Search for Life in the Universe

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Search for Life in the Universe

Chapter 5The Nature of Life on Earth

(Part 2)

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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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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

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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

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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

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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

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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?

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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

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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?)

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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• 20,00025,000 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?

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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

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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

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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

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Extremophiles• Volcanic vents:

– Water temperature reaches 400C (750F), 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

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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