A region of a mouse chromosome was subjected to micrococcal nuclease hypersensitivity analysis over stages of development. In early stages, the region had regularly spaced nucleosomes. In later stages, the nucleosomes were irregularly spaced with several nucleosome-free regions detected.

Based on the above observations, which one of the following is the best possible inference?

Correct option is A

EXPLANATION-

Option (a) - The observations of a developmental stage-dependent change in nucleosome arrangement (from compact to open) match the definition of facultative heterochromatin, which undergoes transitions between active and inactive states.

Incorrect options- 
Option (b) - Regularly spaced nucleosomes in early stages mean the DNA is less accessible to transcription factors and RNA polymerase. This arrangement correlates with transcriptional repression, not activation.
So, early stages are more likely to be transcriptionally silent, not active.

Option (c)- The irregular spacing and nucleosome-free regions do not imply a failure in nucleosome assembly. Rather, they suggest active chromatin remodeling, often associated with gene expression (e.g., by chromatin remodeling complexes or transcription factors that displace nucleosomes).
Nucleosomes are still present, just dynamically rearranged.

Option (d) - Micrococcal nuclease hypersensitivity assays are specifically used to infer chromatin accessibility, which strongly correlates with gene expression. Open chromatin (nucleosome-free regions) = more likely active transcription.
So, nucleosome positioning does provide insights into expression states.

So, the correct option is (a).