Where in the cell are proteins made?

• A. Ribosome
• B. Nucleus
• C. Golgi apparatus
• D. Mitochondrion

Answer: (A)

Proteins are made by ribosomes, the molecular machines that translate mRNA into polypeptide chains. They read the genetic code carried by mRNA and link together amino acids in the correct order to form a protein. This can happen with ribosomes floating freely in the cytosol or with ribosomes attached to the rough endoplasmic reticulum, the latter often producing proteins destined for secretion or for membranes. The nucleus isn’t the site of protein assembly; it’s where DNA is transcribed into RNA. The Golgi apparatus modifies, sorts, and ships proteins after they’re made. Mitochondria have their own small ribosomes and perform limited protein synthesis for mitochondrial proteins, but the main production of cellular proteins occurs on ribosomes.