Provide a definition of chromatin remodeling, and give two exam- ples of this phenomenon.

Gene expression regulation is a crucial process in the life of a cell, governing the production of proteins and ultimately orchestrating cellular functions. It encompasses a diverse set of mechanisms through which a cell controls the amount and timing of specific gene products (RNA and proteins) it manufactures. This regulation can occur at multiple levels within the cell: from transcriptional control, where the DNA is transcribed into RNA, to translational control, where RNA is translated into proteins.

One of the most fundamental ways to regulate gene expression involves chromatin remodeling. This adjustment of chromatin structure either exposes or hides sections of DNA from the transcription machinery, enabling or disabling the transcription of genes. For instance, a tightly packed chromatin, also known as heterochromatin, commonly leads to gene silencing. Conversely, a more loosened structure, or euchromatin, is associated with active gene transcription. Key players in this process include ATP-dependent chromatin remodelers and histone modification enzymes, which we'll discuss in greater detail in following sections.

ATP-dependent chromatin remodeling refers to a dynamic mechanism by which chromatin remodelers utilize the energy from ATP hydrolysis to reshape chromatin architecture. These complex molecular machines slide nucleosomes along the DNA, eject them entirely or vary their composition — activities that fundamentally change the accessibility of certain DNA regions.

For example, during gene activation, remodelers might relocate nucleosomes that are blocking promoter regions of genes, allowing for the transcription factors to bind and initiate the transcription. The ATP-dependent nature underlines the involvement of cellular energy, highlighting this process as deliberate and regulated, an assertion of energy resources for the precise control of gene expression. This ensures that genes are expressed at the right time and location, corresponding to the cell's needs. Importantly, mutations or dysfunctional ATP-dependent chromatin remodeling can lead to improper gene regulation, which has been implicated in various diseases, including cancer.

Histone modification acts as a direct switchboard for gene expression through chemical changes to histone proteins. Different types of modifications, such as acetylation, phosphorylation, and methylation, can be added or removed from histones, yielding different outcomes on gene transcription.

For instance, histone acetylation typically loosens the DNA-histone interaction, making DNA more accessible for transcription. This is often considered a hallmark of gene activation. On the other hand, methylation can either activate or repress gene expression depending on which histone and what residue is modified. These modifications serve not only as signals for the recruitment or release of other proteins that affect transcription but also as a form of cellular memory, maintaining a record of active and inactive genes during cell division, contributing significantly to cell differentiation and the maintenance of cell identity.

The arrangement of nucleosomes is vital in dictating chromatin conformation and hence, gene accessibility. Nucleosomes, each composed of DNA wrapped around a core of histone proteins, can be considered as 'beads on a string' where their distribution either blocks or grants access to specific DNA sequences for transcription factors and RNA polymerase enzymes.

Changing the spacing between these nucleosomes or altering their structure can have profound effects on gene expression. Certain genomic regions, such as enhancers and promoters, need to be nucleosome-free or have a specific nucleosome arrangement to function properly. Regulatory proteins and chromatin remodelers often target these nucleosomes, adjusting their positioning with precision to accommodate the needs of the cell. This nuanced regulation of nucleosome layout is fundamental to how genes are turned on or off and is intrinsic to the larger framework of cellular homeostasis and response to environmental signals.