Scale bar represents 10µm
The nucleus is the largest organelle in the human cell and contains almost all of the DNA (the rest being found in the mitochondria). To fit the DNA, it is winded around protein complexes called histones to form the more condensed chromatin. The nucleus contains the nucleoplasm, a semifluid liquid consisting of molecules and proteins such as enzymes, transcription factors and nucleotides, and is surrounded by the nuclear membrane which separates it from the rest of the cell. This ensures a highly controlled DNA replication and transcription without interference of the translation that takes place in the cytoplasm.
The chemical 4',6-diamidino-2-phenylindole (DAPI) is used to counterstain the nucleus, as it binds strongly to tightly packed DNA, but not the nucleoli because of its much lower amount of DNA. The immunofluorescent staining of the nucleus varies between different antibodies, in some cases the whole nucleus is stained and in other cases only the nucleoplasm. The staining characteristics can vary from a smooth to a punctate staining.
Read more about the proteome of the nucleus.