ONE of the great transitions in the history of life took shape when naked genetic material clothed itself and protocells were born. But try to recreate this moment and you quickly hit a stumbling block. A lipid-coated protocell doesn’t have the machinery to easily divide in two when the genetic material replicates.
Now Kunihiko Kaneko and Atsushi Kamimura, both of the University of Tokyo, Japan, have devised a model which may solve the problem. They took inspiration from living things, in which DNA and RNA code for proteins, and proteins catalyse replication of the genetic material. They suggest a similar self-perpetuating system in which a cluster of two types of molecules catalyse replication for one another and demonstrate a rudimentary form of cell division.
Kaneko and Kamimura argue that natural selection acts on the cluster, improving copy fidelity and boosting the complexity of the protocell. “This may solve the origin of heredity and origin of compartmentalisation simultaneously,” says Kaneko.
In their model, as with DNA, the genetic material – or heredity carrier as they call it – replicates much more slowly than the other cluster molecules and also takes longer to degrade, so it allows an entourage of the other molecule to accumulate. After the heredity carrier replicates, the copies drift away from one another, and the molecules between them break down, automatically creating two separate entities. In effect, when the heredity carrier replicates and separates, the whole cluster divides.
Kaneko and Kamimura’s cluster model isn’t plagued by “parasitic molecules” either. These are selfish entities which may be catalysed by one of the molecules but do not in turn help it replicate. For example, a parasite molecule that only mimicked a heredity carrier wouldn’t have a team of catalysers, so it wouldn’t replicate as well as the true heredity carrier, and would therefore die off (arxiv.org/abs/1005.1142v1).
“This is an interesting model, although there are a couple challenges for a realistic application,” says biologist Irene Chen of Harvard University. In real life, membrane lipids around an RNA molecule don’t typically catalyse RNA replication, but she speculates that replacing the lipids with hydrophobic peptides might work in the way Kaneko and Kamimura propose. However, there is another challenge to overcome: you need access to a high concentration of chemical building blocks to generate the crowded cluster around the heredity carrier, Chen says.
She suggests that periodic wetting and drying – as you might find in tidal regions or clay ponds – could increase these concentrations. You would of course need experiments to demonstrate that real chemicals associated with life can divide as the team suggest, since Kaneko and Kamimura don’t name specific molecules for their heredity carrier or cluster species.
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