When planet Earth was still in its infancy, the precursor of life may have self-replicating strands of RNA. Indeed, scientists presume that this "RNA world" laid the foundation for everything we see today.
Thanks to an evolutionary plot twist, however, everything changed. Instead of RNA, DNA has become the most fundamental fabric of our own genetic information. What happened?
The Blueprint Of Life
The secret lies in a strange feature present in DNA but not in RNA.
Biochemist Hashim Al-Hashimi of Duke University investigated the atoms that make up DNA molecules and found a strange quirk: DNA can "shapeshift" in a way that RNA can't.
This dynamic nature makes DNA more resilient and a more efficient repository of the genetic code that makes us human.
According to Al-Hashimi, DNA is effective because of a strange structure called Hoogsteen base pair, which was discovered by Karst Hoogsteen in the 1960s.
The Hoogsteen base pair isn't something we learn in high school biology, particularly because scientists still struggle in capturing and studying the structure. It is believed that the Hoogsteen base pair only cropped up on very rare occasions.
Detecting Hoogsteen Base Pairs
About five years ago, Al-Hashimi and colleagues took a glimpse at DNA molecules in a nuclear magnetic resonance (NMR) machine, which allows scientists to look at what's going on at the molecular level.
The researchers discovered that the nucleic acid base pairs that comprise the steps of the spiral staircase of DNA continually shift between two forms.
What's more, most of the time, the spiral staircase are connected in the specific way that Francis Crick and James Watson described a half-century ago.
But at any moment, approximately 1 percent of all the base pairs create a Hoogsteen pair: one of the nucleic acids is flipped 180 degrees. The entire double helix structure gets bent.
"It becomes this very dynamic entity," says Al-Hashimi.
Hoogsteen base pairs often emerge when DNA has been weakened in some way, researchers said. For instance, carcinogenic chemicals will target DNA by adding a methyl group to one of the nucleic acids that make up a base pair.
Al-Hashimi says it is like hitting an errant nail into a board that comprises half a step on a staircase. By "turning over" the nucleic acid so that the "nail" isn't sticking out anymore, the Hoogsteen base pair can tend to the damage.
Examining The RNA
Researchers wondered whether the RNA would behave in a way similar to that of the DNA, especially because the RNA also has a helix structure.
When they added a methyl group to an RNA strand, it was like "dropping a nuclear bomb on the helix." The entire helix began to unravel, Al-Hashimi says.
As it turned out, RNA cannot form Hoogsteen base pairs. As they shift to tend to the damage, they just fall apart.
Al-Hashimi says that if human genomes were made up of RNA, there is a chance that they would not be enough to sustain chemical damage inflicted on them all the time.
Therefore, the DNA's incredible ability to absorb damage is one big reason why our genomes evolved into DNA-based structures.
These findings are still "speculative," but they could explain how the RNA got "demoted" from the main foundation to a messenger.
Details of the new study are published in the journal Nature Structural & Molecular Biology.