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Introduction
Amongst the various aspects of science and technology, scientific research in genetics has tremendously advanced. Genetics is the science in which we study about heredity; physical and chemical properties of hereditary material and transmission of this hereditary material from one generation to the next. Therefore, DNA (deoxyribonucleic acid) is a one of the most vital molecule of a living system. It carries the genetic information necessary for the organization and functioning of most living cells and control the inheritance of characteristics. It is the biochemical molecule that makes chromosomes and genes (Benjamin. 1994).
Structure of DNA
DNA basically consists of a chemically linked sequence of subunits. Each subunit contains a nitrogenous base (a heterocyclic ring of carbon and nitrogen base), a pentose sugar (a five-carbon sugar in ring form), and a phosphate group. The nitrogenous bases fall into the two types; purine and pyrimidine. Pyrimidines have a six-member ring while purines have a fused- five and six -members ring.
Each nucleic acid is has 4 basic types.. The same two purines, adenine and guanine, are present in both DNA and RNA. The two pyrimidines in DNA are cytosine and thymine. The bases are usually referred to by their initial letters; so DNA contains A, G, C and T. Two types of pentose are found in nucleic acids. They distinguish DNA and RNA and give rise to the general names for the two types of nucleic acids. However, in case of DNA, the pentose is 2-deoxyribose.
Where nucleotides provide the building blocks from which nucleic acids are constructed (Stanley and Freifelder. 1994). The nucleotides are linked together into a polynucleotide chain by a backbone consisting of an alternating series of sugar and phosphate residues. Therefore, DNA has 2 strands and these twin strands, in the form of a double helix. They are composed of successive units of the sugar de-oxyribose, phosphate and the bases adenine, cytosine, guanine and thymine, through which the twin strands are cross-linked: adenine to thymine and cytosine to guanine. In nature, base pairs form only between A and T and between G and C; thus the base sequence of each single strand can be deduced from that of its partner.
Historical Development of DNA
The idea that genetic material is nucleic acid had its roots in the discovery of transformation by Griffith in1928. Another great step forward was the recognition of deoxyribonucleic acid (DNA) as the chemical substances responsible for heredity in all cells. It was identified as a compound bearing genetic information when, in 1944, Avery, MaCLeod and McCarty discovered that a nonvirulent strain of the bacterium Streptococcus pneumoniae could be transformed in a heritable manner into a virulent strain by simply adding DNA extracted from a dead virulent strain into the medium. That is, the now virulent bacterium could transmit that virulence indefinitely to its progeny. The DNA derived from dead nonvirulent bacteria had not effect under the same conditions (Michael. J. Pelczar 1986).
Therefore, these discoveries marked the introduction of a distinction between the genetic material and the products of its expression, a view that became as implicit basis for subsequent studies.
Mutation, Mutagens and Gene Rearrangements
A mutation is a change in the basic sequence of DNA that usually results in insertion of different amino acids into a protein and the appearance of an altered phenotype. As a consequence of two types of molecular changes, mutation occurs. These changes are “Base Substitution” and “Frame Shift Mutation.” As far as Base substitution is concerned, it occurs when one base is inserted in place of another. It takes place at the time of DNA replication. On the other hand, Frame shift mutation occurs when one or more base pairs are added or deleted which shifts the reading frame on the ribosome are results in incorporation of the wrong amino acids “downstream” from the mutation and in the production of an inactive protein. However, a third type of mutations occurs when transposons or insertion sequences are integrated into the DNA. These newly inserted pieces of DNA can be cause profound changes in the genes into which they insert and in adjacent genes (Levinson and Jawetz. 1993).
Moreover, there are many causes that result in mutation. These include, chemicals, radiations and some agents known as mutagens. Nonetheless, Mutagens acts in different ways that are discussed