RNA Miracles 2: Tracing Molecular Evolution

RNA research is still in its infancy, but there’s already an awesome list of RNA wonders to investigate. 

         RNA, the molecule we thought of as a genetic “helper” for DNA, instead was probably the ancestor/creator of DNA (see my September 7 post). 

         By the late 1960’s, geneticists thought they had solved the genetic puzzle: genes were made of DNA, and each gene coded for a protein that went on to do a cell’s actual work of staying alive.  What a surprise when the Human Genome Project revealed that the vast majority of our genome is made of “junk DNA,” that is, DNA that does not code for any proteins.

          Now we discover that much of this “junk” actually codes for RNA.  What is the purpose of all this RNA?  Investigators have found that RNA functions as if it predates DNA, as if over the 3.5 billion years that life has existed on earth, RNA has been running the business of genetics.

         DNA apparently can’t make a move without the help of RNA or RNA/protein complexes.  RNA finds its way all around a cell’s DNA, stopping and starting gene function.  RNA can cut pieces out of itself and splice those pieces together into new messages, mixing and matching to form a variety of RNA structures, and therefore proteins, out of the same fragments.  RNA constructs the proteins that unwind DNA, strip it of silencers, and run the entire DNA copying process.  With the help of “motor proteins,” RNA can weave itself into three-dimensional molecules in a variety of ways.  And we are just beginning to try to figure out what all those three-dimensional RNA’s do. 

         Some of them resemble parts of larger genetic structures.  For instance, a large RNA/protein macromolecule called a “spliceosome” edits mRNA in cell nuclei.  The spliceosome cuts up mRNA and splices some of the pieces together in a variety of ways, leading to varieties of proteins.  (One example:  From one gene, RNA can produce a variety of vibrating membranes of different lengths for the inner ears of mammals.  The different lengths then transmit differing sound impulses to the auditory nerve.) 

         There is a small 3-D RNA molecule that strongly resembles a portion of the spliceosome.  So much so that it appears the spliceosome may have evolved from just such a 3-D RNA molecule.

         Evolution among biological molecules is awesome.  Not only were there “first” cells containing “first” organelles, but the cells contained “first” molecules.  One of those first molecules may well have been RNA, acting as both gene and enzyme.  Then that early RNA evolved the ability to produce proteins out of amino acids, and DNA out of new versions of its own nucleotides, still retaining, over all those billions of years, its miraculous executive abilities.