Солнечная система и ее тайны

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From Chemicals to the First Cells

How did the transition from chemistry to life take place? An important and fascinating question on its own right, but also a question that is crucial in deciding which cosmic environments to explore for life today. In the previous sections we outlined the molecular building blocks needed in order to have a self-sustaining chemical system capable of Darwinian evolution. We also described the basic process of Darwinian evolution and its molecular carriers. Now we would like to see how they all come spontaneously together and begin functioning as a system that is successful over geological timescales.

The transition from chemicals to life was likely a lengthy and complex path. It would have involved a multitude of chemical pathways and reactions, each consisting of many stages that would lead to the synthesis of the critical building blocks of life. Among those building blocks should have been some simple polymers and semi-permeable membranes that could form compartments. At some point, a simple chemical system of self-replicating polymers within enclosed membranes would begin to exhibit Darwinian evolutionary behavior. This would promote a positive feedback loop to improve the chances of polymer replication, and increase the complexity of the system.

A number of laboratory experiments in the past 10 years have begun to build a real understanding of the chemical pathway that brings chemistry to life. First, the building blocks (the monomers) of all critical biopolymers in Earth life are now known to form spontaneously under a range of cosmic conditions. In particular, the amino acids and nucleotides are found in meteorites, which come from primitive, undisturbed solid material left over from the formation of the Solar System. Amino acids and RNA nucleotides are also shown to form in the lab, under conditions similar to those that existed on the young Earth.

An important breakthrough was achieved in 2009 concerning the synthesis of RNA, when Powner, Gerland, and Sutherland found a way to assemble its nucleotide building blocks outside a cell. The traditional view had been to search for a path to combine a ribose sugar to a nucleobase. Instead, Powner et al. showed that the final RNA product could be synthesized with the help of an intermediary compound that contributes atoms to both the sugar and the base parts of the nucleotide.

Once the nucleotides are available, polymerization of an RNA strand is the next hurdle. Laboratory experiments show that polymerization can occur when certain clay minerals are dissolved in water. Clays, which are the combination of finely-ground rocks and water, are likely to be common on any rocky planet with liquid water. They have been shown to help with the formation of fatty acid membranes, which fold into bubbles (vesicles). These vesicles are able to play the role of pre-cells for RNA strands. Fatty acids are simple molecules with a hydrophobic tail and a hydrophilic head. Because of this property, they are able to form semi-permeable membranes that bend to form microscopic bubbles (see animations below):

The development of these three components - formation of nucleotides, polymerization into RNA strands, and formation of vesicles with semi-permeable membranes - all under similar and plausible conditions, is tantalizing as a possible pathway from inorganic chemistry to the first cells. For the cells to be capable of Darwinian evolution, it is necessary that a self-replicating

RNA emerge, and that the membrane enclosing the vesicles allows the cells to grow and divide (replicate) without breaching the membrane. The latter vesicle functions were demonstrated in the Szostak lab at Harvard, which has graciously provided us with the videos you see on this page.

Today's complex cells use protein enzymes to catalyze the replication of RNA. The method of RNA self-replication necessary for Darwinian evolution of the prebiotic cells remains a mystery. However, Altman and Cech have discovered that a certain class of RNAs can function as chemical catalysts; they are called ribozymes. More recent work has gone further into understanding prebiotic RNA replication.

The above sequence of pathways and steps describes a plausible transition from planetary chemistry to life in an early form known as the "RNA World." We will discuss this more in the next unit.

Below you can view a video interview with Jack Szostak. Professor Szostak is the 2009 Nobel Prize winner in Medicine for his work with telomers. His lab is currently working on understanding cell membranes and cellular division. Here he discusses how a clear division between "alive" and "not alive" is misleading, and argues for the idea of a pathway from one to the other.

Солнечная система и ее тайны