| 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
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).
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