Biology:Homeotic gene
Homeotic genes are genes which regulate the development of anatomical structures in various organisms such as echinoderms,[1] insects, mammals, and plants. Homeotic genes often encode transcription factor proteins, and these proteins affect development by regulating downstream gene networks involved in body patterning.[2] Mutations in homeotic genes cause displaced body parts (homeosis), such as antennae growing at the posterior of the fly instead of at the head.[3] Mutations that lead to development of ectopic structures are usually lethal.[4]
Types
There are several subsets of homeotic genes. They include many of the Hox and ParaHox genes that are important for segmentation.[5] Hox genes are found in bilateral animals, including Drosophila (in which they were first discovered) and humans. Hox genes are a subset of the homeobox genes. The Hox genes are often conserved across species, so some of the Hox genes of Drosophila are homologous to those in humans. In general, Hox genes play a role of regulating expression of genes as well as aiding in development and assignment of specific structures during embryonic growth. This can range from segmentation in Drosophila to central nervous system (CNS) development in vertebrates.[6] Both Hox and ParaHox are grouped as HOX-Like (HOXL) genes, a subset of the ANTP class (named after the Drosophila gene, Antennapedia).[7]
They also include the MADS-box-containing genes involved in the ABC model of flower development.[8] Besides flower-producing plants, the MADS-box motif is also present in other organisms such as insects, yeasts, and mammals. They have various functions depending on the organism including flower development, proto-oncogene transcription, and gene regulation in specific cells (such as muscle cells).[9]
Despite the terms being commonly interchanged, not all homeotic genes are Hox genes; the MADS-box genes are homeotic but not Hox genes. Thus, the Hox genes are a subset of homeotic genes.
Drosophila melanogaster
One of the most commonly studied model organisms in regards to homeotic genes is the fruit fly Drosophila melanogaster. Its homeotic Hox genes occur in either the Antennapedia complex (ANT-C) or the Bithorax complex (BX-C) discovered by Edward B. Lewis.[10] Each of the complexes focuses on a different area of development. The antennapedia complex consists of five genes, including proboscipedia, and is involved in the development of the front of the embryo, forming the segments of the head and thorax.[11] The bithorax complex consists of three main genes and is involved in the development of the back of the embryo, namely the abdomen and the posterior segments of the thorax.[12]
During development (starting at the blastoderm stage of the embryo), these genes are constantly expressed to assign structures and roles to the different segments of the fly's body.[13] For Drosophila, these genes can be analyzed using the Flybase database.
Research
Much research has been done on homeotic genes in different organisms, ranging from basic understanding of how the molecules work to mutations to how homeotic genes affect the human body. Changing the expression levels of homeotic genes can negatively impact the organism. For example, in one study, a pathogenic phytoplasma caused homeotic genes in a flowering plant to either be significantly upregulated or downregulated. This led to severe phenotypic changes including dwarfing, defects in the pistils, hypopigmentation, and the development of leaf-like structures on most floral organs.[14] In another study, it was found that the homeotic gene Cdx2 acts as a tumor suppressor. In normal expression levels, the gene prevents tumorgenesis and colorectal cancer when exposed to carcinogens; however, when Cdx2 was not well expressed, carcinogens caused tumor development.[15] These studies, along with many others, show the importance of homeotic genes even after development.
See also
References
- ↑ Popodi, Ellen (1996). "Sea Urchin Hox Genes: Insights into the Ancestral Hox Cluster". Mol. Biol. Evol. 13 (8): 1078–1086. doi:10.1093/oxfordjournals.molbev.a025670. PMID 8865662.
- ↑ "Homeotic gene action in embryonic brain development of Drosophila". Development 125 (9): 1579–89. May 1998. doi:10.1242/dev.125.9.1579. PMID 9521896.
- ↑ "Homeotic Mutation". Homeotic mutations. 2013. 510–511. doi:10.1016/B978-0-12-374984-0.00727-0. ISBN 9780080961569.
- ↑ "Setting limits on homeotic gene function: restraint of Sex combs reduced activity by teashirt and other homeotic genes". EMBO Journal 13 (5): 1132–44. March 1, 1994. doi:10.1002/j.1460-2075.1994.tb06362.x. PMID 7907545.
- ↑ "Cdx and Hox genes differentially regulate posterior axial growth in mammalian embryos". Dev. Cell 17 (4): 516–26. October 2009. doi:10.1016/j.devcel.2009.08.010. PMID 19853565.
- ↑ "Hox Genes and Their Candidate Downstream Targets in the Developing Central Nervous System". Cellular and Molecular Neurobiology 25 (3–4): 697–741. 2005. doi:10.1007/s10571-005-3971-9. PMID 16075387.
- ↑ Holland, Peter WH; Booth, H Anne F; Bruford, Elspeth A (2007). "Classification and nomenclature of all human homeobox genes". BMC Biology 5 (1): 47. doi:10.1186/1741-7007-5-47. PMID 17963489.
- ↑ Theissen G (2001). "Development of floral organ identity: stories from the MADS house". Curr. Opin. Plant Biol. 4 (1): 75–85. doi:10.1016/S1369-5266(00)00139-4. PMID 11163172.
- ↑ "The MADS-box family of transcription factors". European Journal of Biochemistry 229 (1): 1–13. 1995. doi:10.1111/j.1432-1033.1995.0001l.x. PMID 7744019.
- ↑ "Homeotic genes have specific functional roles in the establishment of the Drosophila embryonic peripheral nervous system". Development 115 (1): 35–47. May 1992. doi:10.1242/dev.115.1.35. PMID 1353440.
- ↑ "Rescue and regulation of proboscipedia: a homeotic gene of the Antennapedia Complex". Development 113 (1): 257–71. Sep 1991. doi:10.1242/dev.113.1.257. PMID 1684932.
- ↑ "The ABC of the BX-C: the bithorax complex explained". Development 133 (8): 1413–22. Apr 2006. doi:10.1242/dev.02323. PMID 16556913.
- ↑ "trithorax regulates multiple homeotic genes in the bithorax and Antennapedia complexes and exerts different tissue-specific, parasegment-specific and promoter-specific effects on each". Development 117 (1): 119–34. January 1993. doi:10.1242/dev.117.1.119. PMID 7900984.
- ↑ "Unique morphological changes in plant pathogenic phytoplasma-infected petunia flowers are related to transcriptional regulation of floral homeotic genes in an organ-specific manner". The Plant Journal 67 (6): 971–79. July 1, 2011. doi:10.1111/j.1365-313X.2011.04650.x. PMID 21605209.
- ↑ "The Cdx2 homeobox gene has a tumour suppressor function in the distal colon in addition to a homeotic role during gut development". Gut 52 (10): 1465–71. October 2003. doi:10.1136/gut.52.10.1465. PMID 12970140.
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