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Search for Life in the Universe

Search for Life in the Universe. Chapter 5 The Nature of Life on Earth (Part 2). Announcement. Midterm 2 is postponed until March 13 Midterm 2 covers Chapters 4-6 The lecture on March 8 will cover Chapter 7 Chapter 7 is not included in Midterm 2 (it will be included in Midterm 3).

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Search for Life in the Universe

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  1. Search for Life in the Universe Chapter 5 The Nature of Life on Earth (Part 2) AST 248, Spring 2007

  2. Announcement • Midterm 2 is postponed until March 13 • Midterm 2 covers Chapters 4-6 • The lecture on March 8 will cover Chapter 7 • Chapter 7 is not included in Midterm 2 (it will be included in Midterm 3) AST 248, Spring 2007

  3. Outline • Metabolism • Basic Metabolic Needs • Carbon and Energy Sources • Metabolism and Cells • Importance of Water • DNA and Heredity • Deoxyribose Nucleic Acid (DNA) • DNA Replication • Genes, Genomes, & Genetic Code • Ribonucleic Acid (RNA) • Mutations and Evolution • Will Life Elsewhere Use DNA? • Life at the Extremes • Extremophiles • Implications for Extraterrestrial Life AST 248, Spring 2007

  4. Basic Metabolic Needs • Metabolism: chemical reactions within living organisms • Cell: Chemical reactions much faster than in the open • Collects the raw materials for the chemical reactions • Provides the energy for the reactions • Provides enzymes to catalyze the reactions • Instructions for enzymes encoded in DNA • Adenosine triphosphate (ATP) and Diphosphate (ADP): • ATP endothermic reaction  ADP  exothermic reaction  ATP • Every living organism uses ATP • Other molecules would be equally effective • Unique use of ATP points to a single origin of life AST 248, Spring 2007

  5. AST 248, Spring 2007

  6. Carbon and Energy Sources • Carbon: • Heterotroph: eat other organisms • Autotroph: self-feeding by converting atmospheric CO2 • Energy: • Photoautotrophs (plants): photosynthesis: CO2 + H2O + sunlight  sugar • Photoheterotrophs (rare prokaryotes): carbon from food but make ATP using sunlight • Chemoheterotrophs (animals): energy from food • Chemoautotrophs (extreme prokaryotes): energy from chemicals and not sunlight AST 248, Spring 2007

  7. Metabolism and Cells • Metabolism: • Four forms of metabolism defined by: • Sources of carbon (direct or indirect) • Sources of energy (light or chemical) • The four forms of metabolism are quite general and should apply to life anywhere • Cells: • Needed environment for metabolism at acceptable rate • Origin of Life (on Earth and elsewhere): • Look for cells as sites of metabolism AST 248, Spring 2007

  8. Importance of Water • Importance: • Contact: organic chemicals float in the cell and find each other • Transportation: bring chemicals in and out of cells • Participant in reactions: • ATP • Photosynthesis • Necessity: • Life on Earth: all use water • Dormant without water: for a limited time only • Elsewhere: need a liquid • What are the alternatives? AST 248, Spring 2007

  9. Deoxyribose Nucleic Acid (DNA) • Structure: • Double helix of nucleotides: like two intertwined Slinky toys • Composition: • Deoxyribose (sugar) • A phosphate group, linked to the deoxyribose of the next nucleotide • One of four bases, which determine the genetic code • Four bases: • Adenine (A): A  T • Guanine (G): G  C • Thymine (T): T  A • Cytosine (C): C  G • Pairing forced by chemical shapes AST 248, Spring 2007

  10. AST 248, Spring 2007

  11. DNA Replication • Steps: • Complete double helix • Strands separate into 2 helices • Free floating bases join open strands • Fill in deoxyribose and a phosphate group • Two identical copies of the DNA in the cell • Cell division: one copy to each daughter cell • Complexity: • Enzymes: a dozen or more needed • Heredity: ensured by exact copying, but • Errors: occur occasionally  evolution • Origin of Life: need simpler mechanism (RNA?) AST 248, Spring 2007

  12. AST 248, Spring 2007

  13. Genes, Genomes, & Genetic Code • Gene: • A sequence of DNA bases  instructions for a single function • Basic unit of heredity • Genome: • Complete sequence of DNA: contains all genes • Human genome: • ~3 billion bases • 20,00025,000 genes • Genetic code: • Rules for reading DNA • Genetic words: 3 bases in a row • Total number of words: 43 = 4 x 4 x 4 = 64 • Redundancy: only 20 amino acids used to make proteins • Only first 2 bases matter in most cases  Originally 2-base words? AST 248, Spring 2007

  14. AST 248, Spring 2007

  15. Ribonucleic Acid (RNA) • Implementation of the genetic code: • Many enzymes involved • Ribonucleic acid (RNA) crucial in protein synthesis • RNA: • Similar to one strand of DNA • Ribose replaces deoxyribose • Uracil replaces thymine • Protein synthesis • Transcription: RNA assembled along one strand of DNA • Go to assembly point of amino acids • Translation: RNA attaches amino acids into chains making proteins AST 248, Spring 2007

  16. Mutations and Evolution • DNA is the mechanism for Darwin’s theory of evolution • DNA replication is fast and accurate: • Minutes for some bacteria • A few hours for the entire 3-billion human genome • Less than one mutation (error) per billion bases • Causes of mutations: • Ultraviolet (UV) light • Chemical agents (carcinogens) • Nuclear radiation (mostly natural cosmic rays) • Effect of mutations: • Harmless • Fatal • Evolution AST 248, Spring 2007

  17. Will Life Elsewhere Use DNA? • Heredity and evolution are essential • DNA does the job on Earth today • RNA may have been the first mechanism • Who’s to say that the same complex mechanism is universal? • Some type of molecule has to provide the mechanism for heredity and evolution AST 248, Spring 2007

  18. Extremophiles • Volcanic vents: • Water temperature reaches 400C (750F), possible because of the large pressure • Black smokers: mixed with nasty volcanic chemicals • Antarctic dry valleys: • Microbes in small pockets of water in rocks • Lithophiles (rock lovers): • Several kilometers below the surface • Chemical energy from rocks • Carbon from CO2 filtering down • Endospores (e.g., anthrax) • Can lay dormant for long periods • Can survive lack of water, extreme heat and cold, and poisons • Some can survive in vacuum AST 248, Spring 2007

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  23. Implications for Extraterrestrial Life • Oxygen for eukarya on Earth for only ~10% of its life • What is the probability that eukarya-like organisms would develop? • We are more likely to find extremophiles elsewhere • Extremophiles may be the norm, not the exception AST 248, Spring 2007

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