STRUCTURE OF DNA

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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