|Gene transcription, from The Mermaid's Tale, Weiss & Buchanan, 2009|
A few of the reasons are
- genes are interrupted coding sequences (they have non-coding 'introns'),
- introns are spliced out of mRNA by particular DNA sequence motifs,
- genomes evolve by duplication of whole segments,
- much more DNA is transcribed into RNA than was thought;
- some of this RNA has complimentary sequences to protein codes and there is an elaborate mechanism for this, which is never translated into protein, to inhibit 'real' genes (this known as microRNA);
- some non-protein RNA--copied from genes without the usual gene processing sequence elements, is nonetheless found attached to ribosomes in the cell as if it were being translated anyway;
- gene usage is determined in part by the way DNA in the gene's part of a chromosome is packaged and chemically modified ('epigenetics');
- important aspects of variation are due to mutations that happen during the lifetime of the organism;
- each cell uses only some of its genes, and sometimes only one of its two copies of a gene that it is using;
- genes can be assembled via effects of genes from other chromosomes or even make mRNA that is a composite of pieces from two different chromosomes ; and (so we don't have to keep on and on),
- some mRNA is 'edited' after being transcribed, in replicable ways sometimes conserved among distantly related species, in which one nucleotide copied from the DNA template is replaced by a specific other nucleotide, thus changing the function, including the protein code, of the mRNA.
There are many answers, and we regularly harp on them. Here, the main point is that these discoveries are real, their importance highly variable and mainly unknown, and that they always add to, but rarely if ever reduce, the complexity between DNA and the traits that it affects. Promises of simple prediction have been aided and abetted by the addictive discoveries that really work like the genes Mendel studied in peas. We do a lot of hand-waving to dismiss complexity, but we cling like drowning sailors to life-rafts to the simple, Central Dogma, in the fervent hope that we'll find the Big Gene Story. Yet those who are thinking about science itself, rather than about what they have to do to maintain their careers, know very well that we know very little about the nature of genetic function.
Dogma should not be part of science. But historians and philosophers of science have shown that it certainly is. A new book, for example, shows how this has affected immunology for decades, as investigators clung to a 'fictive' theory, the idiotype network theory, even though it never had much basis (the book, The network collective: Rise and fall of a scientific paradigm, edited by Klaus Eichmann, is reviewed in the July issue of Bioessays). There's no place for dogma in science, nor for the tribalism that accompanies it. But, well, we're only human so purging the motivations that drive dogma, and the careers that are made on its basis, may not be in the cards.