nucleic

As an adjective nucleic

is referring to the nucleus of something (as nucleic acids are found in the nucleus of cells).

As a noun nuclein is

(biochemistry) a phosphorus-rich protein found in the nucleus of a cell, later specifically nucleohistone or nucleoprotamine; also, any similar compound present in the cell nucleus.

As an adjective nucleic

is referring to the nucleus of something (as nucleic acids are found in the nucleus of cells).

As a noun nuclei is

.