Describe how epigenetic modifications influence gene expression without changing the DNA sequence.

• A. DNA methylation and histone modifications alter chromatin structure, affecting promoter accessibility and transcription; these marks can be heritable through cell division.
• B. DNA sequence changes are required to alter gene expression.
• C. Epigenetic marks always reset after each generation.
• D. Epigenetic modifications only occur during embryogenesis and then stop.

Answer: (A)

Gene expression can be turned up or down without changing the DNA sequence by marks that modify how the genome is packaged and read. DNA methylation at CpG sites tends to repress transcription by attracting proteins that compact chromatin or block transcription factor binding. Histone modifications, like acetylation or methylation of histone tails, change chromatin structure to make promoters more or less accessible to the transcriptional machinery. Because these marks are maintained during DNA replication, daughter cells inherit the same expression state through cell divisions, giving a stable, heritable pattern at the cellular level. Environmental signals and developmental cues can influence these marks, altering gene activity without changing the underlying sequence. The other statements don’t fit because gene expression can be altered without sequence changes, epigenetic marks aren’t universally reset every generation, and they are not limited to embryogenesis.