Final answer:
Genomic imprinting and X-chromosome inactivation are examples of cis-epigenetic mechanisms. These processes lead to changes in nucleosome packaging and gene expression by altering chromatin structure. These epigenetic modifications affect gene accessibility and can occur at multiple levels of gene expression regulation.
Step-by-step explanation:
Genomic imprinting and X-chromosome inactivation are examples of cis-epigenetic mechanisms that occur during development. In females, one of the two X chromosomes is inactivated through a process called X-chromosome inactivation, which is an epigenetic regulation mechanism ensuring that females, like males, have one functional copy of the X chromosome in each body cell. This inactivation is achieved through epigenetic changes, leading to alterations in nucleosome packaging and consequently gene expression.
Epigenetic modifications can change the configuration of chromatin from a tightly packed structure to a more relaxed one, which affects gene accessibility for transcriptional machinery. For instance, when the X chromosome is inactivated, it is highly condensed, forming a Barr body, and most of its genes are not expressed. Conversely, the active X chromosome has a looser structure, allowing genes to be transcribed. This type of regulation can occur at the epigenetic, transcriptional, and translational levels.