An article by Professor Steve Jones, View From the lab: mutations suggests reasons for the utility of a front loading paradigm. Current approaches take note of an existing balance between the potential for change and the need to maintain genomic function by preserving stability.
Mutations are essential to an evolutionary process. If there are none, change is not possible. While too many leads to the demise of an organism, or a species, a balance allows for adapative changes. Mutations are kept in check by multiple DNA repair mechanisms tageting different types of damage. Jones describes the repair processes:
"The chances of physical error as each DNA molecule is copied are such that mistakes – mutations- should build up with great speed and stop most of the dividing helices in their tracks. Even those that make it would be so damaged that their carriers would not survive.
Fortunately, biological sums are never simple. The genetic material is indeed much damaged by the laws of chemistry, which work as inexorably in our cells as in a test tube. In cells, though, most of the mistakes are put right. The thousand natural shocks that life is heir to are fixed in many ingenious ways. Special enzymes clean up the mess as they snip out a mutated segment, join together broken bits of the molecule, or replace a faulty piece with the correct version. The process is like the spell-check in a word-processing program: make a mistake and the machine puts it right. More than 100 genes are now known to be involved in the DNA repair business. Without them we would not survive.
To check the spelling in a document, one needs a back-up based on the correct versions of words (a dictionary) – but where is the dictionary for DNA? In fact, the genome is built on back-ups, with repeat copies of genes that can be used to check when one has gone wrong. Even the double helix is a sort of spell-checker, for editorial enzymes can compare one DNA base with its opposite and undamaged number to check that it fits."
How did repair mechanisms evolve? This is another way of asking the question: How did a balance between change and stability evolve? Since genes coding for enzymes, enabling DNA repair, provide the needed stability and since they themselves had to evolve, the more basic question is: Is evolution possible without a means to limit damage to genomic sequences having selective value? What would have prevented the decay of the very coding sequences needed to confer selective value to enzyme repair genes or, for that matter, genes involved in the replication function? Evolution is required to attain the idealized balance between stability and change. Yet natural forces inhibit an evolutionary process until genomic repair systems are in place. More from Professor Jones:
"Damaged DNA is fixed with great efficiency, in many unexpected ways. Why, then – given that nearly all errors are put right – is it not repaired absolutely, with no mistakes at all? Why is the mutation rate not even lower than it is?"
That's an interesting question that will be left for comments. Jones of course knows why the rate cannot be zero. Why not proceed to the logical next step and hypothesize the front loading of the repair mechanisms needed to maintain the previously cited balance between genomic change and stability. Front loading by design not only explains balance, it explains how a natural barrier to it is overcome.