(A) P, Q

(B) Q, S

(C) R, S

(D) P, S

Correct Answer

✅ Option (D): P and S

Detailed Explanation

Plastid genomes (chloroplast genomes) share many features with prokaryotic genomes due to their endosymbiotic origin.

Statement P

“Plastid genome is circular in nature with genome size of 120–160 kb.”
✅ Correct

• Plastid genomes are circular DNA molecules.

• Genome size typically ranges between 120–160 kilobases (kb) in most higher plants.

✔ Statement P is true.

Statement Q

“The plastid ribosomes are with sedimentation coefficient of 80S.”
❌ Incorrect

• Plastid ribosomes are 70S, similar to bacterial ribosomes.

• 80S ribosomes are found in the eukaryotic cytosol, not plastids.

✘ Statement Q is false.

Statement R

“The gene for the small subunit of RuBisCO is located in the plastid.”
❌ Incorrect

• RuBisCO has two subunits:

  • Large subunit (rbcL) → encoded by plastid genome

  • Small subunit (rbcS) → encoded by the nuclear genome

• Hence, this statement is incorrect.

✘ Statement R is false.

Statement S

“rRNAs in the plastid genome are arranged in one transcription unit.”
✅ Correct

• Plastid rRNA genes (16S, 23S, 4.5S, 5S) are organized in a single operon-like transcription unit.

• This arrangement is similar to prokaryotic rRNA operons.

✔ Statement S is true.

Why Option (D) Is Correct

• P is correct (circular genome and size)

• S is correct (rRNA operon arrangement)

• Other options include at least one incorrect statement

✔ Therefore, the correct answer is Option (D): P and S.

Quick Exam Memory Tips

• Plastid genome → Circular, 120–160 kb

• Plastid ribosome → 70S

• RuBisCO small subunit → Nuclear gene

• Plastid rRNA → Single transcription unit

Exam Relevance

🔹 Frequently asked in CSIR-NET Life Sciences
🔹 Important for cell biology & plant molecular biology
🔹 High-yield true/false MCQs