AAMC FL Practice Exam

Where is protein synthesized before it undergoes post-translational modifications?

1. In the nucleus

2. In the Golgi apparatus

3. In the ER

4. In the cytoplasm

The correct answer is: In the ER

Protein synthesis occurs primarily in the endoplasmic reticulum (ER), particularly the rough ER, which is studded with ribosomes. These ribosomes translate mRNA into polypeptide chains, which then enter the lumen of the ER. This initial synthesis is crucial because it sets the stage for subsequent processing. After synthesis, proteins undergo post-translational modifications, which can include glycosylation, phosphorylation, and cleavage. While some of these modifications can begin in the rough ER, they are often completed in the Golgi apparatus, where proteins are further processed, sorted, and packaged for transport to their final destinations. Thus, the correct answer highlights the rough ER's role as the site where amino acids are assembled into proteins before any modifications take place, making it essential in the pathway of protein synthesis.