DNA Biology Practice Test

Chromatin architecture controls access to DNA for transcription. The way DNA is packaged—how nucleosomes are positioned and which histone marks decorate their tails—determines whether transcriptional machinery can reach promoter and enhancer regions. When nucleosomes cover a promoter, or when repressive histone marks are present, the DNA is less accessible and transcription is repressed. In contrast, remodeling that shifts or removes nucleosomes near a gene, along with histone acetylation and other activating marks, loosens the chromatin and makes the DNA accessible to RNA polymerase II and transcription factors, enabling transcription. Euchromatin is the open, accessible form associated with active transcription, while heterochromatin is the condensed, less accessible form associated with repression. DNA methylation can further reinforce repression by promoting a closed chromatin state. This combination of nucleosome positioning and histone modifications, rather than DNA sequence alone, sets how readily a gene is transcribed.