Primary CellInstructor: Prof. F. MoorePresented by: Z. MokhtarzadehMay 2013
Origin of the Universe13.7 billion years agoThe ―Big Bang‖ led to theformation of the stars of the―universe‖Matter and energy veryrapidly distributed throughoutuniverseTemperatures droppedLight elements (hydrogen andhelium) were produced in thefirst few minutes of the Big Bang4.6 billion years agoPlanets of our solar systemwere formed including our earth
A billion Year Old EarthBy 3.5 billion years ago, when the Earth was a billion years old, it had a thickatmosphere composed of CO2, methane, water vapor and other volcanic gasesBy human standards this earlyatmosphere was very poisonousIt contained almost no oxygenToday our atmosphere is 21% oxygen
A billion Year Old EarthBy 3.5 billion years ago, the Earth also had extensive oceans and seas of salt water,which contained many dissolved elements, such as iron.
Earth ~3.5 billion years ago Lifearose, but how did this happen?
No Life 0.5-1 billion years Life?How did we get here?4.5 billion years ago ~3.5 billion years ago
Early Ideas about Origins of Life▪ Spontaneous generation - life arising from nonliving matter▪ Belief in it goes back to ancient Greek philosophyExamples:▪ mud producing fish▪ grain producing mice▪ decaying meat producing maggots
Redi’s Experiment▪ Effort to disprove spontaneous generation▪ Decaying meat in uncovered control jars vs. covered experimental jars.▪ Results: maggots and flies filled the open jars but not covered jars.Showed only flies produce flies.
The common belief in spontaneous generationwas stopped dead in its tracks in 1862 by LouisPasteur with his famous demonstration thatnutrient fluids, sterilized and sealed againstcontamination, could be kept indefinitelywithout the generation of microbial or otherforms of life.In a sense, Pasteur was almost too good. Hisexperiment made scientists reject the idea thatlife could have arisen spontaneously at anytime under any circumstances.Pasteur’s Experiment (mid-1800s)
Primordial Soup theoryA.I.oparinJ.B.S. HaldaneIn 1920s Oparin, a Russian and Haldane, an Englishman,independently developed a hypothesis that forced reconsiderationof spontaneous generation. They agreed that spontaneousgeneration of life is not possible under present earth conditionsbut suggested that the earths surface and atmosphere were fardifferent during its first millions of years of existence at present.Primordial conditions would favor spontaneous generation of liferather than inhibiting it.
The composition of early atmosphereA.I.oparinJ.B.S. HaldaneEarth‘s early atmosphere had a composition very different than today‘satmosphere•No free O2•More reducing than present atmosphere•Initially thought to contain H2O, H2, CH4, NH3
"Primordial soup" theoryOparin and Haldane thought that with the mix of gases inthe atmosphere and the energy from lightning strikes,amino acids could spontaneously form in the oceans. Thisidea is now known as "primordial soup“. Oparinsuggested that the organic compounds could haveundergone a series of reactions leading to more and morecomplex molecules. He proposed that the moleculesformed colloid aggregates, or coacervates, in an aqueousenvironment. The coacervates were able to absorb andassimilate organic compounds from the environment .Theywould have taken part in evolutionary processes,eventually leading to the first lifeforms.The Oparin-Haldane hypothesis was NOT widely accepted at first because of the weight ofevidence against spontaneous generation and the lack of an effective way to test the hypothesis.
Miller–Urey experiment1950s: Stanley Miller & Harold Urey recreated theassumed early atmosphereContained H2O, H2, CH4, NH3Lacked free O2Energy input in forms of heatand electrical sparksMimic geothermal heat andlightning
Results▪ After a week 15 amino acids in themixture▪ Other biologically importantmolecules had been formedincluding ethanoic acid, lactic acidand urea▪ Later similar experiments weredone using CO2 that producednucleotides.
1) Uncertainty about the early atmosphere was really of that type. The environmentproduced by Miller was more reducing than we now believe the earth‘s earlyatmosphere to have been.2) How to produce polymers (proteins, nucleic acids)? The gas-discharge experimentsonly produce monomers (if conditions are right), but none produce the long chainmolecules that are the ultimate basis for life on the earth.3) With oxygen we have ozone. The ozone layer blocks out a lot of ultra violet light. Ultraviolet light destroys ammonia. Ammonia was one of the gasses used in theexperiment. This creates a problem, does it not?This scenario has recently been criticised for several reasons:
In 1986, the geophysicist Louis Lerman suggested that the key process that formed the chemicals neededfor life took place within bubbles of the ocean‘s surface.Louise Lerman’sbubble model
Chemical Evolution▪ First cells may have originated by chemical evolution involving 4 steps:1) Abiotic (Non-biological) synthesis of small organic molecules (monomers)2) Monomers joined together to form polymers (proteins, nucleic acids)3) origin of self-replicating molecules that eventually made inheritance possible4) packaging these molecules into pre-cells, droplets of molecules withmembranes that maintained an internal chemistry
PolimerizationJoining of monomers into polymers such as protein and nucleic acids. This polymerization in living cells is catalyzed by enzymes Early polymerizations must have occurred without the aid of enzymes Is this possible?
