Which of the following is a purine nitrogenous base?

What are the length of one full turn of the $DNA$ helix and the distance between the two strands (polynucleotides)?

If a double stranded $DNA$ has $20\; \%$ of cytosine,what will be the percentage of adenine in it (in $; \%$)?

$A$ $DNA$ polymer has a free phosphate group at one end of the sugar,which is called the $P$ end of the polynucleotide chain. Similarly,at the other end of the polymer,there is a free $OH$ group of the sugar,which is called the $Q$ end of the polynucleotide chain.
$\quad P \quad Q$

The term 'gene' refers to

What is the width of a $DNA$ molecule having five turns?