What is the primary function of the rough endoplasmic reticulum in eukaryotic cells?

The rough endoplasmic reticulum (RER) plays a vital role in the production and processing of proteins within eukaryotic cells. It is called "rough" due to the presence of ribosomes on its cytoplasmic surface, which are the sites of protein synthesis. The ribosomes translate mRNA into polypeptide chains that are then inserted into the lumen of the RER.

Once inside, these polypeptides undergo crucial modifications, such as folding, glycosylation (the addition of sugar groups), and assembly into functional proteins. This processing is essential for the proteins to attain their proper structure and function before being transported to their final destinations, which could include secretion outside the cell or delivery to various organelles.

The other options do not align with the primary function of the rough endoplasmic reticulum. While storage of DNA occurs in the nucleus, lipid synthesis takes place primarily in the smooth endoplasmic reticulum, and cell division is managed by structures such as the mitotic spindle and centrosomes, not the endoplasmic reticulum. Thus, the emphasis on protein synthesis and processing accurately reflects the essential role of the rough endoplasmic reticulum in cellular function.