Cell Structure: Question 9

Worked solution

Step 1: Recall the basic structure of the nucleus

The nucleus is bounded by a nuclear envelope, which is a double membrane (two membranes, one inside the other), not a single membrane. This double membrane contains nuclear pores, gaps through which molecules such as messenger RNA and ribosome subunits can pass between the nucleus and the cytoplasm.

Step 2: Continuity with the rough endoplasmic reticulum

In places, the outer membrane of the nuclear envelope is continuous with the membrane of the rough endoplasmic reticulum, so the space between the nucleus’s two membranes connects directly with the space inside the rough ER. This links the nucleus into the cell’s wider internal membrane system rather than leaving it as an isolated compartment.

Step 3: Why the other options are wrong

• B: wrongly describes a single membrane with no pores; the true structure is a double membrane containing pores.
• C: correctly describes the double membrane and pores, but wrongly claims the nucleus contains 70S ribosomes attached to its own DNA. That description applies to mitochondria and chloroplasts, which contain their own small circular DNA and 70S ribosomes, not the nucleus.
• D: wrongly claims there is no nuclear membrane at all; this describes a prokaryotic cell’s nucleoid region, not the nucleus of a eukaryotic cell.

Final answer

• The nucleus is bounded by a double membrane containing pores, continuous with the rough endoplasmic reticulum in places, option A.