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Structure

The nucleus, being the largest organelle, varies in diameter from 10 to 20 micrometres. It is enclosed by a double membrane called the nuclear envelope. The inner and outer membrane fuse at regular intervals, forming nuclear pores. The nuclear envelope regulates and facilitates transport between the nucleus and the cytoplasm, while separating the chemical reactions taking place in cytoplasm from reactions happening within the nucleus. The outer membrane is continuous with the rough endoplasmic reticulum (RER) and may be studded with ribosomes. The space between the two membranes (called the "perinuclear space") is continuous with the lumen of the RER.

Inside the nucleus is one or several nucleoliIn biology, the nucleolus is, strictly speaking, a "suborganelle" of the cell nucleus, which is an organelle. It is aconsequence of rRNA synthesis: nucleolar organizers special regions on some chromosomes that contain multiple copies of the genes encoding surrounded by a fibrous matrix called the nucleoplasmSimilar to the cytoplasm of a cell, the nucleus contains nucleoplasm . The nucleoplasm is a highly viscous solid containing the chromosomes and nucleoli. Chromosomes contain information encoded in DNA attached to proteins called histones and are usually a. The nucleoplasm is a liquid with a gel-like consistency (similar in this respect to the cytoplasm), in which many substances are dissolved. These substances include nucleotide triphospates, enzymes, proteins, and transcription factorsIn the context of genetics, a transcription factor is a regulatory protein that initiates the transcription of certain genes upon binding with DNA. The binding of a transcription factor to a specific DNA sequence can result in either an increased rate of. Genetic material ( DNADeoxyribonucleic acid DNA is a nucleic acid which carries genetic instructions for the biological development of all cellular forms of life and many viruses. DNA is sometimes referred to as the molecule of heredity as it is inherited and used to propagate) is also present in the nucleus, and is in a loosely coiled form called chromatin (as opposed to its highly coiled and compacted form, the chromosome).

There are two types of chromatin: euchromatin and heterochromatin. Euchromatin is the least compact form of DNA, and the regions of DNA which constitute euchromatin contain genes which are frequently expressed by the cell.

In heterochromatin, DNA is more tightly compacted. Regions of DNA which constitute heterochromatin generally contain genes which are not expressed by the cell (this type of heterochromatin is known as facultative heterochromatin) or are regions which make up the telomeres and centromeres of the chromosomes (this type of heterochromatin is known as constitutive heterochromatin). In multicellular organisms, cells are highly specialised to perform particular functions, hence different sets of genes are required and expressed. Therefore, the regions of DNA that constitute heterochromatin vary between cell types.

Nucleoli are densely-stained structures at which ribosome subunits are assembled.