Final answer:
Histones are proteins that promote the establishment of a heterochromatic state in the nucleus. They play a crucial role in DNA packaging and gene regulation. Heterochromatin contains genes that are not expressed, while euchromatin contains genes that are actively transcribed.
Step-by-step explanation:
The proteins that promote the establishment of a heterochromatic state are called histones. Histones are evolutionarily conserved proteins that associate with DNA in the nucleus to form chromatin. They play a crucial role in DNA packaging and gene regulation.
When DNA is wrapped around histone proteins, it forms nucleosomes in a 'beads on a string' structure, which is the first level of chromatin organization. Multiple nucleosomes coil together to form a 30 nm fiber, which is the second level of chromatin organization. The higher-order packing of the 30 nm fiber leads to the formation of metaphase chromosomes seen during mitosis and meiosis, which is the third level of chromatin organization.
Heterochromatin is a tightly packaged region of chromatin that usually contains genes that are not expressed. Histones and other proteins associated with heterochromatin make it difficult for transcription factors and RNA polymerase to access the DNA, thus silencing gene expression. In contrast, euchromatin is a less dense region of chromatin that contains genes that are actively transcribed. Euchromatin is more accessible to transcription factors and RNA polymerase.