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Origin of Life History of Life on Earth • Timeline: Big Bang: Universe expanded explosively between 10 billion and 20 billion years ago, as evidenced by the red shift, which reflects the velocity of galaxies moving away from each other. The sun was formed 5 billion years ago, and the planets were formed soon thereafter. Evidence of life is found in ancient rocks in Australia: simple cells similar to modern bacteria that are about 3.5 billion years old. These may represent one of the early experiments in life or may be in the ancestral line of modern life forms. Current evidence suggests that the life forms now in existence had a single common ancestor at least as far back in time as the appearance of RNA-directed protein synthesis What was the prebiotic Earth like? • Stage 1 – no atmosphere protects the earth’s surface, so meteorite impacts are abundant and cause the earth’s crust to melt – radioactive decay heats the core -volcanoes erupt. • Stage 2 – volcanic heat and meteorite impacts release geologic H2, H2O, CO2, methane and ammonia – atmosphere with dense clouds forms – rainfall begins – the Earth’s surface begins to cool. • Stage 3 – The atmosphere and oceans are bombarded with energy – volcanic heat – violent storms – lightning bolts – UV radiation. Nucleotides and organics are concentrated in places where there is water, high heat, and mineral catalysis – perhaps an environment like deep-sea hydrothermal vents have today. When and how did life begin on Earth? • Hypothesis 1: conditions on the early Earth, as described on the previous slide - favored the evolutionary origin of prelife forms. • Hypothesis 2: Life forms originated somewhere else in the universe and migrated to the early Earth in primitive, resistant forms (Panspermia – this hypothesis was advanced by Arrhenius, the scientist who discovered the correlation between temperature and the rates of chemical reactions) Although Hypothesis 1 is the reigning one at this time, Hypothesis 2 cannot be refuted Some Supporting Evidence: • Comets are half frozen water containing organic compounds (and cells?) • Asteroid analysis yields organic molecules, e.g., the Murchison, Australia (1969) asteroid contained 74 amino acids, 250 hydrocarbons, all 5 bases of DNA/RNA. • Many contemporary bacterial species could survive interstellar conditions indefinitely in the ensporulated form, so long as they had some protection from cosmic rays • Panspermia doesn’t solve the question of how life originated – it just moves the question to another location. Hypothesis and Experiment Oparin (1924) hypothesized that the primordial atmosphere had the components that could have combined to produce organic compounds…. Stanley Miller, a graduate student in the lab of Harold Urey at the University of Chicago, began a series of experiments that showed that organic synthesis under conditions of the early atmosphere was conceivable (1950s). (inset) Spontaneous chemical synthesis • After one week, the initial amino acids began to be polymerized into simple carbohydrates and peptides. What had been transparent reactants took on a reddish tinge – primordial slime…. The reactions of the citric acid cycle are conventionally known as the intermediary reactions of oxidative metabolism. However, in the prebiotic Earth there was little free oxygen, so instead of starting with acetate and proceeding to CO2, the reactions could have started with CO2 and generated acetate – as well as regenerating all the intermediates. Now that we have some ingredients, what next? “Metabolism-first world” “RNA-first world” Small organic molecules Nucleic acids Catalytic RNA Networks of reactions – non-enzymatic catalysts Natural selection favors cyclic reaction sequences that provide pathways to lower energy levels for high-energy electrons while regenerating their own constituents RNA-directed RNA replication DNA takes over information storage; proteins take over catalytic functions Evidence for the metabolism-first world • Given appropriate physical conditions (reducing atmosphere, heat, U.V. light, catalytic surfaces), considerable organic synthesis can go on without enzymatic catalysis – for example, the first 3 reactions of the reverse citric acid cycle can be driven by UV light. Evidence for RNA as the link between random organic molecules and life…the RNA World hypothesis Experiments that extend the Miller/Urey studies revealed that RNA was produced from the random oligonucleotides. RNA complementarity elicits copy formation (adenine-uracil, cytosine-guanine) RNA has the ability to