Which of the following provides information about the structure,dynamics,and evolution of chromosomes?

Histones are enriched with $-$

In $B-DNA$,one full turn of the helical strand contains

$A$ genome is defined as the set of chromosomes corresponding to the:

Assertion $(A) :$ The distance between two polynucleotide chains in $\text{dsDNA}$ remains constant.
Reason $(R) :$ Always a purine comes opposite to pyrimidine or vice versa.

In $DNA$,how are the nucleotides linked together?