Molecular biologists are investigating the functional significance of non-coding RNA molecules. During an experiment, they isolate large complexes containing both protein and RNA from exocrine pancreatic cells. The complexes are found both freely floating in the cytoplasm and bound to the endoplasmic reticulum. The RNA found within these complexes is primarily synthesized at which of the following intracellular sites?
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The large complexes containing both protein and RNA that are found freely floating and bound to the endoplasmic reticulum are most likely ribosomes. Ribosome biogenesis occurs primarily within the nucleolus, a dense round structure inside the nucleus that is in direct contact with the rest of the nucleoplasm. The nucleolus contains ribosomal DNA coding for the 28S, 5.8S, and 18S ribosomal RNA (rRNA) components, and it is here where most of the rRNA is transcribed. After synthesis, rRNA is combined with ribosomal protein components that are translated in the cytoplasm and then imported into the nucleus. The immature 60S and 40S ribosomal subunits are then exported from the nucleus to fully mature in the cytoplasm.
(Choice A) The rough endoplasmic reticulum is identifiable due to its characteristic long, folded membranes that are coated with ribosomes, giving it a speckled or rough appearance.
(Choice C) The nucleus is identifiable as a membrane-bound structure that contains the nucleolus, electron-lucent euchromatin, and electron-dense heterochromatin (condensed DNA typically found around the periphery). However, most of the ribosomal DNA is found in the form of tandem repeats within the nucleolus; only the 5S rRNA is transcribed outside of the nucleolus.
(Choice D) Mitochondria are organelles with dual phospholipid bilayer membranes that can be recognized by the presence of foldings (ie, cristae) in their inner membranes. Mitochondria have their own ribosomes, but they are typically found within the mitochondrial matrix or bound to the inner mitochondrial membrane.
(Choice E) Pancreatic zymogen granules are electron-dense structures containing digestive proenzymes that are released from the cell via exocytosis.
Educational objective:
The nucleolus is a dense intranuclear body visible by light and electron microscopy that functions as the primary site of ribosome synthesis and assembly. All ribosomal RNA except 5S rRNA is transcribed in the nucleolus.