serve as a catalyst: “ribozymes”, doing many of the things we expect proteins to do. Promotion of synthesis resulted from RNA catalysts that drew energy from a triphosphate group (like ATP). Self-Replication: Replication is an intrinsic property of DNA and RNA Evolution between RNA and amino acids led to the Genetic Code F The 20 amino acids used in o r nucleotides, e.g.: triplets of proteins are specified by The genetic code that led to the RNA World evolved very early – probably even before there were cells because it is essentially the same in all life forms. Q: If there is RNA, capable of copying itself, carrying out some life functions, and ultimately, coding for proteins, what else is needed? A: ? ? A: A membrane to keep the system together, so that the enzymes do not diffuse away from the reactants… Voila – we have a prokaryotic cell! Compartmentalization: The Role of Membranes • Systems of replicating RNA and the proteins they produced needed to stay together for the system to flourish… • Either before or after the development of the genetic code, a membrane-isolated system may have developed. • How? Coacervates or proteinoids • When lipids and amino acids mix in water, tiny bubbles, called proteinoids or coacervates, about the size of bacteria, form. They are surrounded by a double membrane similar to cell membranes; they can grow by adding material and bud off or divide. Simple chemical reactions can occur inside them. Phospholipid membranes are self-assembling The intrinsic property of membrane phospholipids that drives selfassembly into bubble-like spheres is an amphipathic structure, with water-insoluble hydrophobic hydrocarbon chains and water-soluble heads that contain phosphate. Shifting gears… Now that we have at least a hypothetical scheme for cellular evolution, let’s look at the scale that we will be operating in to examine cellular function: Size Comparisons Macromolecules: measured in Angstroms (10-10m) Subcellular structures: measured in nanometers (10-9m) The largest viruses, bacteria and cells: measured in micrometers or microns (10-6m) Molecular structures Size range comparison Relating this to visible objects… A BB pellet is an atom A marble is a simple molecule A cat is a chain molecule (like a microfilament or microtubule) A tractor-trailer is a molecular structure (such as a lysosome or Golgi) An oceanliner is a cell A mountain range is an organ (like the heart) And a continent is a whole body – around 5 trillion cells Cells Alive clip • http://www.cellsalive.com/howbig.htm Cell Types • • 1. 2. 3. 4. • 1. 2. 3. Archaea (Archaeobacteria): genetically distinct line that often exists in extreme environments. Eubacteria (Prokaryotes): These include the bacteria and cyanophytes (blue-green algae) no nucleus. genetic material (single circular DNA) not enclosed within a membrane. Recombination happens through transfer of plasmids (short circles of DNA that pass from one bacterium to another). Ribosomes, but few other organelles. Cell wall made up of peptidoglycan. Eukaryotes: cells found in multicellular organisms, algae, protozoa. cells with a nucleus (the genetic material is surrounded by a membrane) multiple chromosomes, linear and complexed with regulatory proteins. Many organelles, with internal compartmentalization of functions. Archaea are found in extreme environments such as hot springs. Prokaryotes Cell types What about viruses? • Dogma: Non-living, replicating genetic material enclosed in a protein coat, completely dependent on living cells for their existence… • Viruses infect all three branches of “life”. • The overwhelming number of viruses are not harmful to their hosts and peacefully co-exist – We have more viral genes than human genes in us, if you take a whole body and process it for genes. • Viruses may have been a step in the sequence of evolution of cells, or a parallel line of evolution that has parasitized all three branches of cellular organisms. A newly discovered virus called Mimi is visible with the light microscope and possesses 10 times as many genes as a typical DNA virus, including genes for DNA repair, protein translation and other proteins. Summary • Whether life on earth resulted from reactions in gaseous bubbles, deep in marine volcanoes, or as a gift from outer space, it required molecules that could store and pass on instructions, mechanisms to utilize energy for growth and replication, and membranes that could keep functional elements together. • The enormous diversity that has resulted is exhibited by functional differences in the cells of multicellular animals – the subject of this course.