Chromatin Remodelling Explained

Chromatin remodelling is a dynamic process that involves changes in the structure of chromatin, which is the complex of DNA and proteins (primarily histones) that makes up chromosomes. These changes are crucial for regulating access to the DNA, thereby controlling gene expression, DNA replication, repair, and recombination. The question asks about statements related to this process in human cells.

Analyzing Statements on Chromatin Remodelling

Let's evaluate each statement:

• Statement A: Local chromatin conformation may play more important role than the local DNA sequence of the promoter.
  This statement is generally considered true. While the DNA sequence of a promoter region provides binding sites for transcription factors, the local chromatin conformation (e.g., whether the DNA is tightly packed into nucleosomes or is more open and accessible) can significantly impact whether these factors can bind and initiate transcription. An open chromatin structure is often permissive for gene expression, while a closed structure is repressive, regardless of the underlying DNA sequence signals.
• Statement B: Histones in nucleosome can undergo many different covalent modifications, which in turn, alter the chromatin architecture locally.
  This statement is true. Histone modifications, such as acetylation, methylation, phosphorylation, and ubiquitination, occur on the tails of histone proteins or within the core. These modifications can alter the charge and structure of histones, affecting their interaction with DNA and other proteins. This leads to changes in the local chromatin architecture, influencing processes like transcription. This is part of the "histone code" hypothesis.
• Statement C: Chromatin remodelling is a develop- mentally regulated passive process which does not require ATP.
  This statement is false. Chromatin remodelling is indeed developmentally regulated and plays a critical role during development. However, it is primarily an active process, largely driven by ATP-dependent chromatin remodelling complexes. These complexes use the energy from ATP hydrolysis to slide, eject, or restructure nucleosomes, thereby altering DNA accessibility.
• Statement D: Several histone variants exist, which replace the standard histones in specific types of chromatin.
  This statement is true. In addition to the core histones (H2A, H2B, H3, H4) and linker histone H1, there are various histone variants (e.g., H2A.Z, H3.3, CENP-A). These variants can be incorporated into nucleosomes in a non-replication-dependent manner and confer unique structural and functional properties to the chromatin regions where they are found. For example, H2A.Z is often found at regulatory regions, while CENP-A is specific to centromeres.

Identifying the Correct Combination

Based on the analysis, statements A, B, and D are correct, while statement C is incorrect because chromatin remodelling is an active, ATP-dependent process, not a passive one.

Therefore, the correct combination of statements is A, B, and D.