The nucleus is an important eukaryotic cell organelle. It functions as the administrative centre and coordinates and controls the cell functions such as protein synthesis, metabolism and cell division. The nucleus is the seat of the genetic material, the DNA. The nucleus is the largest of all cell organelles and occupies about 10 per cent of the cell volume. Cell division starts with the division of the nucleus and the rest of the cell divides only after nuclear division. The nucleus is a spherical organelle and is the centre of nuclear transcription. Usually, there is one centrally placed nucleus in every cell, but some organisms like the slime moulds and a group of algae called the Siphonales have more than one nucleus in their cells (multinucleated cells). On the other hand, cells like human red blood corpuscles do not have a nucleus (enucleated cells). There are no membrane-bound sub-particles or sub-compartments inside the nucleus. However, there are many nuclear bodies made of unique proteins present inside the nucleus. Let’s have a look at nucleus structure and functions.
Let’s deep dive into the nucleus structure.
The eukaryotic nucleus is a large organelle with a double membrane cover and has some important particles inside.
Also referred as nuclear membrane, the nuclear envelope is a double-layered cover consisting of two membranes – an inner membrane and an outer membrane which separates the genetic material present inside the nucleus from the cytoplasm surrounding it. The inner and outer membranes of the nuclear envelope are separated by a perinuclear space, which measures 10 -50 nanometer. This membrane regulates the movements of macromolecules between the nucleoplasm and cytoplasm. The outer membrane of the nucleus is continuous with the Rough Endoplasmic Reticulum (RER) membrane. This layer has ribosomes distributed on it. The pores on the nuclear membrane are called nuclear pores that allows free diffusion of certain molecules. The nuclear pores are made of proteins called Nucleoporins, which helps in facilitating the transport of only water-soluble molecules to pass through them, while larger molecules like nucleic acids and bigger proteins are prevented from passing through the pores. Located at the place where the two nuclear membranes fuse is an eightfold structure. This ring-like structure is called nuclear basket and is present is every nuclear pore. The cell nucleus of the mammals usually contains 3000-4000 pores. The molecules that can pass through the pores are usually transported with the help of the transport factors known as karyopherins. These karyopherins are of two types – importins, which help to carry the molecules into the cell, and exportins which help to move the molecules out of the nucleus. While most of the karyopherins attach directly to the molecules, some of them use adaptor proteins to interact with the cargo molecules that need to be transported.
Nucleoplasm, alternatively called karyoplasm, is the gelatine-like substance inside the nucleus. It is made of water, ions, and other molecules. The main function of these substances to help the nucleus to maintain its shape. It also supports the nucleoli. Hyaloplasm, the fluid part of the nucleoplasm, helps in the transportation of materials needed for proper cell functioning. It also dissolves unwanted enzymes and nucleotides.
These structures provide mechanical support to the nucleus. These are a network of cytoskeletal structural components called intermediate filaments. The nuclear lamina is composed of Lamin proteins and they also anchor the chromosomes.
The nucleus is the seat of the genetic material, which is present as linear DNA molecules organised into structures called chromosomes. The number of chromosomes is specific to every species. For example, humans, the homo sapiens have 23 pairs of chromosomes. DNA, the genetic material of the cell, is a double helix. The long DNA strands wrap around histone proteins and form a structure that resembles beads on a string. This structure is called the nucleosome. Nucleosomes folds into a compact, dense particle called chromatin fibre. Chromatin reinforces the DNA molecule and it plays an important role in regulating gene expression and protecting DNA from damages. The chromatin fibres coil and condense to form the chromosomes.
There are three important subnuclear organelles – nucleolus, Cajal bodies (CBs) and speckles.
Nucleolus (plural: nucleoli) is a dense substance found inside the eukaryotic nucleus. These structures are not bound by any membranes. Nucleoli are responsible for the manufacturing of ribosomes, the protein-producing structures found in the eukaryotic nucleus. The number of nucleoli is specific for each species. Nucleoli disappear during cell division and reappear when the chromosomes are arranged in the nucleolar organizing regions after cells division.
Cajal bodies and Speckles
These are spherical non-membranous bodies found inside the nucleus. The possible function of Cajal bodies is to modify RNA after its transcription.
Speckles, or interchromatin granule clusters, are abundant in pre-RNA splicing factors.
Let’s have a look at the function of the nucleus
The nucleus is the place where genetic transcription happens. The primary function is to mediate DNA replication during the cell cycle and to control gene expression.
Compartmentalization of the Cell
The nuclear envelope gives a definite structure to the eukaryotic nucleus and separates the contents from the rest cytoplasm that encloses the nucleus. The nuclear envelope prevents the translation of unspliced mRNA by separating the nuclear process from the cytoplasmic process.
DNA transcription occurs in the nucleus. DNA transcription involves the unwinding of DNA double helix, synthesis of RNA, the coiling of DNA and the actual transcription process.
Transporting various molecules in and out of the nucleus through the nuclear pores is an important function of the nucleus. Important proteins are carried into the nucleus with the help of importins. Molecules like the RNA are transported out of the nucleus with the help of exportins.
Assembly and Disassembly
During cell division, the nucleus is integrated and disintegrated to divide the contents and the genetic material. The cell nucleus is also associated with programmed cell death or apoptosis. The nuclear lamina and the nuclear envelop dissolves and then formed again during the cell cycle. Duplication of the DNA and RNA replication takes place in the nucleus. Thus, nucleus integrates and disintegrates various structures during different phases of the cell cycle.
Before getting transported to the cytoplasm, the primary transcript, or the newly synthesised mRNA go through post-transcriptional modification. This happens inside the nucleus and involves a series of biological processes.
A part of ribosome biogenesis takes place in the nucleus. Over 200 different proteins work in coordination to synthesis the four eukaryotic rRNAs, which then together with the ribosomal proteins synthesise the pre-40S subunit and the pre- 60S subunit. These subunits are then transported to the cytoplasm for maturation.
The largest organelle and the seat of the genetic material, the nucleus is a very important organelle with clearly defined structure and function. Eukaryotic nucleus has evolved as a very important organelle in multi-cellular forms of life. The nucleus is definitely a feature of an advanced form of life and supports the complex functions of the cells. Nucleated cells have many advantages over those cells without a nucleus.