A molecular model for DNA structure first proposed by J.D.Watson and F.H.C Crick (1953) has been universally accepted.
They were awarded Nobel Prize for their discovery. The work of Crick and Watson has been supported by X-ray diffraction picture obtained by Wilkins and his associates.
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According to this model, a DNA molecule is composed of two chains that are spirally coiled and form a double helix, much like a spiral stairway.
The distance between the two chains is uniform and it is maintained by base pairing. The back bone of the chain is made up of alternating sugar and phosphate molecules. At specific intervals, the chains are joined by ‘steps’.
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Each step is made up of two nitrogen bases. Each nitrogen base is attached to the sugar molecule of the opposite chain. Between themselves, the nitrogen bases of a set are joined by hydrogen bonding.
The pairing of the nitrogen bases is always specific. It is made up of a purine and a pyrimidine i.e., a double ring and a single ring.
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Two purines would occupy too much space and cannot be accommodated between two chains; similarly two pyrimidines would form a short step and leave a gap between two chains. Hence the pairing is always between a purine and pyrimidine.
Because of this pairing, in a DNA molecule, the total purine content is always equivalent to total pyrimidine content.
The purine, pyrimidine base pairing is further specific. It is always Adenine with Thymine and Guanine with Cytosine. Adenine and Thymine are joined by two hydrogen bonds through atoms attached to position 6′ and 1Cytosine and Guanine are joined by three hydrogen bonds at positions 6′ 1′ and 2′, the bonding with hydrogen is weak; but this facilitates easy separation of two strands during replication.
According to Crick and Watson, the helix has a diametre of 20 A0 and there is one complete turn to the helix at every 34 A0 and has a stack of 10 nucleotides at every turn.
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One unique character of DNA is its unity and yet infinitesimal variety. The DNA structure mentioned above is uniform in all organisms, whether it is man or virus. At the same time it is also true that DNA is the genetic material that decides the characters of individual organisms. Then how does DNA decide the individuality?
The answer lies in the base pair sequence. The combination of A/T, G/C is uniform, but their sequence is not; for instance in one individual it may be AT, G/C, GC, AT, GC AT etc. There is no limit to the variety in which base pair sequences are arranged.
This is the secret of individual differences. In no two individuals, the base pair sequence is same.
Unusual bases in DNA:
The normal bases in DNA are A, T, G and C. In some viruses like PBS1 and PBS2 uracil replaces thymine. In some bacteriophages C is replaced by 5 hydroxy methyl cytosine.
Single strand DNA:
Although DNA in most organisms is double stranded, in some phage viruses, single stranded DNA is found. The following table summarizes the differences between double stranded DNA and single stranded DNA.