Which of the following is a purine nitrogenous base?

What is the ratio of purines to pyrimidines in a $DNA$ molecule containing $77$ nucleotides?

What is the type of coiling in $DNA$?

What is the distance between two successive nitrogen base pairs in a $DNA$ double helix?

Read the following statements and choose the set of correct statements:
$(a)$ Euchromatin stains light.
$(b)$ Heterochromatin is transcriptionally inactive.
$(c)$ Histone octamer contains eight molecules of histone proteins.
$(d)$ Packaging of chromatin at higher level requires additional set of proteins that are referred to as non-histone chromosomal $(NHC)$ proteins.
$(e)$ In a human diploid cell,$6.6 \times 10^9$ base pairs are present,not $6.6 \times 10$ nucleosomes.
Choose the correct answer from the options given below:

"Nu body" was shown by