Which cellular structure is specific to eukaryotic cells and is not found in prokaryotic cells such as bacteria?

Introduction / Context:
This question tests your ability to distinguish between eukaryotic and prokaryotic cells. While both cell types share some basic features, one of the most important differences is the presence or absence of a true membrane bound nucleus. Recognising that only eukaryotic cells possess a nucleus enclosed by a nuclear envelope is key for understanding cell classification and evolution.

Given Data / Assumptions:
- Eukaryotic cells include cells of animals, plants, fungi and protists.- Prokaryotic cells include bacteria and archaea.- Several structures are listed: nucleus, DNA, cytoplasm, cell membrane and ribosomes.- You are asked to identify which structure is specific to eukaryotes and absent from prokaryotes.

Concept / Approach:
Both eukaryotic and prokaryotic cells contain DNA as genetic material, are surrounded by a cell membrane and have cytoplasm with ribosomes for protein synthesis. The crucial difference is the organisation of the genetic material. In prokaryotes, DNA is located in a nucleoid region that is not separated by a membrane. In eukaryotes, DNA is enclosed within a double membrane structure called the nucleus or nuclear envelope. Therefore, the nucleus is specific to eukaryotic cells. The strategy is to eliminate all structures common to both cell types and select the one that appears only in eukaryotes.

Step-by-Step Solution:
Step 1: Recall that prokaryotes and eukaryotes share basic features such as a cell membrane, cytoplasm, DNA and ribosomes.Step 2: Recognise that the nucleus surrounded by a double membrane is not present in prokaryotic cells.Step 3: Evaluate each option and ask whether bacteria possess that structure.Step 4: Confirm that bacteria have DNA, cytoplasm, a cell membrane and ribosomes, but do not have a true nucleus.Step 5: Select option A, the nucleus surrounded by a double membrane, as the eukaryote specific structure.

Verification / Alternative check:
Standard cell diagrams in biology textbooks clearly show a nucleus in eukaryotic cells, while bacterial cell diagrams lack a membrane bound nucleus.Descriptions of prokaryotes often begin by stating that they lack membrane bound organelles, including a nucleus.These facts verify that the nucleus is unique to eukaryotes among the structures listed.

Why Other Options Are Wrong:
Option B is wrong because DNA is present in both eukaryotic and prokaryotic cells, although it is organised differently.Option C is wrong because cytoplasm is common to both cell types and surrounds internal components in all cells.Option D is wrong because a cell membrane made of a phospholipid bilayer surrounds both eukaryotic and prokaryotic cells.Option E is wrong because ribosomes exist in both groups and, although they differ slightly in size, are not unique to eukaryotes.

Common Pitfalls:
Students sometimes mistakenly believe that prokaryotes lack DNA or ribosomes, which is incorrect.Another pitfall is to assume that any complex looking structure must be present only in eukaryotes, without checking whether bacteria also have a simpler version.Remember that the defining feature is not the presence of DNA but the presence of a membrane bound nucleus that encloses it.

Final Answer:
The structure specific to eukaryotic cells and absent in prokaryotic cells is the nucleus surrounded by a double membrane.