Gard model

The GARD model (Graded Autocatalytic Replication Domain)

In the context of abiogenesis, the Lipid-world [1] suggests assemblies of simple molecules, such as lipids, can store and propegate information, thus undergo evolution.

These 'compositional assemblies' have been suggested to play a role in the origin of life (OOL). 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.

Thus, the model is viewed as an alternative or an ancestor to the RNA world hypothesis.

Contents

The GARD model

GARD is a general kinetic model for homeostatic-growth and fission of compositional-assemblies, with specific application towards lipids.[2]

The composition vector of an assembly is written as: v=n_1\ldots n_{N_G}.Where n_1\ldots n_{N_G} are the molecular counts of lipid type i within the assembly, and NG is how many different lipid types exist (repertiore size).

The change in the count of molecule type i is described by:

\frac{dn_i}{dt} = (k_f*\rho_i*N-k_b*n_i)*(1%2B\sum_{j=1}^{N_G}\beta_{ij}*\frac{n_j}{N})

k_f and k_b are the basel forward (joining) and backward (leaving) rate constants, βij 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.

The assembly current size is N=\sum_{i=1}^{N_G}n_i.. The system is kept away from equlibrium 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.
The model is subjected to a Monte Carlo algorithm based simulations, using Gillespie algorithm.

Selection in GARD

In 2010, Eors Szathmary and collaborators have chosen GARD as an archtypal metabolism-first realization.[3] They have introduced selection coefficient into the model, which increase or decrease the growth rate of assemblies, depanding 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.

External links

References

  1. ^ Segre, D.; Ben-Eli, D.; Deamer, D.; Lancet, D. (2001). "The lipid world". Orig Life Evol Biosph 31: 119–145. doi:10.1023/A:1006746807104. PMID 11296516. http://www.springerlink.com/content/rq23036351237250/. 
  2. ^ Segre, D.; Ben-Eli, D.; Lancet, D. (2001). "Compositional genomes: Prebiotic information transfer in mutually catalytic noncovalent assemblies". Proc Natl Acad Sci U S A (Elsevier): 219–230. doi:10.1073/pnas.97.8.4112. http://www.pnas.org/content/97/8/4112.abstract. 
  3. ^ Vasas, V.; Szathmary, E.; Santos, M. (2010). "Lack of evolvability in self-sustaining autocatalytic networks constraints metabolism-first scenarios for the origin of life". Proc Natl Acad Sci U S A 107 (4): 1470–1475. doi:10.1073/pnas.0912628107. http://www.pnas.org/content/107/4/1470.abstract.