Nucleus – definition
In cell biology, the nucleus is the large, membrane-bounded organelle that contains the genetic material in the form of multiple linear DNA molecules organized into structures called chromosomes. In cell biology, the nucleus function is to act as the control center of the cell. This is because it contains the genetic material that codes for the vital functions of the cell. The nucleus is the organelle responsible for maintaining the integrity of DNA and for controlling cellular activities such as metabolism, growth, and reproduction by regulating gene expression. The nucleus is the largest cytoplasmic structure in animal cells. In mammalian cells, the average diameter is 6 µm. There are cells though that lack nuclei, such as human red blood cells. There are also certain cells that contain relatively more nuclei, e.g. osteoclasts.
In biology, the term nucleus usually refers to the cell nucleus, which is defined as the organelle inside the cell containing the chromosomes. Not all cells though have a nucleus. When a cell lacks nucleus the cell is described as anucleated. Apart from this definition, the nucleus is also used in other biological fields. For instance, in botany, the nucleus may also refer to the central kernel of a nut or seed, or the center of a starch granule. In neuroanatomy, a nucleus is a group of cell bodies of nerve cells in the brain or spinal cord.
In other scientific fields, the term nucleus may refer to the core or the central part around which other parts are grouped or gathered. For instance, in Physics, a nucleus refers to the positively-charged center of an atom that usually contains the protons and neutrons. In Chemistry, a nucleus is a fundamental arrangement of atoms that occur in compounds through substitution of atoms without a change in structure. In Astronomy, a nucleus is the center of the head of a comet or the central or brightest part of a nebula or galaxy. In Meteorology, a nucleus is a particle on which water vapor molecules accumulate in free air to form water drops or ice crystals.
The term nucleus came from the Latin nucleus, meaning “kernel” or “core”, a diminutive of nux (“nut”). The plural form is nuclei. Nuclear is the descriptive term that relates to a nucleus.
Nucleus vs. nucleoid
The nucleus is absent in prokaryotes and the lack of this organelle is used as a basis to distinguish whether a cell is a prokaryote or a eukaryote. Only eukaryotes have a nucleus; prokaryotes lack a nucleus. However, prokaryotes have a region in their cell where the genetic material is located. This region is called a nucleoid. It is nucleus-like and not bound by a nuclear envelope that separates the genetic material from the cytoplasm.
Nucleus vs. nucleolus
The nucleus should not be confused with another cytoplasmic structure, nucleolus. Both of them are present in eukaryotic cells. However, they differ in structure and function. The nucleus is a double-membraned organelle whereas a nucleolus is a round granular structure and not membrane-bound. Nevertheless, the nucleolus is found inside the nucleus. Thus, the nucleolus is one of the nuclear components and sometimes classified as one of the nuclear bodies. Apart from the nucleolus, the other components of the nucleus are chromatins (chromosomes), nuclear bodies (e.g. Cajal bodies and gems (Gemini of Cajal bodies), polymorphic interphase karyosomal association (PIKA) domains, promyelocytic leukaemia protein (PML) bodies, splicing speckles, paraspeckles, perichromatin fibrils, and clastosomes), nuclear membrane, and nucleoplasm. The nucleolus, in turn, is composed of proteins, DNA, and RNA. While the nucleus is largely involved in gene regulatory functions, the nucleolus functions primarily for the creation of ribosomes essential for protein synthesis. In higher eukaryotes, the nucleolus is comprised of a fibrillary center involved in the transcription of the rDNA, the dense fibrillary component that contains the fibrillarin protein associated with rRNA processing, and the granular component that contains the nucleophosmin protein for ribosome biogenesis. (1)
Nuclear parts and function
The nucleus structure is a double-membraned organelle of the eukaryotes. It has three main components: nucleolus and other chromatins (chromosomes), nuclear bodies, nuclear matrix, nucleoplasm, and nuclear envelope.
A complex of nucleic acids (e.g. DNA or RNA) and proteins (e.g. histones) is called chromatin. During cell division, the chromatin condenses to become a chromosome. The basic structural unit of chromatin is a nucleosome. Each nucleosome is made up of a DNA segment wound around the histone protein cores. The main function of chromatin is to package DNA into a smaller volume to fit into the cell. There are two major forms of chromatin: the euchromatin and the heterochromatin. The euchromatin is structurally loose to allow transcription and replication whereas the heterochromatin is more condensed and therefore less active.
The nuclear DNA comprises the large fraction of the cell’s genome (the small fraction comes from the extranuclear DNA in the mitochondria and/or chloroplasts). The DNA outside the nucleus is called extranuclear DNA. These extranuclear DNA, such as cpDNA in chloroplasts and mtDNA in mitochondria, occur in multiple copies since there are several chloroplasts and mitochondria while there is usually just one nucleus inside a cell. A cell would, therefore, contain several copies of mtDNA and cpDNA, often in thousands. Nuclear DNAs are compacted into chromatin structures through histones whereas mtDNA and cpDNA are not.
