Scientists have come closer to uncovering the mysteries of life’s origins by creating an enzyme called ribozyme, which contains the unique property of being able to create mirror-image copies of itself.
The new enzyme is called a ribozyme because it is made from ribonucleic acid (RNA). Modern DNA-based life forms appear to have evolved from a simpler “RNA world,” and many scientists suspect that RNA molecules with enzymatic properties were Earth’s first self-replicators.
The new ribozyme works essentially in that way. It helps knit together a “copy” strand of RNA, using an original RNA strand as a reference or “template.” However, it doesn’t make a copy of a molecule completely identical to itself. Instead it makes a copy of a mirror image of itself—like the left hand to its right—and, in turn, that “left-hand” ribozyme can help make copies of the original.
No one has ever made such “cross-chiral” enzymes before. The emergence of such enzymes in a primordial RNA world—which the new study shows was plausible—could have overcome a key obstacle to the origin of life.
Biology on Earth evolved in such a way that in each class of molecules, one chirality, or handedness, came to predominate. Virtually all RNA, for example, are right-handed and called D-RNA. That structural sameness makes interactions within that class more efficient—just as a handshake is more efficient when it joins two right or two left hands, rather than a left and a right.
“Scientists generally are taught to think that there has to be a common chirality among interacting molecules for biology to work,” said Joyce.
It seems likely, however, that simple RNA molecules on the primordial Earth would have consisted of mixes of both right- and left-handed forms. Despite this reasoning, 30 years ago Joyce, then a graduate student, published a paper in Nature showing that self-replicators would have had a tough time evolving in such a mix. Any strand of RNA that gathered stray nucleotides onto itself would eventually have incorporated an RNA nucleotide of the opposite handedness—which would have blocked further assembly of that copy.
“Since then we’ve all been wondering how RNA replication could have started on the primitive Earth,” Joyce said.