The modern mechanistic theory of morphogenesis ascribes a role of prime importance to DNA, for four main reasons. First, many cases of hereditary differences between animals or plants of a given species have been found to depend on genes, which can be “mapped” and located at particular places on particular chromosomes.
Second, the chemical basis of genes is known to be DNA and their specificity is known to depend on the sequence of purine and pyrimidine bases in the DNA.
Third, it is known how DNA is able to act as the chemical basis of heredity: on the one hand, it serves as a template for its own replication, owing to the specificity of the pairing of the bases in its two complementary strands; on the other hand, it serves as the template for the sequence of amino acids in proteins. It does not play the latter role directly; one of its strands is first “transcribed” to give a single-stranded molecule of “messenger” RNA from which, in the process of protein synthesis, the sequence of bases is “read off ” three at a time.
Different triplets of bases specify different amino acids, and thus the genetic code is “translated” into a sequence of amino acids, which are linked together to give characteristic polypeptide chains, which then fold up to give proteins. Finally, the characteristics of a cell depend on its proteins: its metabolism and its capacities for chemical synthesis on enzymes, some of its structures on structural proteins, and the surface properties that enable it to be “recognized” by other cells on special proteins on its surface.
Within the mechanistic framework of thought, the central problem of development and morphogenesis is seen as the control of protein synthesis. In bacteria, specific chemicals called inducers can cause specific regions of the DNA to be transcribed into messenger RNA, on which template specific proteins are then made.
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The classic example is the induction of the enzyme β-galactosidase by lactose in Escherichia coli. The “switching on” of the gene takes place through a complicated system involving a repressor protein that blocks transcription by combining with a specific region of the DNA; its tendency to do so is greatly reduced in the presence of the chemical inducer.
By a comparable process, specific chemical repressors can “switch off ” genes.
In animals and plants a range of developmental genes have now been identified that are concerned with the regulation of the overall body plan and the number, identity, and pattern of body parts. These genes are usually called the “genetic toolkit.”
The most surprising discovery of developmental biology in the 1990s was that these tool-kit genes are remarkably similar, indeed almost identical, in widely different organisms. For example, the “homeobox” family of genes that affect the patterning of the body axis in fruit flies, mice, and humans are very similar, and yet the body forms of these organisms are obviously very different.
As the molecular biologist Sean B. Carroll and his colleagues have put it, “The conservation of the genetic toolkit provokes many developmental and evolutionary questions. How do such different structures as the insect compound eye and the vertebrate lens-type eye develop when their formation is controlled by such similar, even functionally interchangeable genes?”
This convergence of developmental and evolutionary biology has created a new field called evolutionary developmental biology (“evodevo” for short).
Most tool-kit genes code for proteins that affect the activity of other genes involved in the developmental process, and are part of “signaling pathways.” Some of them code for receptor proteins on cell surfaces that bind to specific molecules that act as signals.
In the early days of molecular biology, there seemed to be a simple, straightforward picture: one gene was transcribed into one messenger RNA molecule, which coded for one protein. But the picture has grown more complicated. Messenger RNA can be made up of pieces transcribed from different regions of the DNA, and subsequently joined together in a specific way.
Moreover, the synthesis of proteins is also controlled at the “translational level”; protein synthesis can be “switched” on and off by a variety of factors even in the presence of appropriate messenger RNA.
The different proteins made by different types of cells thus depend on the way in which protein synthesis is controlled. The only way in which this can be understood mechanistically is in terms of physical and chemical influences on the cells; patterns of differentiation must therefore depend on physical and chemical patterns within the tissue.
These are concentration gradients of specific chemicals called morpho-gens; these include diffusion-reaction systems with chemical feedback, electrical gradients, electrical or chemical oscillations, mechanical contacts between cells, or various other factors or combinations of different factors. The cells must then respond to these differences in characteristic ways.
One way of thinking about this problem is to regard these physical or chemical factors as providing “positional information” that the cells then “interpret” in accordance with their genetic program by “switching on” the synthesis of particular proteins.
These various aspects of the central problem of the control of protein synthesis are at present under active investigation. Most biologists hope that the solution of this problem will provide, or at least lead toward, an explanation of morphogenesis in purely mechanistic terms.