Raritan Valley Community College Section 22.1 4 Overlapping Stages LECTURE SLIDES Prepared by Aditya Aiyer Raritan Valley Community College Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Section 22.1 Overview Origin of Life Stage 1* Reduced Atmosphere Hypothesis Extraterrestrial Hypothesis Deep Sea Vent Hypothesis Stage 2* Stage 3* Stage 4*

Origin of Life The universe created 13.7 billion years ago Our solar system created 4.6 billion years ago The Earth formed 4.55 billion years ago 50 million years Earth cooled enough for outer layers to solidify and oceans to form Life begins to emerge

Stage 1: Origin of Organic Molecules There were specific conditions on early Earth may conducive to prebiotic formation of organic molecules In prebiotic/abiotic synthesis, methane, carbon dioxide, water vapor, and hydrogen gases formed where no oxygen oxidized these gases or living organisms metabolized these gas Scientists referred to this accumulation as a “prebiotic soup”

Stage 1 cntd Scientists formulated hypotheses on where and how organic molecules first formed Widely debated hypotheses include the Reducing Atmosphere, Extraterrestrial, and Deep Sea Vent Hypotheses

Reducing atmosphere hypothesis Reducing atmosphere rich in water vapor, H2, CH4, and NH3 gases Ammonia and Methane gases reduced other molecules Oxygen cannot oxidize gases and no living organisms can metabolize molecules Formed organic monomers essential to macromolecule formation: amino acids, sugars and nitrogenous bases First attempt to apply scientific experiments to understand origin of life Since 1950s, about early Earth atmosphere changed where prebiotic synthesis could be replicated under neutral conditions

Electrical discharge Electrodes Gases H2O To vacuum H2 CH4 Cold water Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Electrical discharge Electrodes Gases H2O To vacuum H2 CH4 Cold water NH3 Condenser Precipitating droplets Boiling water Trap Sample containing organic molecules such as amino acids

Extraterrestrial hypothesis Asteroids and comets brought organic carbon compounds to Earth Includes amino acids and nucleic acid nitrogen bases Some scientists argue that most of organic content would be disintegrated in intense heating and collision against atmosphere Deep-sea vent hypothesis Biologically important molecules may have been formed in the temperature gradient between extremely hot vent water and cold ocean water Most tested and accepted among the three hypotheses

b: © Woods Hole Oceanographic Institution/Visuals Unlimited Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cold H2O Hot H2S gas Hot H2O Vent H2O temperature suitable for organic chemistry Ocean floor Crack in Earth’s crust (a) Deep-sea vent hypothesis (b) A deep-sea vent community b: © Woods Hole Oceanographic Institution/Visuals Unlimited

Stage 2: Complex Organic Molecule Synthesis Experimentally, prebiotic synthesis of polymers is not possible in aqueous solutions Hydrolysis competes with polymerization Certain experiments have shown formation of complex nucleic acids and proteins on a solid surface, particularly clay

Stage 3: Formation of boundaries Protobiont Collection of prebiotically synthesized organic macromolecules surrounded by a boundary, such as a lipid bilayer or membrane, that allowed it to have chemical properties different from that of its surrounding environment

4 defining characteristics of protobionts - Inner contents of protobiont and environment separated by a boundary like membrane - Nucleic acid polymers like RNA inside protobionts store information - Nucleic acid polymers like RNA had enzymatic functions within protobionts - Nucleic polymers like RNA allow protobionts the capability of self-replication

Living cells may have evolved from Two Different Protobiont Structures Coacervates Clusters of charged polymers Inner enzymes in coarcervates could perform rudimentary metabolism Liposomes Vesicles surrounded by a lipid bilayer Clay, or solid surface, can catalyze formation of liposomes that grow and divide Liposomes can enclose RNA, as RNA is considered to be inner contents of protobionts

Solid droplet of protein and carbohydrate Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Skin of water (a) Coacervates 57 µm Solid droplet of protein and carbohydrate Hollow sphere of phospholipid filled with water Phospholipid bilayer (b) Liposomes 200 nm a: © A. I. Oparin. From the Origin of Life, New York: Dover, 1952; b: © Mary Kraft

Stage 4: RNA was Inner Content of Protobionts Most scientists believe RNA to be the first macromolecule contained in protobionts RNA has 3 major functions - Ability to store information - Capacity for self-replication - Enzymatic function as ribozymes DNA and proteins do not share all 3 of these important functions

Chemical selection Original RNA population gradually changes over time by mutations, ultimately producing a new mutant RNA population with a different chemical composition One example comprises of 2 mutations - One mutation caused first mutant RNA population to acquire enzymatic function to attach nucleotides, thereby increasing rate of replication - Another mutation caused new mutant RNA population to acquire enzymatic function to create nucleotides, which does not require prebiotic synthesis of nucleotides