DNA and RNA have nearly identical building blocks yet DNA seems “chosen” to be the holder of genetic information, with RNA merely being the middleman, the transcriber of the genetic code into tangible proteins. Now scientists are asking the question, why DNA and not RNA?
Ever since Watson and Crick and probably even before then, scientists held an immense curiosity concerning the shape and function of nucleic acids that contain the code for life. Duke University’s Hashim M. Al-Hashimi and his team of researchers have a rich history of studying the structure of DNA and RNA, and previous findings indicate DNA’s flexibility as the quality making it superior.
"There is an amazing complexity built into these simple beautiful structures, whole new layers or dimensions that we have been blinded to because we didn't have the tools to see them, until now," said Al-Hashimi, lead author of the recent Duke study published in Nature Structural and Molecular Biology.
The traditional double helix of DNA, portrayed in images and depictions of the nucleic acid for generations, is what scientists believes keeps the genome stable and strong, protecting against things like cancer and aging. But can’t RNA form a double helix as well? It can, but adapting to this formation makes RNA rigid, fragile, and “unaccomodating” to nucleotide binding.
In the past, Al-Hashimi’s research led him to discover the change in structure DNA goes through when dealing with so-called “chemical insults” - being bound by a protein or receiving damage to its traditional structure. DNA responds to changes by “contorting itself into different shapes to absorb chemical damage to the [nucleotides].” Once DNA is able to shed the bound protein or repair any damage, it reverts back to the traditional, Watson and Crick-style double helix.
In his most recent study, Al-Hashimi and his team searched for changes in RNA nucleotide binding pairs as a response to similar chemical insults, expecting a reaction like that of DNA. They used a sophisticated imaging technology called NMR relaxation dispersion to observe changes in individual guanine and adenine bases, which make the infamous “spiraling steps” of two model double helices: DNA and RNA.
Surprisingly, there was “no detectable movement” of the base pairs in RNA, while previous studies had clocked DNA bases moving in response to protein binding or chemical damage from the traditional double helix by at least one percent. To confirm, the researchers continued testing more RNA molecules under several different conditions. Still no movement of bases.
Finally, the researchers manually altered the formation of RNA into the structure observed in DNA in response to chemical insult. What they saw next seemed to completely explain why DNA is charged with holding the genetic code. After being altered, RNA base pairs couldn’t reconnect, and the RNA strands fell apart at the site of alteration.
What is the cause of this key structural difference between RNA and DNA? Scientists believe it’s because RNA’s double helix structure is more “compressed” than that of DNA. Scientists also define the difference as a case of “A-form” (RNA) versus “B-form” (DNA). It is this difference that scientists believe adds an extra “dimension” to DNA’s structure, believed to add a higher level of functionality that allows it to adequately carry genetic information.
Source: Duke University
Huiqing Zhou, Duke University