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:
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. Term DNA was given by Zaccharis.
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 ran in opposite directions so that the 3′ end of one chain facing the 5′ end of the other.
3. The sugar-phosphate backbones remain on the outside, while the core of the helix contains the purine and pyrimidine bases.
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 complimentarily 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 carries the genetic information.
7. The base compositions of DNA obey Chargaff s rules (E.E. Chargaff, 1950) according to which A=T and G=C; as a corollary ∑ purines (A+G) = 2 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)
8. The diameter of DNA is 20nm or 20 A. Adjacent bases are separated 0.34 nm or by 3.4 A along the axis. The length of a complete turn of helix is 3.4 nm or 34 A i.e. there are 10bp per turn. (B- DNA-Watson rick DNA)
9. The DNA helix has a shallow groove called minor groove (-1,2nm) and a deep groove called major groove (- 2.2nm) across.
Some other characteristics of DNA:
1. The amount of DNA per nucleus is constant in all the somatic cells of a given species.
2. The total amount of DNA in a haploid genome is a characteristic of each organism and is known as C-value.
3. Only a small fraction of DNA is functional in eukaryotes.
4. DNA is the chemical basis of heredity and is organized into genes or cistrons.
5. DNA replicates to form DNAs and transcribes to form RNAs.
6. DNA replication occurs in the S-phase of cell cycle.
7. DNA replication is semi conservative in which two daughter DNA molecules formed; each receives one of parental strand and one new strand.
8. One strand of DNA directs the synthesis called template strand, or antisense or non-coding strand. The other strand is called coding or non template of sense strand which has the same sequence as the RNA transcript except for T in place of U.
9. DNA has many repeated base sequences, some of which ire mobile.
10. DNA can easily undergo denaturation (melting) and renaturation with any change in pH, temperature and salt concentration. DNA with a high G+ G content are more resistant to thermal melting than A + T rich molecules.
11. DNA can be synthesized in vitro (in the laboratory).
12. DNA can be measured by the unit picogram (lpg= 10 g)
Biological Importance of DNA:
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 offsprings. 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.
3. Heterocatalytic role:
During transcription any one strand of DNA acts as template for the synthesis of RNA. This is called the heterocatalytic role of RNA.
DNA undergoes recombination its meiosis and occasional mutation (changes in nucleotide sequences) which creates variations in population and ultimately contributes to evolution.
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 minisatellites 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):
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 und curing hereditary diseases by replacing a faulty gene with a normal gene (gene therapy), formation of clones etc.