Which of the following organelles is not involved in protein synthesis?

The nucleus is the organelle where the genetic material is housed and where the processes of transcription and replication occur. While the nucleus plays a critical role in the initial stages of protein synthesis by creating messenger RNA (mRNA) from DNA, it does not directly participate in the translation of mRNA into proteins.

Protein synthesis primarily occurs outside the nucleus, beginning in the ribosomes, which are responsible for translating the mRNA into a polypeptide chain, and on the rough endoplasmic reticulum, which is involved in the processing and folding of proteins. The mitochondrion, while primarily known for energy production, has its own ribosomes and can synthesize some proteins essential for its function, but it is not directly involved in the overall process of protein synthesis like the ribosomes and rough endoplasmic reticulum are. Therefore, the nucleus, although essential for the creation of genetic instructions, is not directly involved in the actual synthesis of proteins.