Evolution of Prokaryotic Cells
Key points
- natural selection as driving force: variation & selection
- organic molecules, polymers synthesized far from chemical equilibrium
- RNA world: both informational and catalytic functions
- containment by membranes critical for selection of favorable cooperative interactions
- DNA and protein evolved as more efficient for informational and catalytic RNA functions
- proks have rapid division, rapid adaptation; versatile and diverse
- through photosynthesis proks. changed the worlds environment
Overview:
- Organization of class
- lectures
- problem sets
- exams
- grading
- Cell Bio Fair
- Importance of cell biology
- cells are the organizational structure of life
- cells are between molecules (made from) and organisms (cooperate to make)
- Our understanding is in models of the physical world
- what is an example of a model?
- importance of model organisms
- Origin of proks/euks multi-step model: the steps to proks
- cells are the organizational structure of life
- similar to our models of processes like cell division, but less easily tested
- The central principle is evolution by selection
- 1) variation- genetic info subject to random variation
- 2) selection- genetic info that favors propagation
- Steps from molecules to proks.
- small organic molecules
- polymers: evolve catalytic activities
- replication, templated: RNA: information, function, evolving molecule
- protein synthesis: by RNA, increases catalytic power
- DNA takes over informational role
- membranes encapsulate reactions: selection acts on cooperative rxns
- simple proks, very diverse, self replicate
- metabolic pathways evolve: eg glycolysis
- photosynthesis: CO2 & N2 fixation (first H2S then H20), O2 production
- aerobic proks
Lecture Outline
(Model for origin of cell/life 3.5-4 billion years ago)
- organic molecules
- can be formed surprisingly easy e.g. U tube trap expt.
- found in aqueous solutions: hydrogen cyanide and formaldehyde
- these in turn generate: amino acids, sugars, purines, pyramidines
- polymers/catalytic assembly: "RNA world"
- small molecules polymerize
- once polymerized can act as catalyst
- assumed that some catalyzed self replication
- autocatalysts have some important charact. of living matter:
- reproduction
- far from random group of interacting molecules
- far from chemical equilibrium
- compete for "food" stocks
- under wrong conditions (temp, food) decay to chem. equil. (die)
- RNA most likely autocatalyst, templated reproduction (complementarity)
- sequence/function relationship RNA (preceeding DNA and proteins)
- information passed during replication ie "genotype"
- function: sequence dep. structure gives binding (catalytic) properties,"phenotype"
- acquire catalytic properties other than self replication (e.g. replication of others)
- mistakes during replication are selected=>favorable sequences predominate
- if cooperations are beneficial then replication becomes very efficient
- selection for accuracy, speed, and product stability
- protein synthesis
- would have increased effic.
- started as RNA only synthetic machine
- today involves three types of RNA (info, adaptor, catalytic) + proteins
- genetic code same in all org.=>descent from primitive protein syn. cell
- DNA: double stranded structure: stable, easy to replicate, permits repair
- took over genetic function
- protein became major catalyst, RNA became intermed. see Fig. 1-11
- membrane
- container of reactants: critical for selection of favored enzymes
- selected for on basis of entire set of interactions within
- amphipathic phospholipids self assemble into vesicles
- Prokaryotic cells
- cell wall, PM, cytoplasm containing DNA, RNA, proteins, small molecules
- small
- replicate quickly (20min under optimal conditions) 5 billion cells< 11 h.
- in optimal conditions "survival of fittest" means divides fastest
- adapt to environmental changes quickly
- most abundant type of cell on earth, extremely diverse
- Two types: eubacteria, archaebacteria
- Evolution of metabolic paths (reaction chains)
- originally simple reactions
- then selection for those cell that have most effic. syn->metabolic pathway
- many identities among all organisms => extremely early origin
- Evolutionary relationships in sequences
- use slow changing (core) enzymes for distant comparisons
- use fast changing (specialized) enzymes for recent comparisons
- photosynthesis: CO2 fixation led to abundant energy source & O2 production
- CO2 and N2 very stable=> large energy for incorp. into organic molecules
- CO2 converted by photosyn.; electrons added to CO2 makes it reactive
- first source was H2S then evolved to use H2O
- cyanobacteria (blue-green algea) fix both CO2 and N2
- they make organic molecules which other organism subsist on
- Aerobic Oxidation
- oxygen is highly reactive-> disadv and advantage
- toxic (still toxic to anearobic organisms)
- is used to breakdown glucose to CO2 & H2O and to make ATP