Se trata de un cnidario que habita en aguas dulces. El desarrollo no termina con el nacimiento de la criatura. Abarca todo el ciclo vital. La hidra parece, vista desde esta perspectiva, inmortal. Los organismos en desarrollo, a diferencia de la diosa, son ensayo, error y riesgo.
Las instrucciones contenidas en el genoma no determinan de forma lineal y sin interferencias el fenotipo. El Profesor Newman ha tenido la amabilidad de respondernos unas preguntas.
It has been said that organisms develop from a plan established in the genes. For you, what are genes, in relation to the organism to which they give shape and function? Genes, which are composed of DNA, directly specify the sequences of RNA molecules and indirectly, the amino acid sequences of proteins.
Before there were multicellular forms, single-celled organisms evolved for as much as two billion years driven, in by genetic change, as well as by establishment of persistent symbiotic relationships among simpler cells. The protein and RNA molecules produced by cells associate with each other in a context-dependent fashion or, in many cases, catalyze chemical reactions generating lipids, polysaccharides and other moleculeswhose rates depend on the temperature and composition of the external environment.
So the population of molecules inside the cell can vary extensively even if the genes do not. But we now know that this is not always true — the rate at which a protein is synthesized, which "Reproduccion asexual definicion y tipos de entrevistas" on factors internal and external to the cell, affects the order in which its different portions fold.
So even with the same sequence a given protein can have different shapes and functions. Furthermore, many proteins have no intrinsic shape, taking on different roles in different molecular contexts. So even though genes specify protein sequences they have only a tenuous influence over their functions.
The deployment of information in the genes, moreover, is itself dependent on the presence of certain RNA and protein molecules in the cell. So, to reiterate, the genes do not uniquely determine what is in the cell, but what is in the cell determines how the genes get used.
Only if the pie were to rise up, take hold of the recipe book and rewrite the instructions for Reproduccion asexual definicion y tipos de entrevistas own production, would this popular analogy for the role of genes be pertinent. When it comes to multicellular organisms, which first arose from single-celled ancestors about million years ago, things are even more complicated relative to genetic determination.
Certain unicellular proteins and other molecules mobilize physical effects in the multicellular context that were unanticipated during their earlier evolution. To take a simple example, all single-celled organisms can secrete protein molecules into the environment around them. These molecules will generally float away, but they may serve to attract prey, repel predators, and so forth. The gene involved simply specifies the sequence of the secreted protein.
What function the protein comes to assume in the new multicellular context has nothing to do with the evolutionary history of that gene, or the selection to which it had been subject. There are many other such examples.
The development of developmental biology leads us to consider the evolution of species in a different light.
How are the process of species evolution and the development Reproduccion asexual definicion y tipos de entrevistas organisms mutually affected? Does development explain, rather than Darwinian natural selection, speciation?
What we have been learning about development is that the mechanisms it employs for the production of form can be both nonlinear and conditional.
For most present-day organisms, however, redundant developmental pathways Reproduccion asexual definicion y tipos de entrevistas to stabilize the generation of a standard phenotypic outcome.
But while the organisms of our experience do not often undergo major morphological reorganization, the circuitry that stabilizes development is itself a product of evolution. What are the major milestones of development? What basic steps are taken on the path leading from the zygote to the body?
Confining the question to animal development, my own specialty, we find that the formation of a cluster of relatively similar-sized cells, variously called a blastula, pregastrula, or inner cell mass, is common to all phyla. This can occur, as in mammals, by division of a small zygote i. The cells are generally held together by one or more members of the cadherin family of cell surface proteins. After the cell-cluster stage there is a set of obligatory and optional transformations that occur in the different kinds of organisms, not always in the same order even in closely related forms.
These include cells assuming different biochemical states or types despite having identical genomeswith no one cell type taking over the whole cluster.
This equilibrium is generally enforced by the Notch signaling pathway, which is activated by contact between different cell types. In cases where the different cell types have different adhesive properties, the populations form "Reproduccion asexual definicion y tipos de entrevistas" tissue layers, a phenomenon seen in every type of animal embryo.
The cell clusters that constitute early-stage embryos often develop one or more interior spaces and may also undergo narrowing and elongation. Both these effects typically depend on branches of the Wnt signaling pathway, which variously cause cells to become polarized in cell surface properties or shape.
