DEOXYRIBONUCLEIC ACID
Deoxyribonucleic acid
(DNA) is a nucleic acid that carries the genetic information to the offspring of
an organism. DNA forms the chemical basis of hereditary characters.
It
contains the instruction for the synthesis of proteins in the
ribosomes. Gene is a part of a DNA molecule. DNA is present in the nucleus
(chromosome) and mitochondria of the cell. The DNA present in the nucleus is
responsible for the formation of RNA. RNA regulates the synthesis of
proteins by ribosomes. DNA in mitochondria is called non-chromosomal DNA.
STRUCTURE OF DNA
DNA is a doublestranded complex nucleic acid. It is formed by deoxyribose, phosphoric acid and four types of bases. Each DNA molecule consists of two polynucleotide chains, which are twisted around one another in the form of a double helix. The two chains are formed by the sugar deoxyribose and phosphate. These two substances form the backbone of DNA molecule. Both chains of DNA are connected with each other by some organic bases.
Each chain of DNA
molecule consists of many nucleotides. Each nucleotide is formed by:
1. Deoxyribose – sugar
2. Phosphate
3. One of the following organic
(nitrogenous) bases:
Purines – Adenine (A) –
Guanine (G)
Pyrimidines – Thymine
(T) – Cytosine (C)
The strands of DNA are
arranged in such a way that both are bound by specific pairs of bases. The
adenine of one strand binds specifically with thymine of opposite strand. Similarly,
the cytosine of one strand binds with guanine of the other
strand. DNA forms the component of chromosomes, which
carries the hereditary information.
The hereditary information that is encoded in DNA is called genome. Each DNA
molecule is divided into discrete units called genes.
GENE
Gene is a portion of
DNA molecule that contains the message or code for the synthesis of a specific
protein from
amino acids. It is like a book that contains the information necessary
for protein synthesis. Gene is considered as the basic hereditary unit of the cell. In
the nucleotide of DNA, three of the successive base pairs are together called a
triplet or a codon.
Each codon codes or forms
code word (information) for one amino acid. There are 20 amino acids and
there is separate
code for each amino acid. For example, the triplet CCA is the code for
glycine and GGC is the code for proline. Thus, each gene forms
the code word for a particular protein to be synthesized in ribosome
(outside the nucleus) from amino acids.
GENETIC DISORDERS
A genetic disorder is a disorder that
occurs because of the abnormalities in an individual’s genetic
material
(genome). Genetic disorders are either hereditary disorders or due to
defect in genes.
Causes of Gene
Disorders
Genetic disorders
occur due to two causes:
1. Genetic variation: Presence
of a different form of gene
2. Genetic mutation: Generally,
mutation means an alteration
or a change in nature, form, or quality.
Genetic mutation
refers to change of the DNA sequence within a gene or chromosome of an
organism, which results in the
creation of a new character.
Classification of Genetic Disorders
Genetic disorders are
classified into four types:
1. Single gene disorders
2. Multifactorial genetic disorders
3. Chromosomal disorders
4. Mitochondrial DNA
disorders.
1. Single Gene Disorders
Single gene disorders or Mendelian or
monogenic disorders occur because of variation or mutation in one
single gene. Examples include sickle
cell anemia and Huntington’s disease.
2. Multifactorial Genetic Disorders
Multifactorial genetic disorders or
polygenic disorders are caused by combination of environmental factors and mutations
in multiple genes. Examples are coronary heart
disease, Alzheimer’s disease,
arthritis and diabetes
3. Chromosomal Disorders
Chromosomal disorder is a genetic
disorder caused by abnormalities in chromosome. It is also called
chromosomal abnormality, anomaly or
aberration. It often results in genetic disorders which involve physical or mental
abnormalities. Chromosomal disorder is caused by numerical abnormality or
structural abnormality.
Chromosomal disorder
is classified into two types:
i. Structural abnormality (alteration)
of chromosomes which leads to disorders like chromosome instability
syndromes (group of inherited diseases
which cause malignancies)
ii. Numerical abnormality of
chromosomes which is of two types:
a. Monosomy due to absence of one
chromosome from normal diploid number. Example is Turner’s
syndrome, which is characterized by
physical disabilities
b. Trisomy due to the presence
of one extra chromosome along with normal pair of chromosomes
in the cells. Example is Down syndrome, which is
characterized by physical disabilities
and mental retardation.
4. Mitochondrial DNA Disorders
Mitochondrial DNA disorders
are the genetic disorders caused by the mutations in the DNA of mitochondria
(nonchromosomal DNA). Examples are Kearns-Sayre syndrome (neuromuscular
disorder characterized by myopathy, cardiomyopathy and paralysis of ocular muscles) and Leber’s hereditary
optic neuropathy (disease characterized by
degeneration of retina and loss of vision).
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