Which of the following is a purine derivative?

The reason for the double helical structure of $DNA$ is the operation of

An alteration in the base sequence of a nucleic acid molecule is called:

Which statement is correct for the nucleotides of $DNA$?

In a nucleoside,a base is attached at the $1^{\prime}$ position of the sugar moiety. $A$ nucleotide is formed by linking a phosphoric acid unit to the sugar unit of a nucleoside. At which position of the sugar unit is the phosphoric acid linked in a nucleoside to give a nucleotide?

Which of the following is $NOT$ a salient feature of the Watson and Crick model for $DNA$?