Note for BSC 220 at UA-Biological Evolution Chapter 4 Notes
Note for BSC 220 at UA-Biological Evolution Chapter 4 Notes
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Date Created: 02/06/15
BE Chapter 4 Emergence of Life March 6 2012 MAJOR TRANSITIONS OF LIFE Life on Earth is built on Carbon Carbon is stable but reacts favorably with useful elements such as hydrogen and nitrogen and it easily forms stable complexes making it the molecular backbone of proteins carbohydrates and fats Because Carbon is the basis of life here organic life chemistry is based on Carbon The history of life on Earth includes major transitions One of the first was a change from inorganic to organic evolution namely from the modi cation of inorganic chemicals to modi cations of organic compounds The result was the development of organic building blocks oflife Inorganic to Organic Evolution 4 billion years ago Between 4 and 46 billion years ago the Earth had cooled sufficiently that larger molecules were stable The early Earth s atmosphere essentially lacked free oxygen 02 Instead the atmosphere included some hydrogen methane gas carbon dioxide and nitrogen 0 Could these form the basic organic building blocks of life 0 Yes In laboratory experiments organic compounds formed spontaneously In fact it has proved to be remarkably easy to get organic compounds to form within these simulated primitive atmospheric conditions hot or cold 0 In the young Earth the exposure to heat radiation pressure and even cold caused a slow accumulation of organic chemicals This transition on Earth to organic evolution occurred most likely about 4 billion y a Much earlier than this and the Earth was still being pummeled by large chunks of rocks left over from the formation of the solar system These impacts generated heat that vaporized surface water preventing seas and lakes from persisting The earliest microorganisms were more like macromolecules large chains of replicating carbonbased molecules Strictly speaking these organisms were heterotrophs meaning they did not directly manufacture ingredients needed for their own maintenance and duplication but lived on organic chemicals in their environment Cell Prokaryotic Heterotroph 35 billion years ago The oldest fossils of microorganisms are found in rocks 35 byo in W Australia These microfossils resemble bacteria that exist today The plasma membrane which defines the boundaries of the cell protects the activities within and acts as a selective gatekeeper with the outside environment Also present is a semirigidcell wall usually outside the plasma membrane which stabilizes the shape of the bacterium These early cells were prokaryotic cells meaning that their cellular DNA was free within the cytoplasm and they lacked organelles They reproduce asexually by means of binary ssion whereby a single prokaryotic cell divides in two and again and again Cell Prokaryotic Autotroph 27 billion years ago Photosynthesis converts the sun s energy into chemical energy used to meet the metabolic needs ofthe cell Rocks about 27 byo indicate that free oxygen had already begun to accumulate in the atmosphere Indirectly this suggests that photosynthesis was well established by this time These first photosynthesizing prokaryotic cells were autotrophs their energy came by the direct conversion of the sun s energy into the manufacture of the cells own ingredients 0 Cyanobacteria Free oxygen was of biological origin from the watersplitting step of photosynthesis The free oxygen produced by the first cyanobacteria dissolved into the surrounding water where it reacted with iron to form iron oxide rust When all the dissolved iron had precipitated out oxygen reached saturation levels in the bodies of water and bubbled out into the atmosphere Cell Prokaryote t0 Eukaryote 2 billion years ago Eukaryotic cells have more complex interiors than prokaryotic cells The cellular DNA of a eukaryotic cell is enclosed in a specialized membraneithe nuclear envelopeiand the cell contains numerous organelles membranebound compartments dedicated to particular cellular functions 0 One such organelle is the mitochondrion a small power factory within the cytoplasm where oxygen is consumed in organic fuels to obtain energy used by the cell Plants cell walls composed of cellulose a carbohydrate which encloses individual cells and collectively provides the structural support of the plant body Eukaryotic cells can reproduce asexually by cell division through mitosis a derived form of binary fission and they can also reproduce sexually through fusion oftwo specialized sex cell gametes Each sex cell formed during meiosis contains one copy of the doublestranded DNA Sexual reproduction leads to frequent genetic recombination in turn generating variation the raw material for evolution Multicellularity Singlecelled microorganisms must be a jack of all trades Multicellular organisms take advantage of cell specializations wherein some cells are involved in growth others are involved in energy processing and delievery others are sensory cells responding to environmental challenges Large organisms living todayiplants fungi animalsiare built on the theme of multicellularity MAJOR TRANSITIONS OF LIFE AND CONSEQUENCES Ozone Just as Earth had an impact on life life had an impact on Earth Ozone 03 a derivative of Oz circles the Earth in the atmosphere filtering out much of the incoming radiation that would otherwise bombard life on the surface of Earth with ha1mful UV rays that penetrate skin and over time shred proteins and DNA Atmosphere of young