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Genetics, Epigenetics and Disease a Literature Review By: Anthony M

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Genetics, Epigenetics and Disease a Literature Review By: Anthony M Genetics, Epigenetics and Disease A Literature Review By: Anthony M. Pasek Faculty Advisor: Rodger Tepe, PhD A senior research project submitted in partial requirement for the degree Doctor of Chiropractic August 11, 2011 Abstract Objective – This article provides an overview of the scientific literature available on the subject of genetic mechanisms of disease etiology as compared to epigenetic mechanisms of disease etiology. The effects of environmental influences on genetic expression and transgenerational inheritance will also be examined. Methods – Searches of the keywords listed below in the databases PubMed and EBSCO Host yielded referenced articles from indexed journals, literature reviews, pilot studies, longitudinal studies, and conference meeting reports. Conclusion – Although current research trends indicate a relationship between the static genome and the dynamic environment and offer epigenetics as a mechanism, further research is necessary. Epigenetic processes have been implicated in many diseases including diabetes mellitus, obesity, cardiovascular disease, metabolic disease, cancer, autism, Alzheimer’s disease, depression, and addiction. Keywords – genetics, central dogma of biology, genotype, phenotype, genomic imprinting, epigenetics, histone modification, DNA methylation, agouti mice, epigenetic drift, Överkalix, Avon Longitudinal Study of Parents and Children (ALSPAC). 2 Introduction Genetics has long been the central field of biology and it’s central dogma states that DNA leads to RNA, which leads to protein and ultimately determines human health or sickness1. The Human Genome Project marked a great triumph for humanity and researchers expected to solve the riddle of many complex diseases with the knowledge gleamed from this project. However, many more questions were raised than answered. Several rare genetic disorders including hemophilia and cystic fibrosis were explained by alterations in the genetic code but true genetic diseases only affect about one percent of the human population2. Epidemic disorders such as obesity, cardiovascular disease, metabolic diseases, and cancer were determined to be multifactorial and not entirely genetic in origin1. Epigenetics is the study of changes in phenotype or gene expression caused by mechanisms other than changes in the underlying genotype, i.e., DNA sequence of the organism3. Environmental influences play a crucial role in altering the epigenome. The influence of environmental forces on genetic material revolutionized the central dogma of biology. Epigenetics has been dubbed the new biology and it’s implications on health and disease are profound1. Researchers and physicians are now realizing that to facilitate lasting change in patients lifestyle and environmental factors must be taken into consideration. 3 Discussion Genetics Every human cell is either a somatic cell or a reproductive cell. Reproductive cells are known as gametes and include sperm cells and egg cells. Every other cell is considered a somatic cell. Cells contain a nucleus that houses the genetic material known as deoxyribonucleic acid (DNA) arranged in compact structures called chromosomes4. Each somatic cell contains 46 chromosomes made up of 23 pairs of homologous chromosomes, where one chromosome in each pair is paternal in origin, and the other is maternal in origin4. One chromosome consists of two sister chromatids connected by a centromere. Somatic cell division, or mitosis, is a process that results in two genetically identical cells by cleaving the centromere and dividing the two sister chromatids equally into two daughter cells4. Reproductive cell division, or meiosis, is a process that results in genetically diverse cells with half of the normal number of chromosomes. When reproductive cells combine, the result is a zygote that is genetically disctinct from both the paternal and maternal genome4. Meiosis and sexual fertilization account for the genetic variability between individuals of a species4. The DNA molecule is complexed with many diverse proteins to condense and organize the genetic material. This complex is referred to as chromatin. Histones are proteins that act as a spool for the DNA to coil around. Other proteins form structures similar to scaffolding to aid in looping and folding the DNA. Chromatin morphology changes throughout the cell cycle and is a key factor in genetic accessibilty1. Condensed chromatin is present during cell division to enable the chromosome to be easily divided 4 and prevents most of the genes from being transcribed1. Loosely packed chromatin is present throughout most of the cell cyle and allows the genes to be transcribed1, 4. Every cell