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The Origin of Life Oceans: the Cradle of Life? Cells

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The Origin of Life Oceans: the Cradle of Life? Cells Oceans: the cradle of life? • Highest diversity of life, particularly archae, bacteria, and animals The origin of life • Will start discussion of life in the ocean with prokaryote microorganisms • Prokaryotes are also believed to be the first cellular organisms on earth • How might life on earth have started? • From molecules to the first cell Cells: a sense of scale Head of a needle Fig 25-UN8 Simple biological molecules can 1.2 bya: form under prebiotic conditions First multicellular eukaryotes 535–525 mya: Cambrian explosion 500 mya: (great increase • Conditions on earth during first billion 2.1 bya: Colonization in diversity of First eukaryotes (single-celled) of land by animal forms) fungi, plants years and animals 3.5 billion years ago (bya): • No free oxygen, no ozone layer to absorb First prokaryotes (single-celled) UV radiation • Atmosphere = CO 2, CH 4, NH 3, H 2 t 0 0 0 0 0 0 0 0 n 0 0 0 0 0 0 0 0 • Violent weather = eruptions, lightning, e 0 5 5 0 0 5 0 5 , , , , , , , s 4 1 1 3 3 2 2 e r torrential rains Millions of years ago (mya) P Simple organic molecules Laboratory are likely to “created” have been molecules produced under such conditions • nucleobases Sugars such as ribose Formation of polymers from Spontaneous polymerization of simple organic compounds random polymers • Amino acids (aa) can join by forming peptide bonds • Nucleotides (nt) can join by forming phosphodiester bonds • In present day cells - polypeptides (proteins) are from aa, polynucleotides (RNA and DNA) are from nt. Earliest polymers were of variable length and random Autocatalytic systems sequence • Mechanism of formation • Origin of life requires that some polymers – Heating of dry organic compounds must have a crucial property: the ability to – Catalytic activity of inorganic polyphosphates catalyze reactions that lead to the or other mineral catalysts production of more molecules of itself. • Polymers can influence subsequent • This can be accomplished by reactions by acting as a catalyst polynucleotides Complementary templating Ribozymes • Catalytic RNA molecule • Believed to be the basis for the first autocatalytic systems • Acts as both template and catalyst • Eventually specialization where DNA is the template and proteins are catalysts Self-replicating RNA systems • Start with a catalytic RNA molecule that polymerizes nucleotides to reproduce itself Self-replicating RNA systems The beginning of evolution • 3.5 to 4 billion years ago - mixture of self- • Next step: replicating RNA systems and organic family of molecule precursors mutually • Systems with different sets of polymers supportive competed for the available precursors catalytic RNA molecules. • Success depended on the accuracy and One catalyzes speed with which the copies were made the and the stability of these copies reproduction of the others From polynucleotides to Primative RNA/protein systems peptides • Polynucleotides are well suited for • Would have an advantage over RNA only information and storage but have limited systems due to better catalysis facilitated catalytic properties by proteins • Proteins have much greater catalytic capabilities • In modern cells, synthesis of proteins is catalyzed by ribosomes (proteins and RNA) from RNA templates. Membranes defined the first cell • An illustration of a protocell, composed of a fatty acid membrane encapsulating RNA ribozymes. Membranes defined the first cell • The development of an outer membrane was a crucial event • Proteins synthesized by a certain species of RNA would not increase reproduction of that RNA species unless they remained in the neighborhood of the RNA • Proteins could diffuse away and benefit competing RNA molecules The need for containment is filled by amphipathic molecules • Amphipathic - property where one part of the molecule is hydrophobic and the other part is hydrophilic. • In present-day cells, these amphipathic molecules are primarily phospholipids. Membranes are readily formed • A simple fatty acid (far left) may have been a major componenent of early cell membranes. To the right of from amphipathic molecules the fatty acid is a phospholipid, which is the primary component of modern cell membranes. Vesicles and • Example: phospholipids micelles, shown on the right, are structures that can be – Hydrophobic tail group formed by fatty acids or phospholipids. Mouse-over the vesicle or micelle to see the whole structure – Hydrophilic head group • Mix in water and membranes spontaneously form Summary of the hypothetical evolution of the first cells DNA based systems have All present-day cells use DNA advantages over RNA based as their hereditary material systems • DNA acts as a permanent repository of