Answer and Explanation:
In protein synthesis, the first step is to synthesize messenger RNA, mRNA. The coping process of the DNA section for the desired protein is called transcription, and it happens in the nucleus. After that, it occurs translation, when the formed mRNA moves to the cytoplasm through the nucleus membrane pores.
Protein synthesis is initiated in the cytoplasm when mRNA meets a free ribosome, the primary structure for protein synthesis. Ribosomes are made of protein and ribosomal RNA and can be found in the rough endoplasmic reticulum or floating in the cytosol. Free ribosomes are not attached to any cytoplasmic structure or organelle. They synthesize proteins only for internal cell use. Other ribosomes are attached to the membrane of the endoplasmic reticulum and they are in charge of synthesizing membrane proteins or exportation proteins. Free and attached ribosomes are identical and they can alternate their location. This means that although free ribosomes are floating in the cytosol, eventually, they can get attached to the endoplasmic reticulum membrane.
Synthesis of proteins might start in the cytoplasm with the production of a molecule portion known as a signal sequence. During translation, mRNAs are read in the 5´ to 3´ direction, and tRNA transfer the correct amino acids to build the polypeptide chain. The protein is then synthesized from the amino terminus to the carboxy terminus. Each amino acid is specified by a codon formed by three bases in the mRNA. mARNs have a start and end codon that tells the ribosome where to start and stop adding amino acids. When the ribosome reaches the end codon, it means that protein synthesis is finished.
If the protein is a membrane or exportation protein, the synthesizing protein and its associated ribosome are lead from the cytosol to a specific region in the Rough endoplasmic reticulum where it continues the protein building, and then the protein is translocated to the lumen. Once there, the protein suffers a few modifications, one of them is folding to become functional. Finally, protein is transported by vesicles to the Golgi complex, and from there to its final destiny. In the Golgi complex, it also happens the protein folding and the initial stages of glycosylation.
