Which cellular structure is surrounded by inner and outer membranes, contains chromosomes, enzymes, and RNA, and has a membrane perforated by pores?

• A. Cell nucleus
• B. Organelles
• C. Nucleolus
• D. Inclusions

Answer: (A)

Think of the nucleus as the cell’s command center. It is enclosed by a double membrane—the nuclear envelope—with pores that perforate the envelope to regulate traffic in and out. Inside are the chromosomes and the enzymes and RNA needed for genetic processes like transcription and RNA processing. The nuclear pores allow RNA transcripts and ribosomal subunits to exit while letting in proteins required for these tasks. The outer membrane is continuous with the rough endoplasmic reticulum and the inner membrane is supported by a network called the nuclear lamina, giving the nucleus its shape. The nucleolus sits inside the nucleus and makes ribosomal RNA, but it isn’t bounded by its own membrane or perforated by pores. Inclusions are non-membrane clusters, and general organelles don’t specify this double-membrane, pore-containing envelope with genetic material, so the described features point to the nucleus.