Hollow clusters sometimes fold inward, an effect typically mediated by the stiffening of a portion of the surface by the secretion of an extracellular matrix material, such as collagen, by some of the cells. Embryos also become segmented in some phyla. This morphological motif can arise in two different ways.
In certain insects, including the fruit-fly, standing waves of gene regulatory molecules form in a large elongated egg prior to cleavage. Paired blocks of tissue separate from adjacent unsegmented tissue at a specific phase of the oscillation, but only
Reproduccion asexual definicion y tipos de entrevistas regions beyond the influence of a morphogen gradient like that described above in the response to Question 1 with its high point at one end of the embryo.
This segmentation mechanism employs some of the same pathways mentioned above, e. In all these examples, basic body-plan-generating mechanisms employ ancient molecules that had evolved to serve Reproduccion asexual definicion y tipos de entrevistas functions. These molecules took on new functions in multicellular aggregates by harnessing physical mechanisms relevant to the larger scale, thus constituting dynamical patterning modules. The most fundamental DPM is cell adhesion, which generates multicellular aggregates.
The other DPMs were a straightforward consequence of the changed physical context of multicellularity. But while there was thus no need for prolonged periods of adaptive selection for the basic processes of development and the production of form to emerge, subsequent evolution has refined and coordinated the action of the DPMs, making present-day embryos very different from the first multicellular organisms.
During later stages of embryogenesis, when new cell clusters initiate the development of organs such as the heart, lungs, kidneys and limbs, they employ the same molecules and mechanisms, i. How do cells communicate and coordinate among themselves?
How can such complexity be possible without a conductor? Cells in clusters, such as early-stage embryos or the most ancient multicellular organisms, exhibit self-assembly and self-organization of form and pattern. They have these properties not simply because of their genes, or the proteins and other molecules the genes are involved in producing, but because of the physical effects harnessed by a subset of those molecules and the functions of the resulting DPMs.
An example of self-assembly is the sorting-out of differentially adhesive cells into separate layers of tissue, mentioned above.
This phenomenon resembles the separation of oil and vinegar in a shaken bottle of salad dressing, and "Reproduccion asexual definicion y tipos de entrevistas" for similar reasons. A mixture of two kinds of cells with different types or amounts of homophilic adhesive molecules on their surfaces is physically analogous to a mixture of oil and water. In each case the subunits cells, molecules with the stronger affinity bind to each other, excluding those of weaker affinity.
The result is a two-phase system. In the tissue case the more cohesive phase becomes engulfed by Reproduccion asexual definicion y tipos de entrevistas less cohesive phase, a phenomenon that is also seen with oil and water when they are suspended in a third medium in which gravitational effects are neutralized.
The remarkable thing is that even if the two cell types arise randomly in a cluster, sorting out will occur and the final spatial relationship of the tissues that result will always be the same. Self-organization is more complicated, Reproduccion asexual definicion y tipos de entrevistas the structures produced are not states of equilibrium, but require active, opposed interactions for their establishment. An example is the formation of the skeleton of the vertebrate limb.
This structure is based on the emergence of a series of regularly spaced rods and nodules of cartilage that are later replaced by bone in most species. A common feature of self-assembling and self-organizing systems is that they progress toward stereotypical endpoints: There are analogous outcomes from the action of other DPMs. These conductorless ensemble performances, though they unquestionably occur, violate one of the key precepts of the Darwinian paradigm.
The existence of such phenomena clearly obviates the need for natural selection in the generation of important morphological motifs. The human being does not complete its development in the uterus, rather it continues along the life cycle.
This affects particularly the brain. What does development biology tell us about the human brain? The discussion above of the evolution and development of body and organ form asserts that morphological phenotypes were more plastic earlier on than they are now. The formative processes — the DPMs — being sensitive to external conditions, permitted form to change rapidly with little or no genetic alteration.
However it took extensive genetic evolution to stabilize and canalize the generation of form: The evolution and development of the nervous system have some similarities with the described scenario, but the differences are even more significant. Like morphological development, the nervous system is based on new roles acquired for single-cell functions in the multicellular context. In the case of the nervous system, one ancient cell property predominates — excitability — the capacity to import or export ions through channels that are sensitive to the electrical potential of the cell membrane.