Question

Arrange the following organisms in ascending order of their genomic complexities
(A). Viroid
(B). Virus
(C). Phytoplasma
(D). Bacteria

• A. (A), (B), (C), (D)
• B. (A), (D), (C), (B)
• C. (B), (A), (D), (C)
• D. (C), (B), (D), (A)

Hint:

An easy way to remember: Viroid (no proteins) $<$ Virus (proteins + capsid) $<$ Phytoplasma (cellular, no wall) $<$ Bacteria (cellular + wall).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Genomic complexity refers to the structural organization, genetic composition, and physical size of an organism's genome.
Pathogens range from simple sub-viral infectious agents to fully independent cellular organisms.

Step 2: Detailed Explanation:
Let us compare the genomic complexity of the four listed pathogens in ascending order:
1. Viroid (A): Viroids are the simplest known infectious agents.
They consist solely of a short strand of circular, single-stranded, non-coding RNA (typically 250 to 400 nucleotides) without any protein coat (capsid).
They have the absolute lowest genomic complexity.
2. Virus (B): Viruses are acellular genetic packages consisting of a DNA or RNA genome (typically 3,000 to over 20,000 nucleotides) enclosed in a protective protein coat (capsid).
Their genomes code for essential structural and replication proteins.
3. Phytoplasma (C): Phytoplasmas are specialized, wall-less, non-culturable bacteria belonging to the class Mollicutes.
They are cellular organisms with a fully functional triple-layered cell membrane and a small circular double-stranded DNA genome (typically 530 to 1,350 kilobase pairs) encoding several hundred genes.
4. Bacteria (D): Walled plant-pathogenic bacteria (such as Pseudomonas or Xanthomonas) are complex, free-living, single-celled prokaryotic organisms.
They possess a large circular chromosome (usually 2,000 to over 5,000 kilobase pairs) along with plasmids, encoding thousands of genes for complex metabolic, virulence, and regulatory networks.
Therefore, the ascending order of complexity is: Viroid (A) \(\rightarrow\) Virus (B) \(\rightarrow\) Phytoplasma (C) \(\rightarrow\) Bacteria (D).

Step 3: Final Answer:
The correct option is (A), (B), (C), (D).