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In this article, we propose to discuss the Watson-Crick Model of DNA and biological importance of DNA.
DNA is the largest macromolecule that represents the genetic material of the cell. Chemically, DNA is a double helix of two antiparallel polynucleotide chains.
Each polynucleotide chain is a linear mixed polymer of four deoxyribotides i.e. deoxyadenylate, deoxyguanylate, deoxycytidylate and thymidylate.
Watson-Crick Model of DNA:
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In 1953, J.D.Watson (an American biologist) and F.H.C. Crick (a British Physicist) proposed the three-dimensional model of physiological DNA (i.e. B-DNA) on the basis of X-ray diffraction data of DNA obtained by Franklin and Wilkins. For this epoch- making discovery, Watson, Crick and Wilkins got Nobel Prize in medicine in 1962.
The important features of Watson-Crick model or double helix model of DNA are as follows:
1. The DNA molecule consists of two polynucleotide chains or strands that spirally twisted around each other and coiled around a common axis to form a right-handed double-helix.
2. The two strands are antiparallel i.e. they run in opposite directions so that the 3′ end of one chain facing the 5′ end of the other.
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3. The sugar-phosphate backbones remain on the outside, while the core of the helix contains the purine and pyrimidine bases.
4. The two strands are held together by hydrogen bonds between the purine and pyrimidine bases of the opposite strands.
5. Adenine (A) always pairs with thymine (T) by two hydrogen bonds and guanine (G) always pairs with cytosine (C) by three hydrogen bonds. This complementarity is known as the base pairing rule. Thus, the two stands are complementary to one another.
6. The base sequence along a polynucleotide chain is variable and a specific sequence of bases carry the genetic information.
7. The base compositions of DNA obey Chargaff’s rules (E.E. Chargff, 1950) according to which A = T and G = C; as a corollary ∑ purines (A+G) = ∑ pyrimidines (C + T); also (A + C) = (G + T). It also states that ratio of (A + T) and (G + C) is constant for a species (range 0.4 to 1.9).
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8. The diameter of DNA is 20nm or 20 Å . Adjacent bases are separated 0.34 nm or by 3.4 Å along the axis. The length of a complete turn of helix is 3.4 nm or 34 Å i.e. there are 10bp per turn.
9. The DNA helix has a shallow groove called minor groove (˜1.2nm) and a deep groove called major groove (~2.2nm) across.
Biological Importance of DNA:
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1. Hereditary material:
The genetic information stored in the nucleotide sequence of DNA helps in synthesis of specific proteins or polypeptides and transmit the information to daughter cells or offspring’s. Thus, DNA is called as molecular blueprint or thread of life.
2. Autocatalytic role DNA:
DNA undergoes replication (self-duplication) in the S-phase of cell cycle. During the process each DNA strand of a double helix can act as template for the synthesis of daughter strand.
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3. Hetero-catalytic role:
During transcription any one strand of DNA acts as template for the synthesis of RNA. This is called the hetero-catalytic role of RNA.
4. Variations:
DNA undergoes recombination at meiosis and occasional mutation (changes in nucleotide sequences) which creates variations in population and ultimately contributes to evolution.
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5. DNA controls cellular metabolism, growth, and differentiation.
6. DNA finger printing (=DNA typing or profiling):
Each individual carries specific short nucleotide repeats which are called as mini-satellites or VNTRs (Variable Number of Tandem Repeats). The VNTRs of two individuals are variable and forms the basis of DNA fingerprinting. This technique is used to identify criminals, determine paternity, verification of immigrant etc.
7. Recombinant DNA technology (Genetic engineering):
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It involves the artificial cleaving and rejoining DNA sequences from two or more organisms to create recombinant DNA. This technology is employed for production of genetically modified organisms (GMOs), genetically modified foods (GMFs), vaccines, hormones, enzymes, clones etc. It is also used for construction of probes (short polynucleotide chain attached to a radioactive or fluorescent marker) for diagnosis of diseases and curing hereditary diseases by replacing a faulty gene with a normal gene (gene therapy), formation of clones etc.
