Explain the following salient features of a DNA molecule :

(a) Complimentarity- Adenine (A) , Guanine ( G) , Thymine (T) and Cytosine ( C) are the four bases in a DNA molecule; 'A' always pairs with 'T' with the help of two hydrogen bonds; 'G' always pairs with 'C' with the help of three hydrogen bonds 
(b) It means if one chain has polarity 5' →  3' the other has 3' →  5' ,a polymer thus formed has at one end a free phosphate moiety at 5 ' of deoxyribose sugar (5' end of the polynucleo tide chain), at the other end of the polymer the deoxyribose sugar has a free 3 ' OH ( 3' end of polynucleotide chain )