Biology CSF - Exam 2

stages of evolutionary origin of cells and their general features

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 properies

origin of organic molecules

• conditions on primitive Earth may have been more conducive to spontaneous formation of organic molecules 

• prebiotic or abiotic 

  • little free oxygen gas 

  • formed prebiotic soup in the ocean

prebiotic vs abiotic 

• prebiotic: before life

• abiotic: without life

reducing atmosphere hypothesis

• based on geological data

• atmosphere rich in water vapor, H2, CH4, NH3, (and little O2)

stanley miller used a chamber apparatus to stimulate this 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 1950’s, ideas about early earth atmosphere changes

  • still, similarly results 

stage 1

• extraterrestrial hypothesis

• deep-sea vent hypothesis 

extraterrestrial hypothesis

• meteorites brought organic carbon to Earth

  • includes amino acids and nucleic acid bases

• opponents argue that most of these resources would be destroyed in the intense heating and collisions

deep-sea vent hypothesis

• biologically important molecules have been formed in the temperature gradient between extremely hot vent water and cold ocean water 

• complex biological communities found here that derived energy from chemicals in the vent (not the sun)

• opponents argue that hydrolysis (chemical breakdown of of a compound due to a reaction with water) reactions would be difficult in aqueous environments 

stage 2 

organic polymers

organic polymers

• experimentally, prebiotic synthesis or polymers not possible in aqueous solutions 

  • hydrolysis competes with polymerization 

• experiments have shown formation of nucleic acid polymers and polypeptides on clay surface 

stage 3 

formation of boundaries 

formation of boundaries 

• protobiont 

• four characteristics of a protobiont 

• living cells may have evolved from liposomes 

protobiont

• an aggregate of prebiotically produced molecules and macromolecules

• have acquired boundary, such as a lipid bilayer, that allows it to maintain an internal chemical environment distinct from that of its surroundings

four characteristic of a protobiont

1. boundary separated external environment from internal contents

2. polymers inside the protobiont contained information

3. polymers inside that probiont had catalytic function

4. protobionts capable of self-replication

living cels may have envolved from liposomes 

• vesicles surrounded by a lipid layer 

• clay can catalyze formation of liposomes that grow and divide 

• can enclose RNA

stage 4

• RNA world

• chemical selection

• hypothetical scenario with two steps

RNA world

majority of scientists favor RNA as the first macromolecule of protobionts

three key RNA functions

1. ability to store information

2. capability for self-replication 

3. enzymatic function (ribozymes) 

   1. DNA and proteins cannot do all 3 functions

chemical selection

a chemical within a mixture inside a protobiont/first cell has special properties that cause it to increase in number compared to other chemicals in the mixture 

hypothetical scenario with two steps

1. one of the RNA molecules mutates and has enzymatic ability nucleotides together