What are nucleosomes and why is chromatin structure important for gene expression?

• A. Nucleosomes are RNA molecules that regulate transcription.
• B. Nucleosomes are DNA wrapped around histone octamers; their positioning and modifications regulate access to DNA.
• C. Nucleosomes are lipid structures on the nuclear envelope.
• D. Nucleosomes are RNA-protein complexes involved in splicing.

Answer: (B)

Nucleosomes are the basic packaging unit of DNA in the cell, made of about 147 base pairs of DNA wrapped around a histone octamer (two each of H2A, H2B, H3, and H4). The arrangement of these nucleosomes along the genome, and the chemical marks on histones (and DNA), determine how accessible the DNA is to transcription machinery. When chromatin is in a relaxed, open form (euchromatin), transcription factors and RNA polymerase can bind and start gene expression. When chromatin is compacted (heterochromatin), access is limited and genes are less active or silenced. Enzymes that add or remove chemical marks (like acetylation or methylation) on histones, and chromatin remodelers that slide or reposition nucleosomes, fine-tune accessibility. For example, adding acetyl groups to histone tails reduces their interaction with DNA, making the DNA easier to read and increasing transcription; certain methylation patterns can either promote or repress transcription depending on the site and context. Nucleosome positioning also creates promoter regions that are free of nucleosomes, allowing transcription factors to bind. So the best description is that nucleosomes are DNA wrapped around histone octamers, and where they sit and how they’re modified controls how easily genes can be expressed. The other options describe RNA or lipids or non-nucleosome RNA-protein complexes, which aren’t nucleosomes.