A nuclear body is defined as a non-membraned mostly proteinaceous structure in the nucleus. As already mentioned above, the nucleolus is regarded as one of the nuclear bodies and is most prominent. It is characterized by its round granular appearance. Its function is largely for the synthesis of ribosomes, which in turn are one of the key players in protein synthesis. Other nuclear bodies are Cajal bodies and gems (Gemini of Cajal bodies), polymorphic interphase karyosomal association (PIKA) domains, promyelocytic leukaemia protein (PML) bodies, splicing speckles, paraspeckles, perichromatin fibrils, and clastosomes. Nuclear bodies may be classified into simple (type I and type II) and complex (type III, type IVa, and type V).
The nuclear matrix is analogous to the cytoskeleton in the cytoplasm of the cell. It is a fibrillar network that provides structural support to the size and shape of the nucleus. The nuclear matrix is more dynamic though compared with the cytoskeleton. It includes the nuclear lamina. The latter refers to the dense fibrous network juxtaposes the nuclear envelope.
The nucleoplasm refers to the protoplasm of the nucleus just as the cytoplasm in the rest of the cell. The nucleoplasm is comprised of various materials (e.g. chromosomes, nuclear bodies, and nuclear matrix) contained by the nuclear envelope. The fluid component of the nucleoplasm is called a nucleosol (just as the cytosol to the cytoplasm).
The nuclear envelope (also called a nuclear membrane) is the biological membrane that surrounds the nucleus. Similar to a cell membrane, the nuclear membrane is a bilipid layer. Thus, the nuclear membrane function is similar to a cell membrane as well in terms of regulating the entry and exit of materials. The nuclear envelope has nuclear pores to control the movement of molecules between the nucleoplasm and the cytoplasm. It is impermeable to large molecules. Thus, it separates the contents of the nucleus from the cellular cytoplasm and allows entry of selected molecules. The nuclear transport of large molecules (e.g. proteins and RNAs) occurs via an active transport system carrier proteins while the passage of small molecules and ions occur passively via the nuclear pores.
The nucleus may be described as bi-lobed, tri-lobed, or multi-lobed depending on the number of lobes. The white blood cell is an example of a cell with a lobed nucleus.
The nucleus is called the control center of the cell. This is because it is responsible for maintaining the integrity of DNA. A large fraction of the genome comes from the nuclear DNA. Thus, the nucleus controls much of the cell’s activities such as metabolism, growth, and reproduction. It does so by regulating gene expression. It also mediates the replication of DNA during the S phase of the cell cycle.
Common biological reactions
Gene expression is the conversion of the information from the gene into mRNA via transcription, and then to protein via translation resulting in the phenotypic manifestation of the gene. Transcription (the first step of gene expression) in eukaryotes occurs in the nucleus (in prokaryotes, in the cytoplasm). It is the process of creating a copy of DNA into mRNA through the help of the enzyme RNA polymerase. Although RNA polymerase traverses the DNA template strand from 3′ → 5′, the coding (non-template) strand is usually used as the reference point. Hence, the process proceeds in the 5′ → 3′ direction, like in DNA replication. However, unlike DNA replication, transcription does not need a primer to start and it uses base pairing to create an RNA copy containing uracil instead of thymine. The second step is translation wherein a strand of mRNA is decoded to produce a particular sequence of amino acids. Although this step occurs in the cytoplasm, regulators of the process are produced from the nucleus. For example, rRNAs and tRNAs are produced via transcription only (and thus skipping the translation step) of non-protein coding genes. The rRNAs and tRNAs are important mediators of the translation of the mRNA. Prior to translation of mRNA though, the newly-synthesized mRNA (called pre-mRNA) undergoes post-translational modification, which occurs in the nucleus. These modifications include 5′ capping, 3′ polyadenylation, and RNA splicing. mRNAs that are moved to the cytoplasm from the nucleus without undergoing modifications would be degraded instead of being translated.
DNA replication is the process of copying and duplicating a DNA molecule in a semiconservative way. The copy contains one of the original strands paired with a newly synthesized strand that is complementary in terms of AT and GC base pairing. This occurs particularly during the S phase of the cell cycle. This is essential as it is a preparatory step prior to cell division (i.e. mitosis or meiosis I).
Importance of nucleus
The nucleus is the largest cytoplasmic structure in animal cells. In mammalian cells, the average diameter is 6 µm. There are cells though that lack nuclei, such as human red blood cells. There are also certain cells that contain relatively more nuclei, e.g. osteoclasts. This implicates that the osteoclasts are greatly more active in terms of gene regulation than the red blood cells. Upon maturity, the red blood cells lose their nucleus to provide greater affinity for gases, e.g. oxygen.
- Diploid nucleus
- Gametic nucleus
- Gracile nucleus
- Hypoglossal nucleus
- Lenticular nucleus
- Medial accessory olivary nucleus
- Nuclear actin binding protein
- Nuclear cardiology
- Nuclear envelope
- Nuclear pore
- Nuclear-cytoplasmic ratio
- Pontis nervi trigeminalis nucleus
- Prerubral nucleus
- Principal sensory nucleus of the trigeminus
- Principal sensory nucleus of trigeminal nerve
- Red nucleus
- Solitary nucleus
- Subthalamic nucleus
- Supraoptic nucleus
- Supraoptic nucleus of hypothalamus
- Veins of caudate nucleus
- Ventral anterior nucleus of thalamus
- Vestibular nucleus
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