The Structure of DNA
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Genetics is the study of the qualities that are inherited and transmitted to the offspring from the parents during reproduction. Parents pass traits to their offspring, making the basis of heredity. The inherited traits are coded for in genes, which are the inherited elements. Genes are the functional units of DNA. Through genetics, biologists and other scientists get to understand the processes and principles of heredity, genetic variation and genes.
Genetics is of great interest in the contemporary society. With the completion of the Human Genome Project, a lot of information about genetics has been exposed that is essential to the understanding of the human health. Understanding of the genome and mutations that affect it has led to an explanation of genetically inherited traits. These characteristics were previously mysterious and could not be contained as drugs could not be developed against them. The information gained from genetics has aided the research into these diseases. It has also opened a window to the possibility of finding a cure for the deadly cancer diseases (Sack, 2008).
The building block of all genetic concepts is DNA. DNA is one of the nucleic acids which stores hereditary or genetic information. It is found in the nucleus in eukaryotic organisms and the cytoplasm in the prokaryotic organisms. DNA is composed of nucleotides, which are composed of sugar, a phosphate group and a base. The DNA sugar is deoxyribose sugar. There are four essential bases in the structure of DNA. From the four bases, the numerous different sequences arise which lead to the differences observed among organisms. The bases are Thymine, Adenine, Guanine and Cytosine. The nucleotides bond to each other by a phosphor-diester bond, leading to a ladder-like double helix structure. The double helix structure is formed by the pairing of anti-parallel strands of DNA. The nucleotide bases from the two strands bind to each other through hydrogen bonds. Thymine pairs with Adenine and Guanine pairs with Cytosine (Hawley, 2010).
The DNA is divided into functional regions called genes. Genes have varying sizes, from a few hundred DNA bases to more than two million bases. The genes code for the traits expressed by each individual. Each gene has specific sets of instructions that code for particular proteins or protein functions. The nucleotide sequence of each gene forms the genetic sequence, which is crucial in the central dogma. The central dogma explains that the phenotypic appearance of an individual is a characteristic of their DNA. From the DNA sequence, RNA (another nucleotide) can transcribe and translate the information into proteins. The proteins are the building structure of the body. Therefore, the protein that is expressed in the DNA is the one that will be manufactured and expressed physically as the structure of the individual (Hawley 2010, Sack 2008).
Genes are found in packaged DNA sets called chromosomes. Each chromosome has millions of DNA bases, from fifty to two hundred million in number. Many genes make a chromosome. The chromosomes are the means of transferring genetic information from the parents to the offspring. They are more complex as they are made up of the genes and binding proteins know as histones. The DNA is usually tightly wound around each of the proteins. Each organism has a specific number of chromosomes, which if exceeded or reduced result in an abnormality. The human beings have twenty-three pairs of chromosomes (46 chromosomes). One of the pairs is the sex chromosomes (XX for female and XY for male). The other twenty-two pairs of chromosomes are autosomes (Hawley 2010, Sack 2008).
Traits are inherited from both parents following the Mendelian inheritance laws. The genetic makeup of an individual (genotype) is made of two alleles of each gene. An allele is a copy of a gene that codes for the same trait. Each allele is inherited from either parent. If the alleles are identical, they are called homozygous, and if they are not identical, they are heterozygous. If alleles are indeed identical, they have similar coding sequences at that particular locus. Each gene has a dominant allele that will be expressed in a case of the presence of a heterozygous pair of the alleles (Jobling, Hurles, & Tyler-Smith, 2013).
The genetic material of the human cells (and other eukaryotic cells) except the red blood cells is found in the nucleus. The RBC do not have a nucleus and hence do not carry genetic information. However, some organelles such as the mitochondria and the chloroplasts have their DNA. The organelles contain multiple copies of small chromosomes and are only inherited from the mother. They are found in the ovum during fertilization as the sperm cell only contributes the nuclear genetic information.
The exact location of a gene on a chromosome is called a locus. There are fifty thousand to hundred thousands of genes in the human genome. However, the DNA in the genes is only approximately 2% of the total genomic DNA. Much of the information on the non-coding DNA has not yet unearthed, despite successfully completing the human genome project (Hawley, 2010). Formerly called junk DNA, the non-coding DNA is increasingly being considered to be of essential function in the central dogma. Scientists are, however, working to find out the exact role of the DNA, which has so far remained elusive. The locus of each gene has enabled the formation of the genetic map, as more than 13,000 genetic sites have been correctly identified (Sack, 2008).
The genetic maps have enabled the study of different inherited diseases. The particular location of the gene(s) responsible for the conditions can be identified and studied. The development of the gene maps makes it faster, cheaper and more practical for scientists to identify and diagnose a given genetic disease. Genetic mapping has made it possible to identify most hereditary diseases such as cystic fibrosis, enabling adequate pharmaceutical research to be carried out on them.
Hereditary diseases occur as a result of a mutation in the genes that code for given proteins. The mutation can either occur through deletions, substitutions or insertions of DNA bases at certain points leading to frameshifts in the structure of the gene. These frameshifts will result in coding for abnormal or non-functioning proteins (Loewe, 2008). Without these proteins to act in their usual roles, the body faces challenges adapting to situations that require the proteins. For example, the Duchenne Muscular Dystrophy is as a result of a deletion in a gene that codes for an important muscle protein, dystrophin. The absence of dystrophin results to muscle weakness and inevitable early death (Behrman, Kliegman, & Jenson, 2011). However, some mutations are as a result of adaptation. For example, the mutation to cause sickle red blood cells was an adaptation to prevent malaria in the tropics.
Genetics has not only enabled us to understand more about ourselves, but has also given us more information about our origins. Evolutionary genetics allows the comparison of genetic data of proteins from different organisms and establishing where they diverged or converged in the evolutionary tree. With the development of technology and computers, the branch of bioinformatics has explored worlds unknown before. Accurate data on genetic sequences has been compared with previous generations to establish the relationships between human beings and other organisms (Jobling, Hurles, & Tyler-Smith, 2013). With the understanding of the past and evolution, genetics helps us to predict the future. The future of genetics is one of the exciting branches of science that has fascinated many biologists.
Genetics is not without its shortcomings. The cloning debates and other ethical issues have brought the in-depth study and application of genetics into questions. The knowledge of Genetics might tempt scientists to try to act God in establishing human beings without blemish. The cloning of human being is also a much-debated question as well as the issues of personalized medicine. The study of the genetics of an individual also means that one gets to understand the genetics of the parents, raising ethical questions about the informed consent. Ethical dilemmas also arise when two carrier parents are expecting a child who has been diagnosed with the disease. Do they end the pregnancy or wait for the baby to be born and suffer? In some instances, genetics has led to more questions than answers (Fulda, & Lykens, 2006). Despite its shortcoming, genetics opened multiple doors in the contemporary science.
Behrman, R. E., Kliegman, R., & Jenson, H. B. (2011). Nelson textbook of pediatrics. Philadelphia: W.B. Saunders Co.
Fulda, K. G., & Lykens, K. (2006). Ethical issues in predictive genetic testing: a public health perspective. Journal of medical ethics, 32(3), 143-147.
Hawley, R. S. (2010). HUMAN GENOME. Academic Press.
Jobling, M., Hurles, M., & Tyler-Smith, C. (2013). Human evolutionary genetics: origins, peoples & disease. Garland Science.
Sack, G. H. (2008). Genetics. New York: McGraw-Hill Medical.
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