27. In eukaryotic chromatin, 30 nm fiber (solenoid) can open up to give rise to two kinds of chromatin. In one type (A), the promoter of a gene within the open chromatin is occupied by a nucleosome whereas in the other (B), the promoter is occupied by histone H1. The following possibilities are suggested.
A) The gene in (A) is repressed.
B) The gene in (B) is repressed.
C) The gene in (A) is active.
D) The gene in (B) is active.
Which of the following sets is correct?
(1) A and D       (2) B and C
(3) B and D      (4) C and D

Introduction

Chromatin structure plays a pivotal role in regulating gene expression. The 30 nm chromatin fiber can adopt different configurations, influencing whether genes are active or repressed. Two key players at gene promoters are core nucleosomes and the linker histone H1. Their presence or absence determines chromatin accessibility and transcriptional outcomes.

Nucleosomes at Promoters and Gene Activity

Nucleosomes package DNA into chromatin but do not inherently block transcription if the chromatin is in an open state. Promoters occupied by nucleosomes in open chromatin regions can support active transcription, especially when nucleosomes are remodeled or modified to allow transcription factor binding.

Histone H1 and Gene Repression

Histone H1 binds linker DNA between nucleosomes, promoting chromatin compaction into higher-order structures. Its presence at promoters is associated with transcriptional repression by limiting access to transcriptional machinery.

Chromatin States and Gene Expression

• Open chromatin with nucleosome occupancy (Type A): Generally transcriptionally active.

• Chromatin with H1 occupancy at promoter (Type B): Typically transcriptionally repressed.

Conclusion

The presence of histone H1 at promoters correlates with gene repression, while promoters occupied only by nucleosomes in an open chromatin context tend to be transcriptionally active. This balance is crucial for dynamic gene regulation.

Answer:
(2) B and C

This explanation integrates chromatin biology concepts and recent research to clarify how nucleosome and H1 occupancy at promoters regulate gene expression.