Sunday, May 29, 2011
Dramatic Breakthrough in Accounting for Life's Origin
The matter of the ultimate origin of life, the theory of Abiogenesis (which is often erroneously conflated with the theory of evolution) has been problematical for years. What is sought is a basic explanation for how fundamentally non-living matter could acquire the properties and attributes of life, including being able to reproduce. In principle this isn't that remarkable a stretch, since we already know there exist living entities at the "margins" - the viruses- which display no attributes of life until they become attached to a host. Once in a host, they can appropriate its cell machinery to churn out billions of copies of themselves.
Evolution in such organisms is also no biggie. For example, consider a point mutation in a Type A flu virus. Here, a minuscule substitution of amino bases yields a virus imperceptibly different in DNA structure from a predecessor. This is a case of microevolution brought about by mutation. A new 'flu vaccine must be prepared to contain it. The most that flu vaccines achieve is keeping the selection or s-value fairly constant for a majority of influenza viruses, while not entirely eliminating the associated gene frequencies. Hence, yearly vaccines only attempt to reduce the most virulent strains, such as ‘Type A flu’, to the most minimal equilibrium frequency.
Total elimination is impossible because there are always new gene mutations of the virus to assume the place of any strains that have been eliminated. At the same time, the ongoing enterprise of preparing new flu vaccines is an indirect acknowledgement of microevolution in the flu virus. Amazingly, there are many tens of thousands of uneducated people who actually don't believe such examples qualify as bona fide evolution! It's as if these forlorn people can't process that the success of natural selection is inextricably bound to the fitness (w) and the selective value, s, e.g. via: w = 1 - s.
Meanwhile, we know there are pleuro-pneumonia like organisms or PPLOs for short. The PPLO is as close to the theoretical limit of how small an organism can be . Some figures clarify this. It has about 12 million atoms, and a molecular weight of 2.88 million Daltons . Compared to an amoeba, it weighs about one billions times less.
Now, in a remarkable find published in The New Scientist (Vol. 209, No. 2794, p. 11), two investigators: Kunikho Kaneko and Atsushi Kamimura, have made a remarkable breathrough in devising a testable model that is able to replicate the Abiogenesis process. The two basically solved the problem of how a lipid-coated protocell can divide into two (displaying reproduction) when the genetic material replicates. Recall in an earlier blog where I showed the hypothetical protocell reaction wherein a self-sustaining coacervate droplet can use one or two basic reactions involving adenosine triphosphate (ATP) and adenosine diphosphate:
L*M + R + ADP + P -> R + L + M + ATP
ATP + X + Y + X*Y -> ADP + X*Y + X*Y + P
In the above, L*M is some large, indeterminate, energy-rich compound that could serve as ‘food’. Whatever the specific form, it’s conceived here to have two major parts capable of being broken to liberate energy. Compound R is perhaps a protenoid or lipid-coated protocell, but in any case able to act on L*M to decompose it. The problem with this earlier hypothesis was that such lipid-coated protocells lack the machinery to allow for easy division.
Kaneko and Kamimura solved this by taking their inspiration (for their model) from living things in which DNA and RNA code for proteins and the proteins catalyse replication of the genetic material. This goes back to biochemist Jacque Monod's concept that the organism is a self-constructing machine. Its macroscopic structure is not imposed upon it by outside forces, instead it shapes itself autonomously by dint of constructive internal (chemical) interactions. Thus in the Kaneko- Kamimura model one has a self-perpetuating system in which a cluster of two types of molecules catalyse replication for one another while also demonstrating rudimentary cell division.
In the Kaneko and Kamimura model, as with DNA, the genetic material replicates much more slowly than the other cluster molecules but also takes longer to degrade, so it enables lots of the other molecule to accumulate. Following replication of the heredity carrier the copies drift apart while the molecules between them break down automatically creating two separate entities (see image).
This is an exciting breakthrough but some further investigations are needed, specifically ways to circumvent the problem that (in real life) membrane lipids around an RNA molecule don't typically catalyse RNA replication. However, this isn't insurmountable, because all one need do (theoretically) is replace the lipids with hydrophobic peptides.
We look forward to further work done by Kaneko and Kamimura as well as others in the microbiology field, working at the forefront of Abiogensis.