What is the defining characteristic of rough endoplasmic reticulum (rough ER)?

The defining characteristic of rough endoplasmic reticulum (rough ER) is the presence of ribosomes attached to its cytoplasmic surface. These ribosomes give the rough ER its "rough" appearance when observed under a microscope, as they are small, dense structures where protein synthesis occurs. The primary function of rough ER is to synthesize proteins that are either secreted from the cell, incorporated into the cell’s membrane, or sent to an organelle. The ribosomes on the rough ER translate mRNA into polypeptide chains, which then enter the ER lumen for processing and folding.

The other options highlight aspects that do not accurately characterize rough ER. For instance, rough ER does contain enzymes necessary for post-translational modifications of proteins, rather than being defined by their absence. While rough ER may be involved in the secretion of lysosomal enzymes, this is a more specific function rather than a defining characteristic. Lastly, the storage of DNA is associated with the nucleus, not the rough ER, as the ER does not function in storing genetic material. Understanding the roles and functions of cellular structures like the rough ER is crucial for grasping the overall function of cellular biology.