D2.2 Gene expression (HL)

Acetylation of lysine residues adds a negatively charged acetyl group, neutralising the positive charge on histone tails. Lysine is normally positively charged and attracted to the negatively charged DNA phosphate backbone.

When positive charges on histones are neutralised, the electrostatic attraction between histones and DNA is weakened. This causes the chromatin to adopt a looser, more open (euchromatin) conformation.

Open chromatin allows RNA polymerase and transcription factors to access the DNA template, thereby promoting transcription of genes in that region.

The outcome is increased chromatin accessibility and enhanced transcription — matching the option: "Electrostatic repulsion between histones and DNA increases, opening chromatin and promoting transcription."

The question tests the effect of histone acetylation on chromatin structure and gene expression.

Condensed chromatin is associated with methylation (in some contexts) or deacetylation, not acetylation. Acetylation consistently relaxes chromatin, so this option is incorrect.

Histone acetylation operates at the chromatin/transcriptional level, not the post-transcriptional level. The spliceosome acts on pre-mRNA after transcription and is not directly activated by histone acetylation.

DNA demethylation and histone acetylation are distinct epigenetic mechanisms. Adding acetyl groups to histones does not directly remove methyl groups from cytosine bases in DNA.

The remaining option correctly describes how acetylation neutralises histone positive charges, reducing DNA–histone attraction and opening chromatin for transcription.

The promoter is a DNA sequence located close to the transcription start site where RNA polymerase and general transcription factors assemble. It is essential for positioning and initiating RNA polymerase.

An enhancer is a regulatory DNA sequence that activator proteins bind to, increasing the rate of transcription. Critically, enhancers can be located thousands of base pairs upstream, downstream, or even within introns of the gene they regulate.

Enhancers work through DNA looping: activator proteins bound to the enhancer make physical contact with proteins at the promoter, stimulating RNA polymerase activity. They do not bind RNA polymerase directly.

The option stating that the promoter is the RNA polymerase binding site and the enhancer is a distant activating sequence correctly captures both definitions.

The question asks for the correct distinction between promoter and enhancer function in eukaryotic transcription.

This reverses the actual relationship. Promoters are adjacent to the transcription start site; enhancers can be far from the gene. This option is factually incorrect.

Promoters recruit RNA polymerase and general transcription factors, not repressors. Repressors typically bind to silencer sequences or operators (in prokaryotes). This option misidentifies promoter function.

Enhancers do not bind RNA polymerase directly — they bind activator proteins. Furthermore, enhancers are not restricted to upstream locations; they can be downstream or within introns. This option is doubly incorrect.

The correct option accurately describes the promoter as the RNA polymerase binding site and the enhancer as a distant activating element for transcription factor binding.