Bicoid is an example of a morphogen that is a transcription factor. Although most morphogens are secreted signals, transcription factors also can function as morphogens under circumstances where they are free to diffuse from one nucleus to another. The early Drosophila embryo is such a case, since cells are not fully enveloped by isolating membranes until gastrulation begins. Bicoid activates expression of several gap genes that are involved in establishing subdomains along the A/P axis. Different concentrations of Bicoid activate different gap genes. For example, high concentrations of Bicoid activate hunchback expression in the anterior region of the embryo, moderate Bicoid levels activate Krüppel in the middle portion of the embryo, and low Bicoid concentrations permit knirps expression in more posterior regions (Fig. 3.5; see also Plate 1B, C for actual data). Thus, Bicoid satisfies the three conditions for being classified as a morphogen: (1) It is synthesized in a confined region, (2) it diffuses from its site of synthesis and thereby becomes graded in concentration, and (3) it activates distinct subsets of genes at different concentrations.
As discussed in Chapter 1, transcription factors bind to specific sequences of DNA in the regulatory regions of genes and either increase or decrease transcription (i.e., expression) of these target genes.
Maternal mutants disrupt the formation of the egg in the mother prior to fertilization, whereas embryonic mutants interfere with the response of the embryo to maternally provided information. Genes disrupted in maternal mutants are referred to as maternal genes and those affected by embryonic mutants are referred to as embryonic genes.
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