earth had no 02 and hence no 03 Pollutant The first prokaryotic cells to evolve did so in the absence of oxygen Later photosynthesis made its debut and Oz began to accumulate 0 As it accumulated oxygen became a pollutant to these first prokaryotes Theirs was an anaerobic environment without oxygen 0 For prokaryotes adapted to an anaerobic environment oxygen was destructive o In our oxygenrich environment presently these prokaryotes have retreated to ancient anaerobic environments iswamps hot springs etc 0 Cells amp multicellular organisms that evolved later did so in an atmosphere relatively rich in oxygen and came to depend on oxygen s special energetic contribution Eukaryotic Origins o Organisms cooperate in mutually bene cial associations Why not cells 0 In fact that happened The ancient evolutionary events are termed endosymbiosis wherein eukaryotic cells developed partnerships with prokaryotic organism living within their cell borders 0 The mitochondria and chloroplasts evolved form prokaryotes that initially lived independently but were incorporated into eukaryotic cells providing them with energy factories and photosynthesis CHEMICAL CODINGiFROM GENOTYPE TO PHENOTYPE DNA 0 Nucleotides are one of the most important groups of molecules in the cell 0 These consist of a sugar deoxyribose ribose joined on one end with the chemical phosphate and on the other with an organic base a nitrogencontaining chemical ring 0 This genetic material is DNA 0 The DNA incorporates materials and information outside itself Nutrition environmental in uences and eventually learning contribute to the finished phenotype o The two strands of DNA turn together in a helical pattern forming a doublestranded molecule wrapped in proteins to form a chromosome 0 Each nucleotide in the DNA strand has one of four possible bases thymine T cytosine C adenine A and guanine G RNA 0 The DNA is atemplate To carry out its coded instructions it producesitranscribesia complementary messenger molecule of RNA mRNA 0 mRNA directly serves as a template to guide the assembly of chemicals into useful products 0 The structure of this mRNA is matched to DNA and so carries forward the DNA s coding instructions 0 RNA is also composed of nucleotidesicytosine adenine and guanine but with uracil U in place of thymine 0 Each nucleotide of DNA pairs with one particular nucleotide of RNA 0 Adenine to uracil o Cytosine to guanine o Thymine to adenine o Guanine to cytosine o The mRNA now translates its coded info into proteins Each set of 3 mRNA nucleotides forms a codon a triplet sequence of mRNA nucleotides that specifies a particular amino acid one of the building blocks of protein Genes 0 Gene expression 0 In prokaryotes the DNA is a circular doublestranded helix the mRNA transcribed is ready to function and the protein produced is manufactured within the cytoplasm o In eukaryotes the DNA is a linear doublestranded helix that resides within the nucleus 0 Nuclear DNA 0 Mitochondrial DNA 0 Together proteins carbohydrates sugars and fats are the basic building blocks of cells and in turn of the organism Cell Metabolism o Metabolism is the term for the general chemical processes of breaking down molecules to capture the energy released or the building up of complex molecules which requires energy input 0 The common feature of each process is energyiextracting it or storing it 0 Energy is present in the chemical bonds that bind atoms together As large molecules are broken down these bonds split and the released energy is captured and stored in specialized molecules Metabolic Pathways o In animals there are several major metabolic pathways a series of connected chemical steps that gradually capture in increments energy from molecules 0 These pathways are glycolysis the Krebs Cycle and the electron transport system 0 The harvesting of energy from food is termed cellular respiration Food also yields organic chemicals the basic end products of digestion which are used in turn as chemical building blocks to construct the basic tissues of the animals Carbon Fixation 0 Plants are able to directly manufacture synthesize the various building blocks of their tissues Plants do this in the Calvin Cycle a special metabolic pathway 0 In the Calvin cycle carbon dioxide C02 is snatched from the air and used to synthesize an intermediate sugar This direct incorporation of carbon from C02 into stable organic compounds is termed carbon xation Photosynthesis 0 Plant photosynthesis includes the overall process whereby solar energy is captured chloroplasts and then transformed into organic molecules manufactured from carbon dioxide Calvin cycle The primary organic molecules produced in photosynthesis are sugars destined for use in the cell wall cellulose for storage starch as cellular compounds and in support of cell metabolism WHAT IS LIFE 0 Two features distinguish life from nonlifeireplication and metabolism 0 The earliest life was selfreplicating able to produce copies of itself and capable of metabolism the ability to process energy for selfmaintenance 0 Today in living cells the task of replication falls to DNA and that of metabolism mainly to proteins termed enzymes 0 Some biologists now assume that the first life may not have been DNA but RNA and others believe it was a protein world first An RNA World 0 No selfreplicating RNA molecules exist today but laboratory experiments show that randomly generated RNA sequences can become selfreplicating And some RNA molecules exist today that can act as enzymes
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