Final answer:
Heterochromatin spreads along a chromosome until it encounters a barrier DNA sequence. This process, along with chromatin remodeling, is regulated by epigenetic tags that signal whether a region should be transcriptionally active or silenced.
Step-by-step explanation:
Heterochromatin can spread along a chromosome until it encounters a barrier DNA sequence. This is because the spread of heterochromatin, which is a tightly packed form of DNA, is regulated through various mechanisms including chemical modifications (or epigenetic tags) that can affect both histone proteins and the DNA molecule itself. These modifications, such as methylation or acetylation, can change the tightness of DNA coiling around the histones, signaling whether a chromosomal region should be open for transcription or closed off.
In regions where genes are to be expressed, chromatin remodeling occurs to open the chromosomal structure, allowing transcription factors and RNA polymerase to access the DNA. Conversely, when a gene is to remain silenced, chemical modifications signal for a closed chromosomal configuration, preventing transcription. The presence of specific sequences or structures in DNA, such as barrier DNA sequences or tight histone modifications, ensure that heterochromatin does not spread indefinitely and affects only targeted regions of the chromosome.