DNA (Deoxyribonucleic Acid) is One of the Most Vital Molecules of a Living System

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 as following:

1) Nitrous acid and alkylating agents, which alter the existing base so that it forms a hydrogen bond with the wrong base.

2) Benzopyrene, which is found in tobacco smoke bind to the existing DNA bases and cause frame shift mutations. These chemicals are highly carcinogens as well as mutagen, intercalate between the adjacent bases thereby distorting and offsetting the DNA.

3) Some radiations like X-rays have high energy and can cause damage to DNA by breaking the covalent bond that hold the ribose phosphate chain together or by producing free radicals that can attack the bases.

4) An ultraviolet radiation, which has lower energy than X-rays cause the cross-linking of the adjacent pyrimidine bases to form dimmers, which results in inability of the DNA to replicate properly.

DNA: The Genetic Material

Basically the genetic material functions by virtue of its ability to specify a large variety of proteins. Though the early thoughts related to the nature of genetic material were biased by an erroneous assumption: that the structure of genetic material must be as complex as the proteins as whose production it specifies. This assumption was jettisoned when it was realized that the genetic material carries the information needed to specify the protein named code (Kings and Standsfield. 1985).

Though the sequence of nucleotides in DNA is important not because of its structure but, because it codes for the sequence of amino acids that constitutes the corresponding polypeptides. On the other hand, the relationship between a sequence of DNA and the sequence of the corresponding protein is called the genetic code. DNA also contains certain sequences whose function is to be recognized by regular molecules, usually proteins. Here the function of the DNA is determined by its sequence directly, not via any intermediary code. Both types of region, genes expressed as proteins and sequence recognized as such, constitute genetic information.

Moreover, the genetic material of all known organisms and many viruses is DNA. However, some viruses use an alternative nucleic acid, ribonucleic acid (RNA) as the genetic material. From the discovery that DNA is the genetic material, the concept that a mutation is a change in the sequence of nucleotides follows naturally. The existence of mutations (Stent and Calender. 1978) allows us to compare the properties of a wild-type (normal) gene with a defective gene. By identifying the protein that is altered by a mutation, we may characterize the product of a gene and by analyzing the changes that occur in the phenotype of the organism, we may identify the function of the gene (Bachman.1990).

The structural design of DNA enables it to accomplish the purpose of maintaining and perpetuating its sequence (Adams and Knowler. 1990). Consisting of two strands, each of whose sequence corresponding to other in a predictable manner, an individual molecule of DNA in effect contains redundant information (Weinberg. 1985).