Introduction :
James Watson and Francis Crick a rounded with Nobel Prize for (discovering) DNA structure (1953). X – rays diffraction studies indicated that distances between two paired nucleotides is 3.4 A0 and each turn of the double helix (3600) contains 10 paired nucleotides.
Postulates of Watson and Crick’s DNA model :
- DNA exists as double helix in which two polynucleotide chains are coiled about one another in a spiral way ( a right handed spiral)
- Each polynucleotide chain consists of a sequence of nucleotides linked together by phosphodiester bonds, Joining adjacent deoxyribose sugar moieties.
Adenine is always paired with thymine and guanine with cytosine. Guanine and Cytosine are linked by three weak hydrogen bonds (G C), while Adenine and Thymine by two weak hydrogen bonds ( A = T )
- Two chains of DNA run in “Anti- Parallel Directions”
(The sugar-phosphate backbone of two complementary strands are anti-parallel.
On one its 3’-OH 5 – P and on other its
5’ P 3’ OH (See figure)
This means that the two chains are parallel but their 5’ 3’ directions are opposite. These polynucleotide chains are shown Polarity. One end is called 5’ end and other end called 3’ end.
- [ In each chain, the phosphate component carried by carbon atom at position 5’ of sugar (in one nucleotide unit) is joined by Phosphodiester bond to the hydroxyl component of the carbon atom at position 3’ of the sugar in the next nucleotide uint ]
- These alternating sugar – phosphate – sugar components form backbones of DNA double helix.
- The base pairing is specific, Adenine always pairs with Thymine, (A = T) and Guanine always with Cytosine (G C ).
The two DNA strands are said to be complementary (not similar) i.e. If the sequence of nucleotides on one strand is known, the sequence on the other strand can be accurately predicted due to this property of complementarily.
High Degree of Stability of DNA is due to :-
- A large number of hydrogen bonds.
- Hydrophobic bonding or stacking forces between stacked base pairs.
- Presence of phosphate sugar backbone in DNA helix points out to give hydrophilic ridges with one negative charge per phosphate. The presence of this negative charge is in fact responsible for interaction of DNA with proteins, one of the most important being the histone proteins that help in the packing of DNA.
- This negative charge also helps in attaining different staining patterns of DNA like feulgen staining, silver staining etc.