A researcher isolated chromatin from mammalian cells and digested with micrococcal nuclease in different tubes for 1 h, 3h, 6h and 12h. thereafter, DNA was purified from all the digested chromatin samples, and two independent Southern blot hybridization experiments were performed with probe-I and probe II. The probe-I and probe-II correspond to different loci of the chromosome(s). The images below represent the Southern blot hybridization pattern generated by probe-I and probe-II.

Following statements were made to explain the results of the Southern blot experiments.

A. Size of probe-II is smaller than probe-I

B. Probe-I may correspond to the centromeric region of the chromosome.

C. Probe-I may correspond to a hypomethylated locus of the genome.

D. Probe-II may correspond to an euchromatic locus of the genome.

Which one of the following options represents the combination of all correct statements?

Correct option is B

Let's analyze the Southern blot hybridization results and determine the correct statements.

Understanding the Southern Blot Results

• Probe-I pattern: The band intensity remains similar across all time points (1h, 3h, 6h, 12h).
• Probe-II pattern: The bands decrease in intensity and size progressively over time, with almost no signal at 12h.

This suggests that:

• Probe-I hybridizes to a region resistant to nuclease digestion, implying a tightly packed, possibly heterochromatic or centromeric region.
• Probe-II hybridizes to a region that becomes increasingly digested over time, indicating it is located in euchromatin, which is more accessible to nucleases.

Evaluating the Given Statements

1. Statement A: "Size of probe-II is smaller than probe-I."

   • The size of a probe is independent of the digestion pattern. The observed bands reflect chromatin accessibility, not probe size.
   • Incorrect.

2. Statement B: "Probe-I may correspond to the centromeric region of the chromosome."

   • The centromeric region is heterochromatic, making it resistant to digestion. Since the bands of Probe-I remain unchanged, it supports this possibility.
   • Correct.

3. Statement C: "Probe-I may correspond to a hypomethylated locus of the genome."

   • Hypomethylated regions are typically more open (euchromatin) and should be digested more easily.
   • Since Probe-I remains undigested, this is Incorrect.

4. Statement D: "Probe-II may correspond to an euchromatic locus of the genome."

   • Euchromatin is loosely packed and accessible to nucleases, leading to progressive digestion, which matches the pattern of Probe-II.
   • Correct.

Correct Answer:

• Statements B and D are correct.
• Answer: Option 2 (B and D).