Final answer:
Gene expression involves copying the gene's nucleic acid sequence into RNA, which is facilitated by chromatin state modifications that regulate the accessibility of genes for transcription.
Step-by-step explanation:
When a gene is expressed, C) its nucleic acid sequence is copied into RNA.
Gene expression involves the process of transcribing the nucleic acid sequence of a gene into RNA, predominantly messenger RNA (mRNA). During this process, the chromatin structure, composed of DNA and histone proteins, undergoes specific modifications that allow the transcription machinery access to the DNA sequence. If a gene is active, modifications such as histone acetylation render chromatin to a more open state (euchromatin), which facilitates transcription.
In contrast, genes that are not being expressed are found in denser chromatin areas (heterochromatin), preventing the transcription machinery from accessing the DNA sequence. This regulation is an example of epigenetic control over gene expression, as the underlying DNA sequence remains unchanged, but the accessibility of the sequence for transcription is altered due to chromatin remodeling.