Define Hox genes and its role in embryogenesis.

A detailed description of Homeotic (Hox) genes and embryology. Complete with Collinear Characteristics, function and mechanism and regulation of Hox Genes. Tables and Diagrams depicting the Spatial and Temporal collinearity. Response in MS Word.

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...al 1994).
Spatial Collinearity is defined as the correlation between the physical location of a gene within its own complex and its domain of expression within the developing embryo (Krumlauf 1994). Mammalian Hox genes have four semi-duplicate gene clusters (A to D) whom are presumably generated by the duplication of a single primordial cluster during evolution (Innis 1997). Thus, corresponding genes on the four clusters are considered to be paralogous, with further subdivision of each cluster into 13 groups that contain two to four genes per group (Lufkin 1997, Figure 1). Humans, mice and perhaps all mammals posses the same set of 39 Hox genes (Capecchi 1997). These genes are organized in a fashion where they are aligned along the chromosome in the same order as they are expressed along the antero-posterior axis of the embryo, so that each gene can specify matching segmental identity and positional transformation. Hox genes located at the 3' end of the complex usually have a more pronounced and rostrally expressed anterior border than genes located in sequentially increasing 5' positions (Lufkin 1997). Using the developing midbrain as an example, HoxA-1 (paralogoue group1) is shown to have a rostral border of expression at the 3-4 rhombomere boundary, whereas HoxA-4 (paralogoue group 2) has a more cadually located rostral border at the 6-7 rhombomere boundary (Figure 2). Interestingly, Hox expression is not exact and often offset caudally by several somites relative to expression in the neural tube (Capecchi 1997).

Figure 1. Diagram of murine Hox genes and an E10.5 mouse embryo showing Spatial and Temporal collinearity. All 4 clusters of paralogous Hox genes, from A to D, are shown. Hox genes at the 3' end are expressed earlier in the rostral region of the embryo. Hox genes at 5' end are expressed later in the caudal regions. Note: Locations of embryo segmentations are not in exact scale with Hox gene clusters. (Modified from Lufkin 1997)

Figure 2. Detailed schematics of rhombomeres in mouse embryo. Spatial collinearity is shown by the fact that HoxA-1, which is more 3' within the gene cluster, is expressed more rostrally at the rhombomere 4/5 border and the more 5' HoxA-4 is expressed more caudally at the rhombomere 6/7 border. (Modified from Bonceinelli 1999)

With respect to Temporal Collinearity, Hox genes positioned at the 3' end of the complex are generally activated earlier than the more 5' positioned genes (Gellon and McGinnis 1998). For instance, HoxD1 is usually transcribed earlier than HoxD6 because HoxD1 is positioned more rostrally towards the 3' anterior end ...