Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

→ DNA as an acidic substance present in nucleus was first identified by Friedrich Meischer in 1869. He named it as 'Nuclein'.
→ However, due to technical limitation in isolating such a long polymer intact, the elucidation of structure of DNA remained elusive for a very long period of time.
→ It was only in 1953 that James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed a very simple but famous [Double Helix] model for the structure of DNA.
→ One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chains.
→ However, this proposition was also based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.
→ The base pairing confers a very unique property to the polynucleotide chains.
→ They are said to be complementary to each other, and therefore if the sequence of bases in one strand is known then the sequence in other strand can be predicted.
→ Also, if each strand from a DNA (let us call it as a parental DNA) acts as a template for synthesis of a new strand, the two double stranded DNA (let us call them as daughter DNA) thus, produced would be identical to the parental DNA molecule.
→ Because of this, the genetic implications of the structure of DNA became very clear
→ The salient features of the Double-helix structure of DNA are as follows:
(i) It is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside.
(ii) The two chains have anti-parallel polarity. It means, if one chain has the polarity 5'→3', the other has 3' →5'.
(iii) The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds. As a result, a purine always comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix as shown (Figure).
(iv) The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm (a nanometre is one billionth of a metre, that is 10^-9 m) and there are roughly 10 bp in each turn. Consequently, the distance between a bp in a helix is approximately 0.34 nm.
(v) The plane of one base pair stacks over the other in double helix. This, in addition to H-bonds, confers stability of the helical structure (Figure).