System genes homeobox (Noh), first described in fruit flies Drosophila melanogaster is the paradigm of developmental biology. Knox genes are so named because they encode proteins, transcription factors, contain a conservative linked to a DNA site called homeodomains. (The portion of a gene that encodes homeodomain called homeobox, which gave its name to the family of genes Noh.)
Most animal species have genes Noh, and the homeodomains encoded by these genes are similar; however, different species contain different number of genes Noh; for example, Drosophila contains 8, and people almost 40 genes. In humans, 40 genes Noh organized in four groups: A, b, C and D, in four different chromosomes. The order of individual genes within the groups is preserved across species.
Genes group Noh man was formed by a series of duplications of genes. The initial events dubbed the original ancestral gene Noh tandem along a single chromosome. Subsequent duplication of this one set of genes Noh and moving new sets of genes to different positions of the genome has led, ultimately, to the emergence of four groups of genes in humans (and other mammals), called Noh Oh, Noh, Noh C and Hox D.
Unique combinations of gene expression Noh in small groups of cells located in specific areas of the embryo will determine the fate of these regions. Just as a specific combination of a single cluster of genes Noh Drosophila expressed along the anteroposterior body axis and regulate gene expression and thus the formation of different body structures mammals to perform similar tasks make use of a number of genes Noh from different groups
In the early stages of embryo development transcription factors Noh define the anteroposterior axis: group Noh and Noh, for example, are restrainedly along the axis, defining individual vertebrae and myoblasts. At subsequent stages of development of the group And Noh and Noh D determine the local structure along the axis of the developing limb.
One interesting aspect of gene expression in Noh is that the order of genes in the group corresponds to the position of the embryo in which the gene is expressed, and the period of development. In other words, the position of genes Noh in the band collinear with synchronization expression and position of expression along the anteroposterior axis of the embryo.
For example, in the group of Noh In genes that are expressed first in the anterior part of the embryo, located at one end of the group; the order of the remaining genes in the group corresponds to the order in which they are expressed, corresponding to both the position along the anteroposterior axis of the embryo, and time of expression. Although the organization of genes unusual and atypical organization of genes in the genome, similar to the phenomenon observed in another family of human genes regulating the development, the globin group of genes.
Family genes Noh illustrates several important principles of developmental biology and evolution. First, the group of genes functioning together, performing the same General task at different times and in different places of the embryo. Second, homologous structures are created by sets of homologous transcription factors, derived from common evolutionary ancestors.
For example, insects and mammals have the same basic body plan (head front of the torso, the limbs away from the body, the heart and respiratory system precede the digestive organs), defined by the set of genes derived from a common evolutionary predecessors. Thirdly, although this is not typical of genes involved in development, Knox genes show remarkable organization in the group, consistent with their functioning in the course of development.