Which element is not found in a nitrogenous base?

Degeneration of $DNA$ after heating can be studied by comparing

In sea urchin $DNA$,which is double stranded,$17\%$ of the bases were shown to be cytosine. The percentages of the other three bases expected to be present in this $DNA$ are

Thirty percent of the bases in a sample of $DNA$ extracted from eukaryotic cells is adenine. What percentage of cytosine is present in this $DNA$ (in $\%$)?

How many nucleosomes are found in a helical coil of $30 \ nm$ chromatin fibre?

The telomeres of eukaryotic chromosomes consist of short sequences rich in: