Tuesday, July 27, 2010

The RNA World

The predominant hypothesis for the origin of life on earth is the RNA World Theory.

In this hypothesis, the early earth (about 4 billion years ago) contained water , weather, and simple molecules like methane and ammonia. Inorganic sources of energy (the sun, heat from volcanos, electricity from thunderstorms) catalyzed simple chemical reactions that resulted in slightly more complex molecules, amino acids (the building blocks of proteins) and purines and pyrimidines (the building blocks for the nucleic acids, DNA and RNA). With lots of reactions and lots of time, those building blocks hooked up and formed a self-replicating polymer, a self-replicating RNA.

After all, the conditions for life are two*:

  1. You have the capacity to reproduce.

  2. You carry the instructions for yourself within yourself.

Like many hypotheses (including Darwin's about natural selection), this one was proposed before there was substantial evidence for it. Experiments had shown that building blocks could be coerced by electricity and heat to form polymers (i.e., a primordial soup). We knew RNA could carry instructions, but to catalyze the reactions necessary to reproduce? That's pretty far out.

All organisms studied to date use a protein, polymerase, to synthesize nucleic acid polymers (DNA and RNA). They use charged amino acids to stabilize charged magnesium ions in the enzyme active site. Those charges stabilize the reaction that occurs inside the polymerase, which is to break one bond (between two phosphates) and make another (from one phosphate to a sugar). RNA units don't have charged amino acids, so how could they do all that stabilizing?

Then came the self-splicing intron.

*All of today's living things satisfy these requirements. Bacteria have DNA that encode the proteins to replicate the DNA, and they divide by asexual fission. Viruses carry instructions, but must use proteins from the cells they invade in order to copy it, so they aren't considered alive. Males can't reproduce on their own either, but we make an allowance for them.

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