The cell nucleus is an important organelle found in eukaryotic cells. Its roles include regulating all activity such as cellular metabolism and growth in addition to storing and maintaining the cell’s DNA for transcription and replication.
Cell and Nucleus, Image Copyright: sanjayart / Shutterstock
The cell nucleus comprises numerous components that help it to fulfil its functions. These include the following:
Nuclear Membrane (nuclear envelope)
The nucleus is characterized by having a nuclear membrane around it to separate the nuclear contents from the cytoplasm. The membrane comprises two phospholipid bilayers, the space between which is called the perinuclear space. The outer membrane is continuous with that of the rough endoplasmic reticulum and is similarly studded with ribosomes. The outer and inner membrane of the nucleus is also joined at nuclear pores which regulate the passage of materials between the nucleus and cutoplasm. The inner nuclear membrane has a mesh on its lining which is made of lamin proteins that bind to nuclear components such as chromatin – this is for structural support and possibly has a role in directing materials to the nuclear pores for export.
This is a network that provides support to the whole nucleus. It is made up of intermediate V type filaments composed of lamin proteins.
This is a dense and filamentous meshwork associated with the inner nuclear membrane. The components of the nucleoskeleton have a role in cellular signaling in addition to maintaining nuclear structure and protecting the nucleus against mechanical force.
The nucleoskeleton is primarily composed of intermediate type V filaments, which consist of lamin proteins. The majority of these lamin proteins are bound to the nuclear envelope through interactions with numerous inner nuclear membrane proteins to provide the characteristic stiffness of the nucleoskeleton. There are also other structural proteins in the nucleoskeleton which impact the spatial organization of lamin proteins and other mechanical properties, such as resilience in response to stretch.
Nuclear Pore (Nuclear Pore Complex)
Nuclear pores are openings in the nuclear membrane that are a pathway for molecules to pass in and out of the nucleus and across the membrane. Molecules can include proteins and ribonucleic acid (RNA) involved in gene expression and support for chromosomes.
These pores occur at intervals where the inner and outer membrane of the nuclear envelope fuse. It is estimated that a typical mammalian cell has between 3000 and 4000 nuclear pores on its nuclear membrane and these pores have a diameter of approximately 600 A°.
Most animal and plant cells have a nucleolus but not all.
This is the most prominent sub-structure in the nucleus. It is the location where ribosomes are synthesized in the cell via the transcription and processing of ribosomal RNA (rRNA).
in addition to DNA, the nucleolus is composed of three main parts – the fibrillar centre, the dense fibrillar component and the granular component. The ribosomal RNA (rRNA) genes undergo transcription at the boundary of the fibrillar centers and dense fibrillar component. Pre-rRNA processing follows this in the dense fibrillar component and continues in the granular component, where assembly of the rRNA with ribosomal proteins forms partially completed pre-ribosomal subunits ready for export to the cytoplasm.
There is nearly 6 foot of cellular DNA in the nucleus of every human cell – this DNA is packaged into 46 chromosomes, each approximately 1.5 inches long. The DNA forms a complex with numerous packaging proteins to form a precise and compact fiber called chromatin. Chromatin has subunits called nucleosomes which can either be lightly packed (euchromatic) or tightly packed (heterochromatic).The chromatin fibers usually further fold and the DNA condenses at least by 10,000 times itself onto itself to form chromosomes.
Reviewed by Afsaneh Khetrapal BSc (Hons)