Exam 1 - Exam 1 Chapter 23 The History of Life on Earth The...

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Exam 1 Chapter 23 – The History of Life on Earth The universe began about 15 billion years ago with the sudden appearance – big bang Earth formed 4.5 billion years ago. Life originated in the seas 3.5-4 billion years ago – prokaryotic Over 500 million years for the chemical conditions to become right for life. There are 3 hypotheses to how life began on Earth 1. The first organisms were products of chemical evolution in four stages. a. Abiotic reactions in shallow oceans b. Abiotic reactions in deep thermal vents c. Life cam from outside the earth. Stage 1: Abiotic Synthesis of Organic Monomers Vitalism – organic molecules are thought to be produced only by living organisms Hypothesized that early earth atmosphere was reducing – energy UV radiation and lightning Modern biological macromolecules are all composed of elements that were present on early earth Simulation of early earth could produce living organisms. Stanley Miller and Harold Urey’s 1953 Experiment – apparatus built to mimic early Earth atmosphere with hydrogen, methane, ammonia, and water vapor, along with sparks for lightning and a condenser for rain. Organic molecules collected in the sea. Some evidence that the atmosphere was neutral. Stage 2: Abiotic Synthesis of Polymers – dripping solutions of organic monomers onto hot sand, clay, rock Stage 3: Formation of Protocells (membrane bound sacs combining genetic material and metabotically active molecules) – could have formed spontaneously Protocells have selectively permeable surface, can grow by absorbing molecules from their surroundings, swell or shrink when placed in solutions of different salt concentrations Replication and metabolism are key properties of life. Protocells exhibit simple reproduction and metabolism and maintain an internal chemical environment. Stage 4: Origin of Self-Replicating Molecules RNA was the first hereditary molecule having the ability to copy itself
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First genes were short strands of RNA that could copy themselves without assistance of proteins – ribozymes Ribozymes catalyze RNA splicing and synthesis of new RNA Ribozymes not as stable as RNA or versatile as protein. RNA enclosed in a membrane can keep good protein for itself. Phospholipids form lipid bilayers in water, selected for by natural selection Can’t go from Stage 4 to bacteria. Polynucleotides lack versatility for all chemical functions of a cell. Polypeptides are better at complex cell functions. Partnership formed: polynucleotides direct synthesis of polypeptides Information later stored in DNA because it is more stable. Reverse transcriptase is in retro viruses. Over millions of years natural selection favored the most efficient pre-cells, the first prokaryotic cells evolved, metabolism diversified and oxygen appeared in the atmosphere 2. Life started around deep sea hydrothermal vents or submerged volcanos.
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