Tuesday, November 11, 2003
The Origin of Life
Nicholas Wade addresses biology's most daunting problem -- how and why did life emerge?
Life must have started in the simplest possible way, as a cycle, a natural chemical reaction that repeated itself, spinning off byproducts, some of which stayed around to maintain and develop the cycle. Where did this cycle start? Dr. Wächtershaüser favors some mineral surface like iron pyrites, also known as fool's gold. A natural catalyst, the iron pyrites could have assembled chemicals like carbon monoxide into biological building blocks. At some stage, the little cycle acquired a cover of protective chemicals, to separate its own reactions from the general milieu. When the cover eventually enveloped the cycle and broke free of the mineral surface, the first cell was born.
Dr. Wächtershaüser and others have shown that important components of today's biochemistry can be formed on iron pyrite surfaces, notably pyruvate, the fuel for a basic energy-producing reaction known as the citric acid cycle. A different entry point to the origin of life concerns RNA, the close chemical cousin of DNA. Though DNA gets the attention, it is RNA that performs all of the trickiest operations in the cell, whether retrieving information from the DNA or turning this information into proteins.
Biologists have long supposed that RNA was the pivotal actor in the earliest cells and later delegated most of its information-storage duties to DNA, a less versatile but stabler chemical. The concept gained credence when Dr. Thomas R. Cech and Dr. Sidney Altman discovered independently that RNA could act as an enzyme, a catalyst of chemical activities, as well as store genetic information.
This dual property of RNA seems in principle to resolve one of life's thornier paradoxes, that DNA requires a protein catalyst for its replication, and the protein requires DNA to make it, implying neither could exist without the other's being there first. RNA could have performed both functions. Chemists have not yet devised an RNA molecule that can replicate itself. But they have shown that RNA molecules can copy short pieces of RNA. That bolsters the idea that before DNA there was an RNA world in which RNA, or some similar precursor polymer, ran the show. The subunits of RNA molecules are themselves quite complex chemicals. It is not too easy to see how the first RNA molecules could have come into existence. But a clay called montmorillonite, formed from weathered volcanic ash and familiar in many households as cat litter, has the interesting property of catalyzing the formation of RNA from its subunits.
In an article in Science magazine last month, researchers from the Massachusetts General Hospital reported that the montmorillonite clay had another property of possible relevance to the origin of life. It makes droplets of fat molecules rearrange themselves into small bubbles, similar to the membranes that make up the walls of living cells. Often the clay particles are incorporated into the bubbles, the research team found, with any attached RNA molecules. "Mineral particles may have greatly facilitated the emergence of the first cells," they said.
Life must have started in the simplest possible way, as a cycle, a natural chemical reaction that repeated itself, spinning off byproducts, some of which stayed around to maintain and develop the cycle. Where did this cycle start? Dr. Wächtershaüser favors some mineral surface like iron pyrites, also known as fool's gold. A natural catalyst, the iron pyrites could have assembled chemicals like carbon monoxide into biological building blocks. At some stage, the little cycle acquired a cover of protective chemicals, to separate its own reactions from the general milieu. When the cover eventually enveloped the cycle and broke free of the mineral surface, the first cell was born.
Dr. Wächtershaüser and others have shown that important components of today's biochemistry can be formed on iron pyrite surfaces, notably pyruvate, the fuel for a basic energy-producing reaction known as the citric acid cycle. A different entry point to the origin of life concerns RNA, the close chemical cousin of DNA. Though DNA gets the attention, it is RNA that performs all of the trickiest operations in the cell, whether retrieving information from the DNA or turning this information into proteins.
Biologists have long supposed that RNA was the pivotal actor in the earliest cells and later delegated most of its information-storage duties to DNA, a less versatile but stabler chemical. The concept gained credence when Dr. Thomas R. Cech and Dr. Sidney Altman discovered independently that RNA could act as an enzyme, a catalyst of chemical activities, as well as store genetic information.
This dual property of RNA seems in principle to resolve one of life's thornier paradoxes, that DNA requires a protein catalyst for its replication, and the protein requires DNA to make it, implying neither could exist without the other's being there first. RNA could have performed both functions. Chemists have not yet devised an RNA molecule that can replicate itself. But they have shown that RNA molecules can copy short pieces of RNA. That bolsters the idea that before DNA there was an RNA world in which RNA, or some similar precursor polymer, ran the show. The subunits of RNA molecules are themselves quite complex chemicals. It is not too easy to see how the first RNA molecules could have come into existence. But a clay called montmorillonite, formed from weathered volcanic ash and familiar in many households as cat litter, has the interesting property of catalyzing the formation of RNA from its subunits.
In an article in Science magazine last month, researchers from the Massachusetts General Hospital reported that the montmorillonite clay had another property of possible relevance to the origin of life. It makes droplets of fat molecules rearrange themselves into small bubbles, similar to the membranes that make up the walls of living cells. Often the clay particles are incorporated into the bubbles, the research team found, with any attached RNA molecules. "Mineral particles may have greatly facilitated the emergence of the first cells," they said.
