It's beginning to look like everything you learned in high school about the gene and biological inheritance is quickly becoming obsolete.
A recent article in the New York Times explains why.
According to the Times the snippet of DNA which had once been thought to program for a particular trait is now known to be only part of the story. There are whole complexes of molecules that program for traits and a particular snippet (or exon) may code for as many as six or seven different proteins or no protein at all. In other words, the complexity of life keeps increasing the more we learn about it. The complexity, like some biological fractal, runs all the way down. Here are just a couple of excerpts from the article which suggest the enormous complexity of the mechanisms involved in inheritance:
A single so-called gene, for example, can make more than one protein (transcript). In a process known as alternative splicing, a cell can select different combinations of exons (segments of DNA) to make different transcripts. Scientists identified the first cases of alternative splicing almost 30 years ago, but they were not sure how common it was. Several studies now show that almost all genes are being spliced. The Encode team estimates that the average protein-coding region produces 5.7 different transcripts. Different kinds of cells appear to produce different transcripts from the same gene.
Even weirder, cells often toss exons into transcripts from other genes. Those exons may come from distant locations, even from different chromosomes.
But it turns out that the genome is also organized in another way, one that brings into question how important genes are in heredity. Our DNA is studded with millions of proteins and other molecules, which determine which genes can produce transcripts and which cannot. New cells inherit those molecules along with DNA. In other words, heredity can flow through a second channel.
One of the most striking examples of this second channel is a common flower called toadflax. Most toadflax plants grow white petals arranged in a mirror-like symmetry. But some have yellow five-pointed stars. These two forms of toadflax pass down their flower to their offspring. Yet the difference between their flowers does not come down to a difference in their DNA.
Instead, the difference comes down to the pattern of caps that are attached to their DNA. These caps, made of carbon and hydrogen, are known as methyl groups. The star-shaped toadflax have a distinct pattern of caps on one gene involved in the development of flowers.
DNA is not just capped with methyl groups; it is also wrapped around spool-like proteins called histones that can wind up a stretch of DNA so that the cell cannot make transcripts from it. All of the molecules that hang onto DNA, collectively known as epigenetic marks, are essential for cells to take their final form in the body. As an embryo matures, epigenetic marks in different cells are altered, and as a result they develop into different tissues. Once the final pattern of epigenetic marks is laid down, it clings stubbornly to cells. When cells divide, their descendants carry the same set of marks. "They help cells remember what genes to keep on, and what genes can never be turned on," said Bradley Bernstein of Harvard University.
It's just astonishing, is it not, the miracles of engineering that can be wrought by mere chance and chemistry. If I hadn't read my Richard Dawkins I might be tempted to think there was some merit in the observation by physicist Fred Hoyle that the odds of a living cell arising purely by chance are about the same as a tornado sweeping through a junk yard leaving in its wake a fully-assembled, fully-functional 747 jet airplane.
No snickering please.
RLC