CDS (I) 2023 General Knowledge · Question 41 of 120

Correct — D, nuclei, cytoplasm, mitochondria, chloroplast and ribosomes. RNA is not confined to one compartment of a plant cell, because it is made in one place, works in another, and is a structural component of a third. It is present in the nucleus, where the nucleolus is, in NCERT's words, 'a site for active ribosomal RNA synthesis'. It is present in the cytoplasm, where messenger and transfer RNA do the work of translation. It is present in mitochondria, whose matrix 'possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins'. It is present in chloroplasts, which like mitochondria carry their own genetic machinery — NCERT records that ribosomes are found 'within the two organelles — chloroplasts (in plants) and mitochondria and on rough ER'. And it is present in ribosomes themselves, which 'are composed of ribonucleic acid (RNA) and proteins'. That last item is what makes this option the fullest of the four: it names the structure in which RNA is not merely present but is part of the fabric.

• (a)cytoplasm only — Far too narrow. It would leave out the nucleolus, where ribosomal RNA is made, and the semi-autonomous organelles that carry their own RNA and ribosomes.
• (b)nuclei and cytoplasm only — The intuitive answer, and still incomplete. Mitochondria and chloroplasts have their own genomes and their own 70S ribosomes, so RNA is present inside them independently of the nucleus.
• (c)nuclei, cytoplasm, mitochondria, chloroplast and endoplasmic reticulum — The closest of the three, and the one worth arguing with. RNA is indeed found at the rough endoplasmic reticulum, because ribosomes stud its outer surface — but the RNA there belongs to those ribosomes, and the list that names the ribosomes themselves is the more exact description of where RNA is.

RNA occurs in a cell in several forms. Ribosomal RNA is a structural part of ribosomes and is made in the nucleolus. Messenger RNA is transcribed from DNA in the nucleus and carries the sequence out to the cytoplasm. Transfer RNA brings amino acids to the ribosome during translation. Mitochondria and chloroplasts, which are thought to descend from free-living prokaryotes, keep their own circular DNA and their own 70S ribosomes, and therefore their own RNA, inside the plant cell.

The examiner is testing completeness rather than knowledge of any single fact, so the method is to walk through the compartments and ask whether RNA has a reason to be in each. Nucleus, yes — that is where it is transcribed. Cytoplasm, yes — that is where translation happens. Mitochondria and chloroplasts, yes — each runs its own protein synthesis. Ribosomes, yes — RNA is what they are partly made of. One honest note on option (c), which names the endoplasmic reticulum: RNA is present on rough ER, in the ribosomes attached to it, so the option is not false in the way a distractor usually is. The keyed answer is the better description because it names the ribosome, which is the RNA-bearing structure itself rather than a membrane those structures happen to sit on. If your reading pulled you towards (c), that instinct was sound; the key rewards the more precise list.

• The nucleolus is a site of active ribosomal RNA synthesis and is not membrane-bound.
• Ribosomes are composed of ribonucleic acid and proteins, and are found in the cytoplasm, in chloroplasts and mitochondria, and on the rough endoplasmic reticulum.
• The mitochondrial matrix contains a single circular DNA molecule, a few RNA molecules and 70S ribosomes.
• Chloroplasts and mitochondria are semi-autonomous organelles with their own genetic machinery, which is the basis of the endosymbiotic account of their origin.
• Eukaryotic cytoplasmic ribosomes are 80S while those inside mitochondria and chloroplasts are 70S, the prokaryotic size.

• Restricting RNA to the nucleus and cytoplasm and forgetting the semi-autonomous organelles.
• Treating the endoplasmic reticulum as an RNA-containing organelle in its own right; the RNA there sits in the attached ribosomes.
• Assuming a plant cell differs from an animal cell only by the wall and the chloroplast; the vacuole and the plastid family matter too.

As a where-is-it-found item on a cell component, or as an organelle-to-function match involving ribosomes, mitochondria and chloroplasts.

• practice — not a real PYQ

  The nucleolus within the nucleus is the site of synthesis of which one of the following?

  • (a)Transfer RNA
  • (b)Ribosomal RNA
  • (c)Deoxyribonucleic acid
  • (d)Adenosine triphosphate

  Answer(b) Ribosomal RNA — the nucleolus is a non-membrane-bound body in the nucleoplasm, and cells busy with protein synthesis have larger and more numerous nucleoli.

• practice — not a real PYQ

  Ribosomes found inside the mitochondria and chloroplasts of a plant cell are of which size?

  • (a)70S
  • (b)80S
  • (c)50S
  • (d)30S

  Answer(a) 70S — the prokaryotic size, one of the observations behind the endosymbiotic account of these organelles; cytoplasmic ribosomes of the eukaryotic cell are 80S.