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Re-creating Primordial Life
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20582 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
11 / 1992 |
3,196 Words |
| Author
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Andrew Ellington
Andrew Ellington is assistant professor of chemistry at
Indiana University in Bloomington. |
The question of how life arose on our planet remains one of the great puzzles of science. Trying to answer this question is like trying to determine the shape of an individual piece in a jigsaw puzzle from the completed picture. Since all life-forms, both ancient and modern, descended from nonliving chemical processes, it is tempting to try to deconstruct the complex biochemistry of current organisms into the simple chemical reactions and structures that apparently existed in the environment of the early earth. However, just as there are many ways to cut a picture into a jigsaw puzzle, so too are there many ways to trace back from the staggeringly convoluted array of biochemicals existing today to a few simple prebiotic reactions and compounds.
Many researchers believe that at least some of the key components of modern life, amino acids, nucleotides, and lipids, may have been present in a prebiotic environment. Over time, these building blocks were strung together into long chains (polymers) that could perform more complicated tasks than the simple monomers that composed them. Thus, proteins are long chains of amino acids, nucleic acids are long chains of nucleotides, and starches are long chains of sugars. As life evolved, biopolymers that could assist in their own synthesis or prevent their own breakdown and dissolution were selectively retained, while other strings of monomers "died." To understand the origin of life, we need to understand how these different biopolymers became fixtures in evolutionary history.
Which came first, the egg or the chicken?
In the metabolism of modern life-forms, protein molecules perform most of the catalytic and structural tasks while nucleic acid molecules (DNA and RNA) carry the genetic information required for synthesizing the proteins. Yet the genetic information cannot be translated into new proteins without the aid of protein catalysts; furthermore, nucleic acids cannot even replicate themselves without the aid of protein catalysts. Thus, the continued survival of proteins and nucleic acids requires mutual cooperation.
It is difficult to imagine how even a very primitive organism could live without both nucleic acids and proteins. Yet, since both proteins and nucleic acids are extremely complicated, it is unlikely that life-forms based on their codependence arose by chance: Either nucleic acids or proteins must have come first. This is a classic example of the "chicken and egg" paradox: Nucleic acids are required to encode proteins, but proteins are in turn required to build
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