The cell nucleus is an organelle with a membrane found in eukaryotic cells. It is often referred to as the control center of the cell because it holds the genetic material and regulates cellular activity. The spherical or oval nucleus is surrounded by a double membrane called the nuclear envelope. The nuclear pores in nuclear envelope allow the exchange of molecules between the nucleus and the cytoplasm. Within the nucleus, there are several components, including the nuclear envelope, chromatin, nucleolus and nuclear matrix.
Nuclear Envelope
The nucleus is enclosed by a double-layered membrane called the nuclear envelope which separates the interior of the nucleus from the cytoplasm. It consists of an outer membrane and an inner membrane, with a 20-40nm narrow space between them called the perinuclear space. The outer nuclear membrane has ribosomes attached to the surface and is continuous with the rough endoplasmic reticulum; the perinuclear space also continuous with the lumen of endoplasmic reticulum. The inner membrane is closely associated with the fibrillar netlike nuclear lamina. The nuclear lamina is closely connected to the nuclear network, providing a stable support for the cell nucleus.
The nuclear envelope is selectively permeable and ions, amino acids, nucleosides, etc. can enter the nucleus by diffusion or with the assistance of proteins. Larger molecules enter and exit the nucleus through the nuclear pores which are large protein complexes with about 100nm diameter. They span both membranes and allow the exchange of molecules between the nucleus and the cytoplasm，such as RNAs, proteins and other large complexes of macromolecules. Nuclear pores are abundant in cells with high secretion and division rates, such as cancer cells, liver cells, and pancreatic cells.
Chromatin, Chromosomes
Chromatin consists of histones, DNA and proteins involved in DNA replication and transcription and can be easily stained by basic dyes. Due to the abundance of basic amino acid such as arginine and lysine, the histones tend to bind the phosphate groups in DNA. About every 200 base pairs are tightly wrapped around a histone to form a nucleosome. These nucleosomes are connected by the same DNA strand, forming a bead-like structure called chromatin. The coiled DNA is greatly shortened so that it can stay in the cell nucleus. Chromatin is divided into several cluttered thread-like parts that are difficult to identify individually in the interphase. During cell division, chromatin condenses as rod that can be recognized individually by light microscopy, called chromosomes. Human cells typically contain 46 chromosomes, except for sperm and egg cells, which have 23 chromosomes.
Nucleoli
They are the most prominent part of the cell nucleus and they are easily distinguished by light microscopy when stained. They usually appear as one or more spheres. Ribosomes are produced in the nucleolus which is composed of rRNA, rDNA (which is responsible for transcribing rRNA) and proteins. The rRNAs are transcribed in the nucleolus, folded and combined to proteins to form large and small subunits. They are then secreted through the nuclear pores into the cytoplasm where they assemble with mRNA to be ribosomes. That is why the nucleolus is large in those cells where protein secretion and division are vigorous.
The nuclear matrix or nucleoskeleton
It is a gelatinous fibrillar network with protein and a little RNA，DNA. It connects to the nuclear lamina and provides structural support to the nucleus, maintaining its shape and preventing collapse. The nuclear matrix provides anchors for various nuclear processes, including DNA repair, replication, transcription, regulation, RNA processing. Similar to the cytoskeleton, material in the nucleus can be transported along the nuclear matrix and some enzymes also attach to it.
Overall, the nucleus performs several key functions in the cell. It controls gene expression by regulating the transcription and translation. It is involved in DNA replication during cell division.