Where Does Protein Synthesis Take Place

As it is usually asked, the question “Where does protein synthesis take place?” is slightly incorrect! But if needed to answer it as is it is formulated – the protein synthesis takes place in the cytoplasm. However, you should note that there is small difference between prokaryotes and eukaryotes in terms of actual protein synthesis localization in the cell.

As you know the machinery responsible for the protein synthesis is the ribosome. While the proper functioning of the sub-units together with the constant supply of amino acid residues via the tRNAs, determine the protein chain elongation and finally the termination of the protein synthesis.

Where Does Protein Synthesis Take Place In Prokaryotes?

Besides the absence of nucleus in prokaryotes, the lower level of cell compartmentalization is the main difference compared to eukaryotes. In prokaryote cells the ribosomes are scattered in the cytoplasm, so it is correct to say that the protein synthesis in prokaryotes takes place in the cytoplasm.

protein synthesis takes place in cytoplasmWhere Does Protein Synthesis Take Place In Eukaryotes?

In eukaryotes the ribosomes also reside in the cytoplasm, but could be also found bound to the endoplasmic reticulum (ER). The membranes of the endoplasmic reticulum form a complex and continuous network in the inner core of the cytoplasm and is both to the outer membrane of the nucleus and with the plasma membrane. From cell to cell, the ER membranes are slightly different and their size and structure are  determined by the overall cell’s function. Cells which are involved in synthesis of large volumes of proteins, need a large ER network. There are two kinds of endoplasmic reticulum – smooth and rough. The outer (cytosolic) surface of the rough ER is covered with ribosomes, while the smooth ER lacks any ribosomes.

Does Protein Synthesis Take Place In Rough ER?

Let’s get back to the ribosomes. Both, cytoplasmic and rough ER bound ribosomes translate mRNA into polypeptide sequences. The location where a particular protein is synthesized is determined by the eventual post-translational modifications. Proteins that targets the ER often contain a specific sequence (called signal sequence). This signal sequence is synthesized at the very beginning of translation and once formed determines their movement towards rough ER. The ribosomes that carries such mRNAs and partially synthesized proteins holding a signal sequence, get stuck to the ER, where the complete protein synthesis of these proteins completes. Once the translation is finished, the protein is released in the ER canals and further follows the process of protein secretion.

Does Protein Synthesis Take Place In Mitochondria?

As you probably know, mitochondria also contain ribosomes, which are involved in the protein synthesis of the 13 genes encoded by the mitochondrial chromosome. Recent research shows that some nuclear encoded proteins are also synthesized on the surface of the mitochondria.

So Finally… Where Does Protein Synthesis Take Place?

As it seems, answering the question where does protein synthesis take place is not so trivial, because there are more and more new research evidence for new protein synthesis locations. As the current knowledge postulates – the protein synthesis takes place where the ribosomes are located, since this is the only machinery responsible for this process. This means that the protein synthesis takes place in the cytoplasm, on top of the rough endoplasmic reticulum, in the mitochondria and on the mitochondrial membrane. The protein synthesis definitely does not take place in the nucleus or nucleolus, because as the proteins are macro-molecules, it will be not possible for these protein to leave the nucleus passing through the nucleic envelope.

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