| | Comparative Vertebrate Anatomy |
Homeotic complex (Hox) genes are defined as a group of related genes that specify the anterior-posterior axis and segment identity of metazoan organisms during early embryonic development. These are situated very close to one another on the chromosome in groups of clusters. The orders of these genes located on the chromosomes are the same as the expression of the genes in the development of an embryo, whereas the first gene is expressed in the anterior end of the developing organism. These groups of genes are short sections of DNA that occur in clusters called homeoboxes. A homeobox contains 180 nucleotides that code for a protein domain known as the homeodomain. Hox genes produce protein products belonging to a class known as transcription factors. Transcription factors are capable of binding to DNA, in which they regulate the transcription of genes. This homeobox sequence codes for a 61 amino acid helix-turn-helix protein, resulting in the homeodomain. The homeodomain acts as a switch that turns the gene transcription on and off by binding to specific sequence enhancers of a gene that either activates of represses the gene. These same HOX proteins can act as an enhancer for one gene and a repressor for another. Hox genes have an historical importance in evolution as well as in their relationship to human development, and its specific regulation and their mutations.
Animals have very different body plans. Hox genes play a huge role in the diversification of the segments that develop different animal segments within closely related groups. This diversity of the Hox genes are produced by evolutionary changes in the timing of the gene expression and the different combinations of Hox genes are used together. This is where homologous structures are formed. Homologous structures are things that are similar in evolution, position, or shape but differ in function. Biologists are able to isolate
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