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| In [[evolutionary biology]], the The '''GARD (Graded Autocatalysis Replication Domain) model''' is a general kinetic model for homeostatic-growth and fission of compositional-assemblies, with specific application towards lipids.<ref>{{cite journal |last1=Segre |first1=D. |last2=Ben-Eli |first2=D. |last3=Lancet |first3=D. |year=2001 |title=Compositional genomes: Prebiotic information transfer in mutually catalytic noncovalent assemblies |journal=Proc Natl Acad Sci U S A |volume= |issue= |pages=219–230 |publisher=Elsevier |doi=10.1073/pnas.97.8.4112 |url=http://www.pnas.org/content/97/8/4112.abstract }}</ref>
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| In the context of [[abiogenesis]], the [[Abiogenesis#Lipid world|lipid-world]] <ref>{{cite journal |last1=Segre |first1=D. |last2=Ben-Eli |first2=D. |last3=Deamer |first3=D. |last4=Lancet |first4=D. |year=2001 |title=The lipid world |journal=Orig Life Evol Biosph |volume=31 |pages=119–145 |doi=10.1023/A:1006746807104 |url=http://www.springerlink.com/content/rq23036351237250/ |pmid=11296516}}</ref> suggests assemblies of simple molecules, such as [[lipids]], can store and propagate information, thus undergo [[evolution]].
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| These 'compositional assemblies' have been suggested to play a role in the origin of life. The idea is the information being transferred throughout the generations is '' compositional information '' – the different types and quantities of molecules within an assembly. This is different from the information of an [[RNA]] or [[DNA]], which is the specific sequence of bases in such molecule.
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| Thus, the model is viewed as an alternative or an ancestor to the RNA world hypothesis.
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| ==The GARD model==
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| The composition vector of an assembly is written as: <math>v=n_1\cdots n_{N_G}</math>. Where <math> n_1\cdots n_{N_G} </math> are the molecular counts of lipid type ''i'' within the assembly, and NG is how many different lipid types exist (''repertiore size'').
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| The change in the count of molecule type ''i'' is described by:
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| : <math> \frac{dn_i}{dt} = (k_f \rho_i N-k_b n_i) \left(1+\sum_{j=1}^{N_G}\beta_{ij} \frac{n_j}{N}\right) </math>
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| <math>k_f</math> and <math>k_b</math> are the basel forward (joining) and backward (leaving) rate constants, ''β''<sub>''ij''</sub> is a non-negative rate enhancement exerted by molecule type ''j'' within the assembly on type ''i'' from the environment, and ρ is the environmental concentration of each molecule type.
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| The assembly current size is <math>N=\sum_{i=1}^{N_G}n_i</math>. The system is kept away from equilibrium by imposing a fission action once the assembly reaches a maximal size, Nmax, usually in the order of NG. This splitting action produces two progeny of same size, and one of which is grown again.<br>
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| The model is subjected to a [[Monte Carlo algorithm]] based simulations, using [[Gillespie algorithm]].
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| ==Selection in GARD==
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| In 2010, [[Eors Szathmary]] and collaborators have chosen GARD as an archetypal [[metabolism-first]] realization.<ref>{{cite journal |last1=Vasas |first1=V. |last2=Szathmary |first2=E. |last3=Santos |first3=M. |year=2010 |title=Lack of evolvability in self-sustaining autocatalytic networks constraints metabolism-first scenarios for the origin of life |journal=Proc Natl Acad Sci U S A |volume=107 |issue=4 |pages=1470–1475 |doi=10.1073/pnas.0912628107 |url=http://www.pnas.org/content/107/4/1470.abstract}}</ref> They have introduced selection coefficient into the model, which increase or decrease the growth rate of assemblies, depending on how similar or dis-similar they are to a given target. They found that the ranking of the assemblies are un-affected by the selection pressure, and concluded that GARD does not exhibit Darwinian evolution.<br>
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| In 2012 it was shown that this criticism is errorneous and was refuted.<ref>{{cite journal |last1=Markovitch |first1=O. |last2=Lancet |first2=D. |year=2012 |title=Excess Mutual Catalysis Is Required for Effective Evolvability |journal=Artificial Life |volume=18 |issue=3 |pages=243–266 |doi=10.1162/artl_a_00064 |url=http://www.mitpressjournals.org/doi/abs/10.1162/artl_a_00064 |pmid=22662913}}</ref> Two major drawbacks of the 2010 paper were: (1) they have focused on a general assembly and not on a composome or compotype (faithfuly replicating and [[quasispecies]], respectively); (2) they have performed only a single, random, simulation to test the selectability.
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| ==External links==
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| *Doron Lancet [http://www.weizmann.ac.il/molgen/members/lancet.html homepage] at [[Weizmann Institute of Science]], who is the inventor of GARD.
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| *Origin of life ([http://ool.weizmann.ac.il OOL]) at the Weizmann Institute.
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| ==References==
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| {{Reflist}}
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| [[Category:Evolutionary biology]]
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| [[Category:Origin of life]]
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| [[Category:Stochastic simulation]]
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Hi there! :) My name is Ambrose, I'm a student studying Arts from Bothell, United States.
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