genetic information – Is found principally in a double-stranded form which is more robust and stable – If there is breakage, there is a repair mechanism that uses the intact strand as a template. • DNA templates can become more complex From self-replicating RNA to From procaryotes to eucaryotes present day cells • RNA preceded DNA and proteins, having • 1.5 billion years ago there was a transition both catalytic and genetic properties. from relatively simple procaryotic cells to • Proteins eventually became the major larger more complex eucaryotic cells. catalysts • DNA became the primary genetic material • RNA continues to function in coding for proteins (mRNA) and catalysis (rRNA). The earliest procaryotes were Procaryotes generally have no like present day bacteria obvious internal structures • Bacteria – Simplest organisms – Spherical or rod shaped several microns long – Tough protective coat (cell wall) – Plasma membrane enclosing a single cytoplasmic compartment • DNA, RNA, proteins, and small molecules Originally, there was little need Metabolic reactions evolve for so many metabolic reactions • A bacterium growing in a simple solution • Cells with simple chemistry could survive containing glucose must carry out and grow on the molecules in their hundreds of different reactions. surroundings. • Glucose used for chemical energy and as • Eventually these molecules would become a precursor for all organic molecules the limited and cells devised ways to cell requires. manufacture what they couldn’t acquire. • This led to metabolic complexity. Glycolysis, the oldest metabolic Cyanobacteria can fix CO 2 and pathway N2 • A strong selective advantage would • Degradation of sugar phosphates in the eventually be gained by organisms able to absence of oxygen occurs by glycolysis use C and N directly from the atmosphere. • Glycolysis is similar in all kinds of • CO 2 and N 2 are very stable and require a organisms -- suggesting an extremely large amount of energy as well as ancient origin complicated chemical reactions to convert • Metabolism evolved by sequential them to organic molecules addition of new enzymatic reactions to existing ones. • In present-day organisms hundreds of chemical processes are linked to Photosynthesis and nitrogen Atmospheric oxygen and the fixation course of evolution • Process that used energy from sunlight to convert CO 2 and N 2 into organic compounds. • Cyanobacteria (blue-green) algae are the most self-sufficient organisms that now exist. • The metabolic activity of these organisms set the stage for the evolution of more complex organisms Origin of eucaryotic cells with Aerobic respiration distinct organelles • Accumulation of oxygen in the atmosphere • What happened to the anaerobic led to the ability to oxidize more organisms? completely ingested molecules – Many found low oxygen niches • Also oxygen was toxic to many early • Some acquired aerobic cells as anaerobic organisms intracellular symbionts giving rise to • By around 1.5 billion years ago, organisms eucaryotes with aerobic respiration became widespread Features of eucaryotic cells Features of eucaryotic cells • Outside the nucleus is the cytoplasm where most of the cell’s metabolic • Have a reactions occur. nucleus containing • The cytoplasm contains distinctive most of the organelles cell’s DNA – Prominent organelles are chloroplasts and mitochondria – Each is enclosed in its own membrane Eucaryotic cells depend on mitochondria for aerobic Mitochondria respiration • Similar to free-living prokaryotic cells • Mitochondria • Resemble bacteria in size and shape are found in • Contain DNA, make protein, and virtually all reproduce by dividing in two eukaryotic • Without mitochondria eukaryotic cells cells would be anaerobic organisms including • By engulfing mitochondria, internal oxygen plants and concentrations are kept low animals Acquisition of mitochondria may Some present day organisms have allowed evolution of new may resemble the hypothetical features ancestral eucaryote precursor • Plasma membrane of mitochondria and • They have nuclei but lack mitochondria procaryotes is heavily committed to energy • Example the diplomonad Giardia metabolism • In contrast eucaryote membrane is not. • Eucaryote membrane developed new features – Example: ion channels allowing electrical signaling Giardia • Live as parasites in the guts of animals (including humans) – Low oxygen environment – Rich in nutrients allowing them to survive on inefficient anaerobic metabolism Chloroplasts are descendants of an engulfed procaryotic cell • Similarities to cyanobacteria – Carry out photosynthesis – Structural resemblance (size, stacked membranes) – Reproduce by dividing – Contain DNA nearly indistinguishable from portions of a bacterial chromosome Some present-day cells contain authentic cyanobacteria • Cyanophora paradoxa Fig.
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