Final answer:
Compounds added to histone lysine tails change chromatin structure, affecting transcription by altering nucleosome positioning and accessibility of DNA for transcription machinery, through reversible modifications like acetylation and methylation.
Step-by-step explanation:
Compounds added to histone lysine tails affect transcription primarily by changing the structure of chromatin, which is a complex of DNA and proteins, including histones, in the cell nucleus. Histone acetylation, for example, can result in the loose packing of nucleosomes, making DNA more accessible for transcription. When acetyl groups are added to histone tails, they neutralize the positive charge of the lysines, reducing the affinity of histones for the negatively charged DNA. This leads to a more open chromatin structure and increases transcription. In contrast, de-acetylation reverses this effect, leading to tighter DNA winding and reduced transcription. Histone methylation can have varying effects depending on which amino acids are modified; for instance, methylation of certain lysines on histone H3 can either activate or suppress transcription.
These modifications do not alter the DNA sequence but can influence the accessibility of DNA for transcription by altering the positioning of nucleosomes, making specific genes more or less available to transcription machinery. Therefore, these reversible chemical changes to histone proteins play a crucial role in regulating gene expression by affecting nucleosome spacing and gene accessibility in response to various signals.