OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Where in the cell are proteins synthesized?

1. In the nucleus

2. On the cell membrane

3. In the cytoplasm

4. In the mitochondria

The correct answer is: In the cytoplasm

Proteins are synthesized in the cytoplasm, specifically on ribosomes, which can be found either free-floating in the cytoplasm or attached to the endoplasmic reticulum, forming what is known as the rough endoplasmic reticulum. This process begins with the transcription of DNA into messenger RNA (mRNA) in the nucleus, but the actual assembly of amino acids into proteins occurs in the cytoplasm. Ribosomes then read the mRNA and translate the genetic instructions into polypeptide chains, ultimately folding into functional proteins. Although the nucleus is involved in the initial stages of protein synthesis by housing the DNA and producing mRNA, it does not perform the actual synthesis of proteins. The cell membrane does not contribute to protein synthesis either; instead, it serves primarily as a barrier and a site for communication between the cell and its environment. Mitochondria are known for their role in producing ATP through respiration, although they do contain some ribosomes and DNA, they are not the primary site for general protein synthesis in the cell.