Hox genes underlie metazoan development, but what controls them?
Although metazoan development is conceived as resulting from gene regulatory networks (GRNs) controlled by Hox genes, a better analogy is computer architecture: i.e., a task accomplished in sequential steps linked to an external referent that "counts" each step. A developmental "step" equals the expression of genes in specific cells at specific times and telomeres represent external "counters" wherein "counting" is a function of telomere shortening at each cell division that permits the sequential expression of Hox genes and, ultimately, complex form. Metazoan development thus best resembles a Turing machine, which could be used to model the development of any metazoan.
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