Protein synthesis occurs primarily at two key sites within the cell: the ribosomes and the endoplasmic reticulum.
Ribosomes are the main sites where proteins are produced. They can be found either floating freely in the cytoplasm or attached to the endoplasmic reticulum, forming what is known as rough ER. Ribosomes link amino acids together in the sequence dictated by messenger RNA (mRNA), which is transcribed from DNA. This process is called translation, and it is where the genetic code is converted into a functional protein.
The endoplasmic reticulum also plays a significant role in protein synthesis and processing. The rough ER is specifically involved in the synthesis of proteins that are either secreted from the cell, incorporated into the cell’s plasma membrane, or sent to an organelle. Once the polypeptide chains are formed on the ribosomes, they enter the lumen of the rough ER, where they undergo folding and post-translational modifications.
In summary, the primary sites of protein synthesis are the ribosomes and the rough endoplasmic reticulum, with ribosomes directly synthesizing proteins and the rough ER assisting in their maturation and transport.