Where does actual protein synthesis occur within a cell?

Actual protein synthesis occurs in the ribosome, which is the site where messenger RNA (mRNA) is translated into a polypeptide chain, ultimately forming proteins. Ribosomes can be found floating freely in the cytoplasm or attached to the endoplasmic reticulum, enabling them to synthesize proteins that either function within the cytosol or are secreted from the cell or incorporated into cellular membranes.

The process begins when mRNA, produced during transcription in the nucleus, is transported to the cytoplasm, where ribosomes read the sequence of codons on the mRNA. Transfer RNA (tRNA) molecules bring the corresponding amino acids to the ribosome, where they are linked together in the order specified by the mRNA code. This translation process is crucial for producing the proteins necessary for cellular structure, function, and regulation.

While other cellular components, like the mitochondria and endoplasmic reticulum, play essential roles in energy production and post-translational modifications of proteins, respectively, the actual synthesis of proteins—the assembly of amino acids into polypeptides—occurs specifically in the ribosomes. The nucleus is primarily involved in the transcription of DNA into mRNA, not in protein synthesis itself.