Origin of life lecture

Deep-sea vent hypothesis

Biologically important molecules may have been formed in the temperature gradient between extremely hot vent water and cold ocean water - Supported by experiments - Complex biological communities found here that derive energy from chemicals in the vent (not the sun)

Stage 3: Formation of boundaries (protobiont)

- Aggregate of prebiotically produced molecules and macromolecules that acquired a boundary, such as a lipid bilayer, that allowed it to maintain an internal chemical environment distinct from that of its surroundings

Reducing atmosphere hypothesis

- Based on geological data - Atmosphere rich in water vapor, H2, CH4, NH3 (and little O2) - Chamber simulates atmosphere and bolts of lightning - Formed precursors, amino acids, sugars and nitrogenous bases - First attempt to apply scientific experiments to understand origin of life - Since 1950s, ideas about early Earth atmosphere changed • Similar results with different atmospheric composition

Changes in living organisms the result of

- Genetic changes - Environmental changes • Can allow for new types of organisms • Responsible for many extinctions

Extraterrestrial hypothesis

- Meteorites brought organic carbon to Earth • Includes amino acids and nucleic acid bases - Opponents argue that most of this would be destroyed in the intense heating and collision

protobiont characteristics

1. Boundary separated external environment from internal contents 2. Polymers inside the protobiont contained information 3. Polymers inside the protobiont had enzymatic function 4. Protobionts capable of self-replication

Origin in 4 overlapping stages

1. Nucleotides and amino acids produced prior to the existence of cells 2. Nucleotides and amino acids became polymerized to form DNA, RNA and proteins 3. Polymers became enclosed in membranes 4. Polymers enclosed in membranes acquired cellular properties

Miller / Urey experiment

Experiment that modeled Earth's primordial atmosphere in the lab and produced amino acids. (i.e. organic matter from Earth's primordial conditions)

Prokaryotic cells arose during Archaeon Eon

• Archaeon Eon - when diverse microbial life flourished in primordial oceans • First known fossils 3.5 bya • First cells prokaryotic • All life forms prokaryotic during Archaeon Eon • Hardly any free oxygen so organisms were anaerobic • First cells were heterotrophs • Autotrophs evolved as supply of organic molecules dwindled

Bartel and Szostak Demonstrated Chemical Selection in the Laboratory

• Began with synthesis of 10^15 RNA molecules (long) • Each RNA contained 2 regions - constant region the same in all the molecules and a variable region • Also made short RNAs that were complementary to a portion of the long RNA and had a tag sequence to bind to beads • If the long RNAs mutated and obtained enzymatic activity, the long RNA would be held to the short RNA bound to the beads • Long RNAs that had this ability formed Pool #1 • More long RNAs were made that were variations on Pool #1 • Repeated several times • Pool #10 showed enzymatic ability 3 million times higher that the original random pool • Results showed that chemical selection improves the functional characteristics of a group of RNA molecules over time by increasing the proportions of those molecules with enhanced function

Chemical selection

• Chemical within a mixture of different chemicals has special properties or advantages that cause it to increase in number compared to other chemicals in the mixture • Hypothetical scenario with 2 steps 1. One of the RNA molecules mutates and has enzymatic ability to attach nucleotides together • Advantage of faster replication 2. Second mutation produces enzymatic ability to synthesize nucleotides • No reliance on prebiotic synthesis

Living cells may have evolved from

• Coacervates - Droplets that form spontaneously from the association of charged polymers - Enzymes trapped inside can perform primitive metabolic functions • Liposomes - Vesicles surrounded by a lipid layer - Clay can catalyze formation of liposomes that grow and divide - Can enclose RNA

Stage 1: Origin of organic molecules

• Conditions on primitive Earth may have been more conducive to spontaneous formation of organic molecules • Prebiotic or abiotic synthesis - Little free oxygen gas - Formed prebiotic soup • Several hypotheses on where and how organic molecules originated

Stage 2: Organic polymers

• Experimentally, prebiotic synthesis of polymers not possible in aqueous solutions (controversial) - Hydrolysis competes with polymerization • Experiments have shown formation of nucleic acid polymers and polypeptides on clay surface

Advantages of DNA/RNA/protein world

• Information storage - DNA would have relieved RNA of informational role and allowed RNA to do other functions - DNA is less likely to suffer mutations • Metabolism and other cellular functions - Proteins have a greater catalytic potential and efficiency - Proteins can perform other tasks - cytoskeleton, transport, etc.

Stage 4: RNA world

• Majority of scientists favor RNA as the first macromolecule of protobionts • 3 key RNA functions 1. Ability to store information 2. Capacity for self-replication 3. Enzymatic function - ribozymes • DNA and proteins do not have all 3 functions

Origins of multicellularity

• Multicellular eukaryotes arise 1.5 bya • 2 possible origins - Individuals form a colony - Single cell divides and stays stuck together • Volvocine green algae display variations in the degree of multicellularity • Multicellular animals emerge toward the end of the eon • First animals invertebrates - Bilateral symmetry facilitates locomotion

The Origin of Eukaryotic Cells During the Proterozoic Eon Involved a Union Between Bacterial and Archaeal Cells

• Origin of first eukaryotic cell matter of debate • In modern eukaryotes, DNA found in nucleus, mitochondria and chloroplasts • Examine properties of this DNA and modern prokaryotes • Nuclear genome - both bacteria and archaea contributed substantially - Symbiotic relationship - 2 species live in direct contact - Endosymbitoic - one organism lived inside another • Data supports this origin

Fossils

• Preserved remains of past life on Earth • Many rocks with fossils are sedimentary - Sediments pile up and become rock - Organisms buried quickly and hard parts replaced by minerals • Older rock is deeper and older organisms are deeper in the rock bed

Dates

• The universe began with the Big Bang about 13.7 bya • Our solar system began about 4.6 bya • The Earth is 4.55 billion years old • 4 bya the Earth cooled enough for outer layers to solidify and oceans to form • 4-3.5 bya life emerged