in mitochondria and bacteria (Dyer and Obar, 1985, p. 78). Not only do mitochondria possess their own DNA, but it is circular, as is bacterial DNA; and DNA synthesis is continuous as opposed to that of eukaryotic nuclear DNA. Furthermore, the ratio of guanine-cytosine base pairs in mitochondrial DNA is proportionately higher, as with bacteria, than in eukaryotic nuclear DNA (Margulis, 1981, p. 206). Mitochondrial division resembles bacterial reproduction, according to Margulis. She writes, "Genetic recombination in (mitochondria) is far more reminiscent of phage and bacterial sexuality than it is of eukaryotic nuclear sexual behavior" (Margulis, 1981, p. 218). C. Evolution - Evolution is the cumulative change in the heritable characteristics of a population. i. Conditions for Evolution a. Variation -variation exists in a population of organisms. -variation is a result of sexual reproduction. b. Inheritance -DNA is the molecule responsible for inheritance. Only changes in the DNA are affected by inheritance. c. Selection -Most species produce more offspring than necessary. This results in a competition for survival. THIS IS ONLY A SMALL PART OF EVOLUTION! (SEE DIAGRAM BELOW) d. Time -Changes accumulate over a large amount of time.