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Epigenetic phenomena refer to the change in phenotype, which does not involve DNA mutation but is heritable while genome is the full set of genes in an organism (National Human Genome Institute, 2010). Individual animals develop by a gradual differentiation and elaboration of a fertilized egg which is driven by chemical reactions that switch part of the genome on and off at particular times. Epigenetic changes acquired by a cell are carried on to the next generation of cells, for example from mother to daughter. Epigenetic cannot be understood fully without experimental approaches. Below are experimental approaches that give genome wide information as well as site-specific information.
Approaches that give genome wide information
Genome-wide cytosine methylation studies are an example. These studies are based on the sensitivity of certain restriction enzymes to cytosine methylation and the relative resistance of methyl cytosine to disulfide-induced deamination. Restriction landmark genome scanning (RLGS), a classic example of cytosine methylation, uses a rare-cutting methylation sensitive restriction enzyme together with another enzyme to create a profile of Notldigestion products. Digested Notloverhanging ends are radiolabelled and separated by a two dimensional gel electrophoresis. This technique can resolve about 2,000 sites. MALDI-TOF is enzyme-based technique with high-throughput bisulfate sequencing. It is the technique behind the European Human Genome Project. It is important to note that most of the techniques that use enzymes and methylation differ in terms of how they detect the genomic source of the distinctive fractions and how they select the fraction of the genomes (Jason, D., et al, 2006).
ChiP-chip is another example of an approach that gives genome wide information. This is a high-resolution mapping of chromatin proteins and enzymes. It combines immunoprecipitation (chip) with micro assay analysis (chip). Cells are fixed, chromatin fragmented and protein of interest purified, along with associated DNA. An antibody or an affinity tag is used. The DNA that was co purified with the protein is then detected using DNA microarray and mapped back to the genome (Jason, D., et al, 2006) .These approaches describes differences between tissues and shows cell differentiation. These are important features in improving disease diagnosis, detecting genetic predispositions to disease early, rationing drug design, controlling gene therapy, controlling systems of diseases and creating pharmacogenomics customs drugs (National Human Genome Institute, 2010).
Approaches that give site-specific information
Recombinase-mediated cassette exchange (RMCE) gives site specific information. The approach is used to develop artificial genetic loci that impart specific and reproducible transgenes in higher eukaryotes. This facilitates the analysis of cis-regulatory DNA elements governing expression and position effects, improves human control over the transgenes and accelerates the development of animal models for complex human disease. RMCE is achieved by the clean exchange of a pre-existing â€˜gene cassetteâ€™ for an analogous cassette carrying the gene of interest. It is particularly important in cancer treatment because of its gene analysis properties. It can therefore be used to determine which particular genes lead to tumor development (Feng, 1999).
How environment influences phenotype
Phenotype refers to those traits or characters we observe in an organism. It has been proven that proven that heritable environmentally induced epigenetic modifications normally underlie the reversible and Tranâ€™s generational alterations in an animalâ€™s phenotype Most of these changes are inherited mitotically in somatic cells. The environmental effects on the epigenome can have long-term effects in the gene expression. Environmental factors like nutritional supplements, reproductive factors, behavioral cues and low level radiation can lead to altered epigenetic programming in the prenatal and early post natal development of animals. Xenobiotic chemicals in the environment have some estrogenic characteristics which disrupt endocrine. The epigenome is most vulnerable to environment during embryogenesis DNA programming occurs in very early stages of development. In addition, toxins in the environment cause epigenetic changes to the DNA. These changes are passed on to subsequent generations despite lack of continued exposure. Roger Williams proved in the 1960s that genetic contributions were not enough to generate the difference observed in the phenotypes. Even twins who may have identical genetical make up usually exhibit very different habits when exposed to different environment settings. However, variations can occur without any significant changes in the environment.
Imprint establishment during gametogenesis in mammals
Genomic imprinting is an epigenetic phenomenon in which a gene is differentially expressed depending on the parent from which it was inherited. It occurs in plants and mammals. It should however not be confused with the inheritance of the X and Y genes. It is one of the many processes enabled by the methylation process. Medicinenet.com defines methylation as the addition of a methyl group to a cytosine residue to convert it to a5-methylcytosine (2004). DNA methylation occurs where cytosine (C) lies next to a guanine (G) at a site referred to as CpG islands. CpG islands are near the promoters of a gene.
Chromosomes and genes they contain are inherited in pairs, a copy from each parent. Mostly both alleles are expressed equally. However, one allele is over the other in genomic imprints. It can either be the maternal alleles or the paternal allele that is expressed (Macmillan, 2001). Imprinting occurs during gametogenesis or immediately after fertilization, when the chromosomes are still distinct and the imprint can be reliably passed on to the daughter cell.
Imprinting occurs at a gene promoter.
For an imprint to be formed, some modification occurs at the gene promoter by methylation. This prevents the binding of RNA polymerase at the gene promoter thus gene transcription does not occur. DNA cytosine methyltransferase is vital as to allow the attachment of the methyl to the cytosine. During gametogenesis, methyl patterns shift and these alterations are passed on during replication. This leads to expression of normally silent genes (Macmillan, 2001). Examples of imprinted genes are H19, Igf-2 and Igf-2r.
agouti gene hair color and Bisphenol A,
Pigments are produced in melanocytes. These cells take their positions on the hair follicles. Any shortcomings in melanocyte migration will lead to disorders such hooded, white sporting and spotting lethal.
Bisphenal A is also called BPA It is a main component in the industrial production ,of polycarbonates plastics and resins. Long-term exposure to Bpa causes increased risk of suffering from the breast cancer as well as metastatic prostate cancer in human beings. In addition, it leads to insulin resistance, which causes type 11 diabetes and heart failure. Also in women, it leads to meiotic aneuploidy which is the leading cause of miscarriages. Apart from this, it alters development of body parts like the reproductive tract and the immune system.