Homeotic genes which are found in the fruit fly Drosophila are involved in pattern formation during embryonic development. As based on the sequence similarities more than thirty homeo box-containing genes of the class Antenntapaedia have been identified in mouse as well as man. The genomic organization of this multigene family in gene clusters located on different chromosomes supports the hypothesis that the homeo box gene complexes evolved by duplications. The cluster duplications can be argued that they happened early during vertebrate evolution.
As one base on the comparison of the arthropod as well as vertebrate homeo box gene systems, he considers the function of multi – gene regulatory networks for the specifications of the regional as well as cellular identify during development and in the regional as well as cellular identity during development and in the adult organism. It can be argued that during evolution the homeo box gene network may have been crucially involved in the development of new body plans and behavioral attributes. (Daston, 2005, p. 69) Introduction
The homeo box was first discovered as a common DNA sequences element in home genes of the fruit –fly Drosophila melanogastor. These genes normally regulate the development and morphogenesis of the fly – as well as mutations in such genes lead to characteristic phenotypes, so-called homeotic transformations, for example, the transformation of antennae into legs. Homeo box genes have subsequently been isolated form a variety of organisms including chicken, frogs, mice as well as humans. (Daston, 2005, p. 85) They can normally be grouped into different categories based on their sequences as well as their biological features.
Homeo domains have been identified in ubiquitous transcription factors such as Oct. –1, and in tissue – specific transcription factors. Examples for the latter one Pit – l,a pituitary – specific transcription activator of the gene for growth homeo and Oct – 2 that regulates the transcriptional activity of immunoglobulin genes. These are genes of the so-called class Antennapaedia are the Drosophila homeotic genes, and it is this similarity that suggest that they may also ply a crucial role in development in vertebrate (Leroy, 2003, p.
190) Expression of mammalian homeo box genes The homeo box genes in vertebrates are typically about 10kb (kilo bases) long as well as consist of at least two exons. The genes encode proteins of the average size of 240 amino acids. Normally the homeo domain resides in the carboxy – terminal part of these proteins. The hexapeptide sequence has been implicated such that it mediates dimerization of home demean proteins. (Jenkins, 2000, p. 372) The expression patterns of homeo box genes are quite complex and of late been covered extensively.
The most interesting feature of the expression patterns of class Antennapaedia homeo box genes in developing embryos in the region specific accumulation of their respective mRNAs. Each homeo box genes has a characteristic anterior boundary of expression in the spinal cord, and while the posterior boundary is less well defined, a gradient can be observed in the succession of anterior expression levels. Such graded expression is also found in the mesoderm.
These patterns are reminiscent of those observed in Drosophila in that homeo box expression is detected in structures originating from different germ layers such as neural tissue, which is of ectoderminal origin, and somatic mesoderm, Moreover, as in the fly, the region which is expressed itself is reflected by the chromosomal position of the respective gene. (Plomi, 1986, p. 2500 Duplication of homeo box gene clusters during vertebrate evolution A more detailed calculation of the timing of the duplication events with respect to vertebrate evolution has been derived form analysis of nucleotide sequences of mammalian homeo boxes.
When the human as well as mouse sequences for each of the homeo boxes are compared in pairs, they differ by an average of 11. 5 nucleotides. It is such that more than 90 per cent of these differences represent silent substitutions as they do not result in amino acid differenced. These data show that human as well as mouse homeo box sequences have been very strongly conserved since the divergence of the two species comparisons of cognate homeo boxes located on different clusters in the mouse show that each pair of sequences differs by an average of 32.
4 nucleotides, and 84. 6 percent of these changes account for silent substitutions. If one of the clusters arose from a very recent duplication it should differ by a smaller number of nucleotides from the remaining clusters. But the four clusters in the mouse and human appear to be equally distant by several different criteria. Therefore it is possible that the four clusters arose in a rapid succession of duplications rather than over a long period of time. (Baskin, 2001, p. 124)
Adverse effect of retinoic acid on embryo development and the selective expression of retinoic acid receptor in mouse blastocysts All- trans retinoic acid (RA), the oxidative metabolic of vitamin A, is essential for formal development. More so, high levels of RA are teratogenic in many species. It has of late been shown that excess RA results in immediate effects on the preimplantation embryo as well as on blastocyst development. The study was conducted to clarify the long term survival of mouse blastocyst and the effect of RA on gene expression.
Vitamin A and its physiological metabolites retinoic acid (RA) and other retinoids, exhibits a striking effects on pattern formation and may be one of the morphogens that control embryonic development. While normal embryo development requires retinoids, too much or too little, at the wrong stage or at the wrong time, can seriously affect the developing embryo. It is such that embryos exposses to low concentrations of RA develop normally, whereas those exposed to higher concentrations develop characteristic defects which are dose-dependant.
Embryonic exposure to RA causes a wide spectrum of severe malformation in the offspring of human, chickens, and these include neural tube and central nervous system defects, cleft palate, ear, skeletal defects and other craniofacial malformations, defects of the heart, thymus, and urogenital system and limb and digit deduction or duplication. (Gould, 1987,p. 270) The expression of mRNA encoding three receptors (RAR), A. B. and C has been found in mouse blastocysts and in early post implantation mouse embryos.
These nuclear receptors regulate gene transcription in a ligand-dependent fashion through binding to specific DNA sequences, generally located up[ stream of the promoter of the target gene. Primers and amplification Primers are used for amplification of the retinoic acid receptors (RAR) and B – actin CDNA. For Sets of primers have designed from available mouse RATLa, RARb, and RARc sequences and the B- actin sequence found in GenBank. The primers were chosen with the assistance of two computer programs, Oligo 4. 0 and Primer Express.