Spherical or oval-shaped nucleus serves as the control centers of eukaryotic cells, as they contain genetic material and regulate cellular metabolism.
Nuclear Envelope
It is a double concentric membrane that encloses cell nucleus. Between these membranes is 20-40 nm wide perinuclear space that is continuous with the lumen of endoplasmic reticulum. The outer membrane connects to rough endoplasmic reticulum and cytoskeleton. It’s also a site where ribosomes translate RNA. The smooth inner membrane lacks ribosomes, but lined with nuclear lamina, a network of fibrillar proteins.
A natural selective barrier is created by nuclear envelope. DNA replication and RNA transcription occur within nucleus, while protein translation is restricted to cytoplasm. This separation ensures orderly cellular activities and minimizes damage to genetic material. Additionally, the barrier also regulates material exchange between nucleus and cytoplasm. Nonpolar, lipid-soluble small molecules permeate the double membrane easily. Non-hydrophobic substances travel through thousands of nuclear pores located on cell nuclear surface. These pores are large protein complexes whose diameter is 100 nm approximately, and composed of about 30 different proteins. They penetrate the entire double membrane. From a top-down view, they resemble chrysanthemums very much.
Some hydrophilic small molecules, such as ions and polar substances, passively transport through the hydrophilic channels in nuclear pores. Certain biological macromolecules are exchanged by active transport. Their surfaces have specific amino acid sequences called nuclear localization signals (NLS). Fibers that grow from pores grasp NLS and pull macromolecules into the passage. During the bidirectional active transport, protein in pore changes conformation, GTP is consumed and NLS is discarded. DNA and RNA polymerase are guided into nucleus from cytoplasm. Ribosomal subunits and mRNA are released into cytoplasm. Uncut and capped mRNA can’t pass through nuclear pore. It suggests that it is the final quality control step in mRNA synthesis and processing. Notably, some molecules that inherently lack a signal sequence can freely shuttle across the nucleus after adding an NLS artificially.
Chromatin and genetic material
The main components of chromatin are histones and double-stranded DNA. Additionally, they contain various non-histone proteins involved in regulating gene expression, replication, and repair. Histones were discovered in avian red blood cell by German biochemist Albrecht Kossel in 1884. They contain many basic amino acids, particularly lysine and arginine, which give them a basic nature or positive charge. They attract negatively charged DNA molecules via electrostatic forces. Histones are as abundant in chromatin as DNA. There are five types: H1, H2A, H2B, H3, and H4. The latter four make up an octamer that is wrap tightly 147 base pairs to form a nucleosome. The spacing between each nucleosome ranges from a few to 80 base pairs. The rosary structure shortens DNA to one-third of its original length. It is shortened agan in the secondary structure because nucleosomes cluster together with the help of H1 histones. Genetic material is divided into several parts that seem to mix randomly like spaghetti. However, fluorescence staining shows they prefer specific positions. During cell division, they can be recognized by an optical microscope because chromatin condenses into short rod-like chromosomes.
Without shortening mechanisms, DNA length could reach several meters. It’s obviously impossible to fit into a nucleus only a few micrometers. Histones also regulate gene expression. When nucleosomes are tightly packed, certain transcription-related proteins find it difficult to work. The actively transcribed genes are usually found in loosely packed regions. Methylation or acetylation is a common strategy for eukaryotes to modify histones to regulate chromatin compaction.
Nucleolus and nuclear matrix
A nucleus has one or more spherical nucleoli. They are the most prominent parts and can be easily distinguished under an optical microscope after staining. rRNA is transcribed here and assembled with proteins into large and small subunits. Then, these subunits are transported to cytoplasm to assemble into complete ribosomes on the mRNA. This is why cells with high metabolic activity have larger nucleoli.
The netlike nuclear matrix is protein fibers and attaches to nuclear lamina. One of their main tasks is to prevent nucleus from collapsing. Enzymes associated to genetic material are anchored in here, and move along nuclear matrix under the guidance of motor protein. It’s somewhat similar to cytoskeleton.