Final answer:
The formation of higher-order structure in heterochromatin is enhanced by the addition of reversible changes to histone proteins and DNA, such as histone acetylation and histone methylation, which regulate the compactness of chromatin and hence gene transcription.
Step-by-step explanation:
The molecular processes that will enhance the formation of higher-order structure in heterochromatin include the addition of reversible changes to histone proteins and DNA. Specifically, these processes involve the chemical modification of chromatin proteins and DNA such as histone acetylation, de-acetylation, histone methylation, and phosphorylation.
The addition of more nucleosomes to DNA, as well as the removal of nucleosomes, can also affect the chromatin conformation. In particular, histone acetylation will lead to a looser packing of nucleosomes, thus usually associated with euchromatin, whereas de-acetylation promotes the condensation of the chromatin into heterochromatin. Methylation of histones can either open up or close the DNA to transcription depending on the particular methylation pattern.
It is important to note that these modifications are reversible and do not change the DNA sequence itself. They are part of epigenetic changes that regulate the accessibility of DNA to the transcriptional machinery, therefore controlling gene expression. The formation of heterochromatin, a tightly packed form of DNA, limits the accessibility for transcription, maintaining gene silencing.