Which cell structure is unique to prokaryotes (i.e., absent in eukaryotic cells)?

• A. Endoplasmic reticulum
• B. Nucleoid region containing the primary chromosome
• C. Golgi apparatus
• D. Mitochondrion

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the key difference between prokaryotic and eukaryotic cells. Prokaryotes lack membrane-bound organelles, while eukaryotes have them.
2. Step 2: Identify the structures listed: Endoplasmic reticulum, Golgi apparatus, and Mitochondrion are all membrane-bound organelles found only in eukaryotic cells.
3. Step 3: Recognize that the nucleoid region is a non-membrane-bound area in prokaryotes where the primary chromosome (DNA) is located, which is absent in eukaryotic cells.
4. Step 4: Conclude that the nucleoid region containing the primary chromosome is unique to prokaryotes because eukaryotes have a membrane-bound nucleus instead.
5. Step 5: Therefore, the correct answer is the nucleoid region containing the primary chromosome, as it is the cell structure unique to prokaryotes.