What is the length of a $DNA$ molecule consisting of three turns?

$A$ $DNA$ 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.
$\quad P \quad Q$

Which scientist's observations were the basis for the rule that the ratio between Adenine and Thymine,and Guanine and Cytosine in $DNA$ remains constant and equal to one another?

Give the historical background of $DNA$.

The functional unit of mutation is

When a chromosome,which appears like beads on a string,is observed under an electron microscope,what is this structure called?