There’s scarcely any corner of Earth in which life hasn’t gained a foothold. Living organisms shape the clouds, the climate, even the rocks. But how life first appeared on a barren planet some 4 billion years ago remains one of the biggest questions for science.
Many researchers believe that self-replication — an ability to reliably store and copy genetic information — is life’s most core ability. If that’s true, then the earliest life-forms would presumably have needed the sorts of biomolecules that life uses to store and copy genetic information today. The DNA molecule is the star of this process; it encodes the forms of the protein molecules that enable chemical reactions in cells.
To translate the genetic information in DNA into the chemical structure of proteins, cells use RNA. The trouble is, in modern cells, proteins are involved in every step of this process: They duplicate DNA when cells divide, they transcribe DNA into RNA, and they help to use RNA for making more proteins. This creates a chicken-and-egg problem: Useful proteins can’t be made in cells without DNA, but proteins are needed to make DNA in the first place.
A discovery about RNA in the late 1980s seemed to break the deadlock. Researchers found that, in addition to helping to convert genetic information into proteins, some RNAs can act as catalysts that enable chemical reactions. These were christened ribozymes, in reference to the protein catalysts called enzymes.
The existence of ribozymes suggested that RNA can both carry genetic sequence information and act as a catalyst, thereby doing the jobs of both DNA and proteins. That makes it plausible that in its very earliest days, life — or the complex chemistry that preceded it — was all about RNA. According to this picture, championed by the physicist-turned-biologist Walter Gilbert, life began as an RNA world.
Imagine then that, somewhere on the prebiotic Earth, individual nucleotide molecules appeared and joined together spontaneously and randomly in chains. By chance one momentous day, one of these nucleotide chains acquired the right sequence to act as a ribozyme with the ability to help other RNAs form. Such a prebiotic ribozyme could even act as an autocatalyst, able to replicate itself.
Extract — continue reading at the source.
