In this article we will discuss about some of the important structural features of DNA.
1. DNA exists as a double helix in which the two polynucleotide chains are coiled about one another in a spiral with a diameter of about 20Å (Fig. 1).
2. Concentration of thymine was always equal to the concentration of adenine and the concentration of cytosine was always equal to the concentration of guanine.
3. Each polynucleotide chain consists of a sequence of nucleotides linked together by phosphodiester bonds, joining adjacent deoxyribose moieties (Fig. 2).
4. The two polynucleotide strands are held together in their helical configuration by hydrogen bonding.
5. The two polynucleotide chains generally exist in perpendicular to the axis of the molecule like the spiral staircase.
6. The bases of one polynucleotide chain pairs with the bases of other polynucleotide chain. These pairing always consist of one purine with one pyrimidine. In a strict sense, the base pairing is fixed and specific, e.g. adenine is always paired with thymine and guanine with cytosine.
7. To form the pairing of bases, adenine and thymine form two hydrogen bonds, while guanine and cytosine form three hydrogen bonds.
8. Two polynucleotide strands of DNA is complementary to each other.
9. The base pairs in DNA are stacked 3.4Å apart with 10 base pairs per turn.
10. The sugar phosphate backbones of the two polynucleotide chains of DNA have the opposite chemical polarity (Fig. 3).
11. The stability of DNA double helix is due to the large number of hydrogen bonds between base pairs and due to the hydrophobic bonding or the stacking forces between the stacked base pairs.
12. The structure of DNA molecules changes as a function of their environment or, in other words, DNA molecules exhibit a considerable amount of conformational flexibility.
13. DNA may exist in three forms:
(a) A-form:
Which has 11 nucleotides pairs per turn, (not occurred in vivo), right-handed helix.
(b) B-form:
Which has 10 nucleotides pairs per turn and this form generally occurs in vivo. Watson and Crick’s DNA model is the B-form DNA, right- handed helix.
(c) Z-form:
This form of DNA is left- handed double helical form (Table 4.1).
14. The exact conformation of a DNA molecule or a segment of a DNA molecule will depend on the nature of the molecules with which it is interacting. This structural variation in the DNA molecule play an important role in the living organism (Fig. 1, 2, 3).