Sidney W.Fox ExperimentIn 1957 Sidney Fox demonstrated that drymixtures of amino acids could be encouraged topolymerize upon exposure to moderate heat.When the resulting polypeptides, or proteinoids,were dissolved in hot water and the solutionallowed to cool, they formed small sphericalshells about 2 μm in diameter—microspheres.
Protocells▪ Proteinaceous microspheres – contain proteins and lipids but no nucleic acids▪ Maintain a localized environment separate from the surroundings▪ Incapable of precise reproduction▪ Exhibit some properties associated with life▪ Metabolism▪ Protocells would eventually pick up RNA and DNA, develop enzymatic capabilities andmembrane organization = primitive cell
RNA or proteins?A raging debate among biologists who studythe origin of life concerns which organicmolecules came first, RNA or proteins. Whichof these arose first is a matter of debateIn all modern organisms, nucleic acids (DNAand RNA) are necessary to build proteins, andproteins are necessary to build nucleic acids -so which came first, the nucleic acid or theprotein?
A Protein WorldThe ―protein-first‖ group argues that without enzymes (which areproteins), nothing could replicate at all, heritable or not. The ―protein-first‖ proponents argue that nucleotides, the individual units of nucleicacids such as RNA, are too complex to have formed spontaneously.While there is no doubt that simple proteins are easier to synthesizefrom abiotic components than nucleotides, both can form in thelaboratory under the right conditions. Deciding which came first is achicken-and-egg paradox.
Iron-Sulfur World▪ A ―metabolism first‖ scenario involves naturallyoccurring iron sulfide (also called pyrite)crystals. These crystals can catalyze bothoxidation-reduction reactions (producingenergy) and polymerizations of amino acids.▪ Works especially well at high temperatures andpressures, such as are found in deep ocean ventscalled ―black smokers‖.▪ An active self-sustaining metabolic system in theabsence of inheritance.
An RNA WorldThe ―RNA world‖ group feels that without a hereditary molecule, other moleculescould not have formed consistently. The ―RNA world‖ argument earned supportwhen Thomas Cech at the University of Colorado discovered ribozymes, RNAmolecules that can behave as enzymes, catalyzing their own assembly. Recent workhas shown that the RNA contained in ribosomes catalyzes the chemical reaction thatlinks amino acids to form proteins. that means that RNA can both store geneticinformation and cause the chemical reactions necessary to copy itself. Thisbreakthrough tentatively solved the chicken and egg problem: nucleic acids (andspecifically, RNA) came first — and later on, life switched to DNA-based inheritance.
The first membranes, the first cells▪ The creation of a cell required a cellmembrane. The evolution of theplasma membrane was a momentousevent because it separated life fromnon-life▪ Lipids spontaneously form bilayer‗vesicles‘▪ These are spherical shells on amolecular scale▪ They can contain self-replicatingRNA strands
Lipids in Membranes▪ In order to spontaneously form a lipid bilayer lipid must have▪ Charges and polar bonds in the head region to interact with water▪ Long fatty acid tails to interact with each other▪ Amphipathic
Lipid Membrane▪ The formation of the membrane performed 3 important tasks▪ the products of the genetic material could be kept close by▪ A cell membrane separates the internal environment from the externalenvironment and regulates the movement of materials into and out of thecell.▪ Chemical reactions became more efficient as reactants could collide morefrequently
A Likely Model▪ Amino acids are formed▪ See the Miller-Urey Experiment▪ Lipid bilayers form▪ These are observed to form spontaneously▪ Self-replicating RNA strings arise▪ This stage is uncertain, but plausible▪ RNA is able to catalyse its own replication▪ RNA strings merge with Lipid bilayers shells▪ First cells form▪ All the components are held together in one place▪ Facilitates chemical processesSpontaneous generation of omino acids,simple carbohydrates, and lipidprecursorsFormation of self-replicating RNAFormation ofproteinsFormation oflipid bubblesEvolution of DNAEvolution of DNA RNA EnzymesProtocellsLiving cells
Unfortunately, our understanding of the origin of life is incomplete.Little is known about how the first cells originated. Current hypothesesinvolve chemical evolution within bubbles, but there is no generalagreement about their composition, or about how the process occurred.Note that while some of these steps have been demonstrated in a lab,nobody has ever made a living cell in a lab.
Refrences▪ Clas blomberg, 2007, phisycs of life: Elsevier, p. 351-354▪ Freeman, Harrington, Sharp, Biological Science: Pearson, p. 99&102▪ Peter H. Raven, George B. Johnson, Jonathan Losos, Susan, 2005, Biology: McGraw-Hill, p64&66▪ WilliamMartin, Michael J. Russell,2002, On the origins of cells: The Royal Society▪ "Did life come from another world?" Scientific American 293, 64 - 71 (2005)▪ Woodward, Robert J., Photo editor,1969, Our amazing world of Nature: its marvelsand mysteries▪ K. Popper, 1990, Pyrite and the origin of life: Nature 344 p. 387▪ Huber, C.; Wächtershäuser, G. (1998). "Peptides by activation of amino acids withCO on (Ni,Fe)S surfaces: implications for the origin of life". Science 281