While culturing a eukaryotic cell line, you add radio-labelled methionine in the
culture media. After addition, you are able to cleanly separate individual
organelles at different times. In which organelle would you detect radioactivity
first?
Rough Endoplasmic Reticulum
Lysosome
Mitochondria
Nucleus
Radioactivity from newly added radio‑labelled methionine would be detected first in the rough endoplasmic reticulum (RER).
Concept and correct option
When radiolabelled methionine is added to the culture medium, it is taken up by cells and incorporated into newly synthesized polypeptide chains during translation. In a typical secretory or membrane protein pathway, these nascent polypeptides are synthesized on ribosomes bound to the rough endoplasmic reticulum, so the earliest detectable radioactivity appears in the RER.
Therefore, among the options:
-
Rough Endoplasmic Reticulum
-
Lysosome
-
Mitochondria
-
Nucleus
the rough endoplasmic reticulum is the first organelle where radioactivity is detected.
Why rough endoplasmic reticulum is first
The rough ER is studded with ribosomes that synthesize secretory proteins, lysosomal proteins and many membrane proteins. In classic pulse‑chase experiments with radioactive amino acids, newly synthesized proteins are first visualized in the rough ER before moving to the Golgi apparatus and then to secretory vesicles or other destinations.
Because methionine is one of the standard amino acids incorporated into most polypeptides, the first site where a significant pool of newly synthesized, radiolabelled proteins accumulates is the rough ER lumen and its membrane‑associated ribosomes. This is why RER is the correct answer in questions about the first organelle to show radioactivity after adding radiolabelled amino acids to a eukaryotic cell line.
Explanation of each option
Rough endoplasmic reticulum
-
The rough ER is continuous with the nuclear envelope and contains ribosomes on its cytosolic surface, making it the primary site of co‑translational insertion of secretory and many membrane proteins.
-
Pulse‑chase autoradiography studies show that right after exposure to radiolabelled amino acids, the label appears in proteins localized to the rough ER, confirming it as the first organelle in the secretory pathway to show radioactivity.
Lysosome
-
Lysosomes receive many of their proteins (including acid hydrolases) via vesicular trafficking from the Golgi apparatus, which itself receives newly synthesized proteins from the rough ER.
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Because lysosomal proteins must first be synthesized on RER‑bound ribosomes, then processed in the Golgi, lysosomes show radioactivity only after a delay and therefore cannot be the first organelle to exhibit the radioactive label.
Mitochondria
-
Mitochondria contain their own ribosomes and mitochondrial DNA and can synthesize a small subset of their proteins internally, but the majority of mitochondrial proteins are made on free cytosolic ribosomes and post‑translationally imported.
-
Methionine added to the medium will also label mitochondrial proteins, but this occurs through cytosolic translation and later import; mitochondria are not the primary co‑translational destination for the bulk of labeled proteins in a typical secretory pathway experiment, so they do not show the earliest organelle‑specific radioactivity peak.
Nucleus
-
Most nuclear proteins (such as transcription factors and histones) are synthesized on free ribosomes in the cytosol and then imported into the nucleus through nuclear pores.
-
Although radiolabelled methionine will be incorporated into nuclear proteins, the nucleus is not the first discrete organelle to accumulate newly synthesized secretory‑pathway proteins; the earliest organelle‑localized radioactivity in such experimental setups is detected in the rough ER, not the nucleus.
Introduction for SEO (exam‑oriented)
Understanding which organelle first shows radioactivity after adding radiolabelled methionine is a classic concept in cell biology and a frequent multiple‑choice question in competitive exams like CSIR NET Life Sciences and GATE. This concept integrates knowledge of the endomembrane system, protein synthesis, and pulse‑chase autoradiography to test whether students can track the pathway of newly synthesized proteins through the cell.