of an organism contains the same genetic code, but humans have approximately 200 different cell types. Cell differentiation is the process by which cells alter their form and function to become specific cell types. Differential gene expression is the expression of different genes by cells with same genome. Epigenetic mechanisms initiate and maintain cellular differentiation3. Skin cells express their genes differently than nerve and muscle cells even though they all have the same genetic code4. Chromatin morphology, histone modification, and DNA methylation play a role in differential gene expression5. When an acetyl group is bound to the tail of a histone it’s charge is neutralized and the histone is less attracted to neighboring chromatin structures. This results in a looser chromatin structure and increased accessability of the genes for transcription. Methyl groups bound to histone tails will have the opposite effect by condensing the chromatin further4. Methyl groups bound to DNA bases result in similar suppression of genetic transcription and these DNA methylation patterns are maintained through successive cell divisions. These patterns are also passed to the next generation4. The inheritance of traits through mechanisms not involving the nucleotide sequence is called epigentic inheritance4. DNA methylation accounts for genomic imprinting throughout the life of an organism and epigenetic inheritance across generations6. 5 The collective DNA or genetic code of an individual is known as the genotype, while the expressed traits of the individual are known as the phenotype4. For certain characteristics the genotype produces a very specific and predictable phenotype, while in other instances the genotype may result in a variety of phenotypes. This phenotypic range is called the norm of reaction4. Characteristics that exhibit a broad norm of reaction, such as skin color, are considered multifactorial because both the genotype and the environment collectively influence the resultant phenotype4. A character is a heritable biological feature, such as hair color, that varies among individuals. Each variant for a character, such as blonde or brown hair, is called a trait. The transmission of traits from one generation to the next is called inheritance, or heredity4. The scientific field of genetics examines heredity and heriditary variation. Gregor Mendel’s experiments and hypotheses formed the foundation for modern genetics and inheritance4. Mendel’s research first described what are now known as alleles, or alternate versions of the same gene, which account for character variants. Mendel noted that for each character, an individual inherits two distinct alleles, one from each parent. Mendel also descibed the occurrence of two different alleles present in the same individual. When this occurs, one allele is dominant and determines the phenotype while the other is recessive and has no noticeable effect on the phenotype. Mendel also origninated the concept that became known as the law of segregation. This law states that the two alleles for a heritable character segregate during gamete formation4. An organism having a pair of identical alleles for a character is said to be homozygous for the gene 6 controlling that character4. An organism that has two different alleles for a gene is said to be heterozygous for that gene4. Genetic Disorders Thousands of genetic disorders are known to be inherited as simple recessive traits4. These disorders range in severity from albinism to cystic fibosis4. Heterozygotes are phenotypically normal due to the presence of the dominant allele but may transmit the recessive allele to their offspring and thus are called carriers. Homozygotes express the disorder phenotypically. The most common lethal genetic disease in the United States of America is cystic fibrosis4. The incidence of cystic fibrosis among those of European descent is one in 2,5004. The incidence of carriers of cystic fibrosis among those of European descent is one in 25, or 4%4. The most common inherited disorder among those of African descent is sickle-cell disease4. The incidence of sickle-cell disease is one in four hundred among those of African descent4. Dominant alleles that cause a lethal disease are much less common than recessive alleles that cause a lethal disease. All lethal alleles occur as a result of DNA mutation in a gamete. Achondroplasia and Huntington’s disease are examples of genetic disorders caused by a dominant allele4. The incidence of Huntington’s disease in the Unites States of America is one in 10,0004. Children born to a parent with Huntington’s disease have a 50% chance of inheriting the allele and the disorder4. 7 Human gender is determined by the presence of sex chromosomes. Females have two X chromosomes present while males have one X chromosome and one Y chromosome present. A gene
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