Question

The correct set of bands that the researcher can use for the pathogen identification is:

• A. Band I and III
• B. Band II and III
• C. Band I and II
• D. Band III and IV

Answer 1

Answer: (A)

Band I and III

Answer 2

The experiment involves separating ribosomal subunits from a mixture of plant and bacterial cells using density gradient ultracentrifugation. In this technique, heavier particles sediment further down the tube, while lighter particles remain higher up. The ribosomal subunits are dissociated before centrifugation.

Here's a comparison of ribosomal subunits in prokaryotes (bacteria) and eukaryotes (plants):

• Prokaryotic Ribosomes (Bacteria): Are 70S ribosomes, composed of a 30S small subunit and a 50S large subunit.
• Eukaryotic Ribosomes (Plant Cytoplasm): Are 80S ribosomes, composed of a 40S small subunit and a 60S large subunit.
• Eukaryotic Organellar Ribosomes (Plant Mitochondria and Chloroplasts): These organelles contain ribosomes that are similar to prokaryotic ribosomes (70S type), consisting of ~30S small subunits and ~50S large subunits.

When subjected to density gradient ultracentrifugation, the subunits will separate based on their sedimentation coefficients (S values), with higher S values settling lower in the tube.

Ordering the subunits from lightest (top of the tube) to heaviest (bottom of the tube):

1. 30S subunits: These are the small subunits from bacteria and from plant organelles (mitochondria and chloroplasts). These will be the lightest.
2. 40S subunits: These are the small subunits from the plant cell's cytoplasm.
3. 50S subunits: These are the large subunits from bacteria and from plant organelles.
4. 60S subunits: These are the large subunits from the plant cell's cytoplasm.

Mapping these to the bands shown in the figure (from top I to bottom IV):

• Band I: Corresponds to the 30S subunits (from bacteria and plant organelles).
• Band II: Corresponds to the 40S subunits (from plant cytoplasm).
• Band III: Corresponds to the 50S subunits (from bacteria and plant organelles).
• Band IV: Corresponds to the 60S subunits (from plant cytoplasm).

The question asks for the bands that the researcher can use for bacterial pathogen identification. Bacterial ribosomes are 70S, which dissociate into 30S and 50S subunits. Therefore, the bands containing bacterial ribosomal subunits are Band I (30S) and Band III (50S). Even though these bands also contain organellar ribosomal subunits from the plant, the subsequent rRNA sequence analysis will distinguish the bacterial sequences from the organellar ones, as stated in the problem description ("Ribosomal RNA (rRNA) sequence analysis can be used for the identification of bacteria").

Thus, the correct set of bands for pathogen identification is Band I and Band III.