Histone modifications primarily affect which aspect of genetic expression?

Histone modifications play a crucial role in regulating gene expression by influencing gene accessibility. Histones are proteins that package and order DNA into structural units called nucleosomes, which subsequently form chromatin. When histones undergo various modifications—such as acetylation, methylation, and phosphorylation—these changes can alter the structure of chromatin, making it either more compact or more relaxed.

When chromatin is relaxed, it allows transcription factors and RNA polymerase access to the DNA, thereby facilitating gene transcription. In contrast, when histones are modified in a way that tightens the chromatin structure, the DNA becomes less accessible, which can inhibit transcription.

This mechanism is a vital part of epigenetic regulation, where modifications do not change the DNA sequence itself but affect how genes are expressed. Since other options pertain to processes that occur after transcription (such as RNA translation and protein synthesis) or to DNA processes unrelated to gene expression control (like DNA replication rate), they do not directly relate to the role of histone modifications in affecting genetic expression.