Nucleus

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Definition

noun

plural: nuclei

nu·cle·us, [ˈnjuː.kli.əs]

(cell biology) The large, membrane-bounded organelle that contains the genetic material, in the form of multiple linear DNA molecules organized into structures called chromosomes

(botany) The central kernel of a nut or seed, or the center of a starch granule

(neuroanatomy) A group of cell bodies of nerve cells in the brain or spinal cord


Details

Cell biology: nucleus

In cell biology, the nucleus is dubbed as the control center of the cell. It 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.


Structure

The nucleus is a double-membraned organelle of the eukaryotes. It has three main components: the nucleolus, the chromatin and the nuclear envelope. The nucleolus is the round granular structure within the nucleus of a cell, composed of proteins, DNA, and RNA, and functions primarily for the creation of ribosomes. The chromatin is a complex of nucleic acids (e.g. DNA or RNA) and proteins (e.g. histones) that condenses to form a chromosome during cell division. 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 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 nuclear matrix (including the nuclear lamina) is the network of fibers in the nucleus (as cytoskeleton is to the cell although nuclear matrix is more dynamic). It provides mechanical support. The nuclear bodies are non-membraned structures inside the nucleus. Apart from the nucleolus, other nuclear bodies are Cajal bodies and gems, polymorphic interphase karyosomal association (PIKA) domains, promyelocytic leukaemia protein (PML) bodies, paraspeckles, splicing speckles, perichromatin fibrils, and clastosomes.


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.


The presence of nucleus is one of the hallmarks of a eukaryotic cell. The prokaryotes do not have a nucleus but a nucleoid region where the genetic material (genophore) is situated.


Common biological reactions

Gene expression

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

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).


Biological functions

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.


Botany: nucleus

In botany, the nucleus refers to the central kernel of a nut or seed, or the center of a starch granule. In neuroanatomy, a nucleus pertains to a cluster of cell bodies of nerve cells in the brain or spinal cord.


Other usage: nucleus

In other scientific fields, the term nucleus may generally 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.


Supplementary

Etymology

  • Latin nucleus (“kernel", "core”), a diminutive of nux (“nut”)

Derived term(s)


Further reading

See also



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