Francis Crick may have been the first to suggest that RNA was a precursor to DNA, and most molecular biologists today would agree with the notion. In fact, it is hard, if not impossible, to see how DNA would have evolved to become the model of molecular complexity it has, were RNA not around to facilitate its evolutionary process. To say that the two are mutually interdependent would be a totally accurate representation.
We could suggest here, as many before have, that DNA is the blueprint and RNA the machine which reads the blue print and constructs polypeptide and nucleotides from it. While the complexity associated with the profusion of chemical synthesis going on in the cells nucleus is staggering, it is important to note that proteins and nucleotides are assembled one by one. There are actually several different types of RNA which perform different functions, but for the purposes of this discussion it is important only to establish that RNA not DNA does the work. DNA simply provides the template that RNA follows in during its thing. So why is DNA important at all then? Well, DNA, as docile as it may seem, is the driving force or at least the stage upon which the act of biological evolution takes place.
DNA’s nucleotides sequences represent a code, each group of three nucleotides called a codon and specifying one of twenty amino acids. A type of RNA called transfer RNA (tRNA) binds itself to the DNA molecule and a messenger RNA (mRNA) molecule. The tRNA, traveling along the DNA chromosome one nucleotide at a time, catalyzes the addition of an amino acid taken from the nucleoplasm to the mRNA corresponding to each codon it encounters. Certain codons represent start and stop points and for the tRNA and once a complete nucleotide sequence has been transferred to the mRNA, it breaks free. Elsewhere in the nucleus the mRNA encounters another type of RNA called a ribosome or rRNA. The ribosome transcribes the still encoded information on the mRNA and assembles a polypeptide chain of amino acids representative of a specify protein. Albeit an abbreviated synopsis of the translation process, this is essentially how RNA makes proteins from the DNA template.
Transcription is a process initiated by proteins RNA translates from DNA, which essentially unzip the DNA molecule’s double helix and then rebuild each single strand into identical copies of the progenitor molecule, adding complimentary nucleotide base pairs taken from the nucleoplasm. The two main proteins in transcription are the helicase enzyme which breaks the hydrogen bonds that hold the two stands of the DNA molecule together, and polymerase enzyme which builds the two new molecules. It is during the transcription phase that DNA plays it roll in evolution. If every time a DNA molecule is transcribed or copied, the resulting daughter molecules were exact replicas of the progenitor, biological evolution would not exist. It so happens however, that polymerase does not always get it right, in fact, it makes mistakes in copying DNA nucleotide sequences on a regular basis. There are four basic nucleotide types normally found in DNA. They are Adenine, Glutamine, Cytosine and Thymine. Another nucleotide called Uracil found in RNA, looks almost exactly like the thymine molecule, with the exception of a methane molecule in place of a single hydrogen atom on the uracil molecule. On an almost regular basis, the polymorase enzyme will pug in uracil nucleotides on the DNA molecule where thymine ones are supposed to be.
Out of the three billion nucleotides representative of the human genome, only about one in every hundred thousand nucleotides is part of a DNA sequence coding for an active gene or protein. Ninty-nine percent of DNA is comprised of what geneticists refer to as junk, and telomeres (sequences that hold each end of a DNA molecule together). The junk DNA is representative of sequences which some time in the past may have coded for proteins, but somewhere along the way have been subject to mutation and become defunct. The process of DNA mutation is called polymorphism, and it happens quite frequently as we have seen in the case of the polymerase uracil substitution error. Remarkably, there are a group of enzyme proteins called DNA glycosylases in the nucleus which repair errors when they occur. Even so, geneticists today have calculated that each time a DNA molecule is transcribed, as many as 100 errors result which are not repaired. In most cases, when DNA transcription errors do effect an active gene, albeit a statistically rare occurrence, inconsistent chemical behavior will almost always lead to the cell’s premature death.
DNA is undergoing constant change, errors are induced, and nucleotide sequences are rearranged every time a living cell undergoes mitosis. On a far greater time scale, the DNA molecules, or chromosomes of any specific species of life, undergo more radical transformations. Estimates based on statistically predictable rates of mutation, suggest that somewhere around 7 million years ago, two DNA molecules in a hominid progenitor successfully fused to become a single chromosome. We can’t know whether this transient primate was male or female, but whichever, we know it survived and bore offspring. For a while, generations of this offspring may have exhibited a strange eukaryote configuration having 47 chromosomes. The reason for this aberration, is that cells contain two copies of each chromosome, one donated by each parent. It’s hard to predict what features this intermediary breed might have exhibited, but it wasn’t around for long. Eventually an offspring was conceived by two parents who both had the mutated merged form of the chromosome and a new species called homo (Latin for mankind, human) with only 46 chromosomes came into existence.
Modern humans, homo sapiens sapiens, represent the only variant of hominid still in existence today, but it is totally predictable that over several million years, polymorphism produced many variations or models, based on the hominid 46 chromosome theme. For instance, we know through mitochondrial DNA analysis, that Homo sapien neanderthalensis which became extinct during the last ice age, is a clearly genetic cousin to homo sapiens sapiens. As with all species, the DNA in human populations alive today continues to mutate and evolve. This is evidenced by populations developing in geological isolation which exhibit unique and divergent physiological effects. It is why some humans have blue eyes and why some have light colored skin while other’s have brown eyes and darker skin pigmentation. It is why some adults can drink bovine milk while is makes others violently ill. It is why most Europeans are susceptible to poison oak and ivy, and Native Americans are not. We can predict with certainty, that should our species survive for a few million years more, some new hominid species will evolve from the human archetype exhibiting qualities very divergent from the existing form.
Neither DNA nor RNA can be identified as the specific mechanism that drives evolution, although this association is often proposed. Instead, it is the totally random effect of polymorphism which brings about evolutionary change in living organisms. Quite incredibly however, DNA provides us with a chronicle of all the polymorphism that has occurred in living creatures over the past 3 or so billion years. It has only been nine years since the entire human genome was completely sequenced, and four years, since the process was completed for the chimpanzees karyotype. While the information gleamed thus far has provided some astonishing new realities, it is only a beginning, and we our bound to learn a hole lot more about the process of evolution in years to come.
The roll of DNA, at least with respect to human understanding of evolution, can be considered no less than profound. This year, 2009, marks the 150th anniversary of the publication of Darwin’s Origin of Species, and the moment when humans first contemplated the implications of evolution. Today, through human understanding of the chemical complexities going on at the DNA macromolecule level, we are privileged to witness first hand the effects of polymorphism, and the fact of evolution at its binary level. And this evidence, gained through experience and observation of DNA and RNA, has provided unequivocal and conclusive proof of elements of Darwin’s theory of variation and natural selection.
What is perhaps most phenomenal about the evolution of the DNA molecule, is the resulting manifestation of existentialism that has evolved from it. It is a truly physically intangible and ineffable perception of self and comprehension of the substance and facilities which manifest the most quintessential occurrence in the universe, life. We humans, and the ability of our sagacious enterprise to comprehend the DNA molecule and its implications with regard to evolution, are the ultimate apotheosis of its quintessential reality.
