Proteins are synthesized inside the cell primarily on the ribosomes. Ribosomes, which are made up of ribosomal RNA and proteins, serve as the sites for protein synthesis. They can be found free-floating in the cytoplasm or attached to the rough endoplasmic reticulum (RER). In eukaryotic cells, the process begins with transcription of DNA to mRNA in the nucleus. The mRNA then travels to the cytoplasm, where ribosomes translate the genetic code into a polypeptide chain, using amino acids brought by tRNA. Ribosomes attached to the rough ER mainly synthesize proteins destined for secretion or for use in membranes, while free ribosomes mostly make proteins for use within the cell. Therefore, the main locations for protein synthesis inside cells are:
- Ribosomes free in the cytoplasm
- Ribosomes attached to the rough endoplasmic reticulum
Additionally, mitochondria and plastids have their own ribosomes and can synthesize some proteins independently. Protein modification takes place later in the Golgi apparatus.
