What forms the backbone of a polynucleotide chain?

Which of the following options is incorrect regarding $DNA$ base pairing rules (Chargaff's rules)?

The octamer that forms a complex unit with $DNA$ is called:

Which of the following is not correct regarding Chargaff's rules for double-stranded $DNA$?

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:

What do the genes found on mitochondrial $DNA$ exhibit?