|MadSci Network: Evolution|
In the search for the origins of life on Earth, the central question has been, "what existed before there were cells?" This has lead to the search for a single substance that fulfills the basic criteria of life and has the capacity to evolve into the cell-based life we know today. The two basic characteristics this substance should have are replication (the ability to reproduce) and enzymatic activity (the ability to create its own environment). DNA can replicate, but that's all it can do. Proteins can change their environments, but they have no mechanism for reproduction. The answer lies in between DNA and proteins.
Start by looking at protein synthesis: transcription involves making an RNA copy of the target gene being expressed, and translation involves ribosomes moving along the mRNA message and converting the purine/ pyrimidine sequence into an amino acid sequence. A closer look at translation reveals that the sequence conversion is carried out by tRNA adapters that "read" the mRNA sequence and create the appropriate amino acid sequence as they line up at the ribosome. In fact, the ribosome itself is made up of several rRNA molecules that are essential for its function. A closer look at transcription in the eukaryotic nucleus reveals that the initial copy of the DNA contains introns which must be spliced out before the mRNA can leave the nucleus. This splicing is carried out by Spliceosomes composed mostly of snRNA. In fact, some eukaryotic introns don't even need Spliceosomes - they are "self-splicing", i.e. they can cut themselves out in the absense of any proteins. These introns along with other catalytic RNA particles including the snRNA's and now even rRNA's have been collectively named "ribozymes", enzymes composed of RNA.
Since many viruses use RNA instead of DNA for their genomes, we now have in RNA a substance that can replicate and store genetic information like DNA, but also has enzymatic activity like protein, fulfilling both of the requirements originally put forth. Scientists now refer to the "RNA World" theory which elaborately explains life before cells, and demonstrates how even today RNA is the central factor in life on Earth. Since there are volumes of data available on the subject, I'll finish with the following set of links:
Great question, and good for you rejecting DNA and protein as possible candidates - it's taken years for scientist to break through the popular dogma that everything is DNA.
Try the links in the MadSci Library for more information on Evolution.