Arikah Map

Cell nucleus

Cell nucleus:The eukaryotic cell nucleus. Visible in this diagram are the ribosome-studded double membranes of the nuclear envelope, the DNA (complexed as chromatin), and the nucleolus. Within the cell nucleus is a viscous liquid called nucleoplasm, similar to the cytoplasm found outside the nucleus.
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The eukaryotic cell nucleus. Visible in this diagram are the ribosome-studded double membranes of the nuclear envelope, the DNA (complexed as chromatin), and the nucleolus. Within the cell nucleus is a viscous liquid called nucleoplasm, similar to the cytoplasm found outside the nucleus.

In cell biology, the nucleus (pl. nuclei; from Latin nucleus or nuculeus, kernel) is a membrane-enclosed organelle found in most eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins such as histones to form chromosomes. The genes within these chromosomes make up the cell's nuclear genome. The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression.

Gene expression involves two major processes, the first of which is transcription, in which DNA is used as a template to produce RNA. When this RNA encodes proteins, it is referred to as messenger RNA (mRNA). The second process is called translation, and involves mRNA being used as a template to produce proteins. Only transcription occurs inside the nucleus, meaning that all mRNA must be exported to the cytoplasm before it can be translated.

The main structural elements of the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle, and the nuclear lamina, a meshwork on the nuclear face of the envelope that adds mechanical support. Because the membranes are impermeable to most molecules, nuclear pores are required to allow movement across the envelope. These pores cross the entire envelope, providing a channel that allows free movement of small solutes. The movement of larger molecules is controlled, and requires active transport, facilitated by carrier proteins. Nuclear transport is of paramount importance to cell function, as movement through the pores is required for both gene expression and chromosomal maintenance.

Although the interior of the nucleus does not contain any membrane-delineated bodies, its contents are not uniform, and a number of subnuclear bodies exist, made up of unique proteins, RNA molecules, and DNA conglomerates. The best known of these is the nucleolus, which is mainly involved in assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA.

The nucleus was the first organelle to be discovered, and was first described by Franz Bauer in 1802.[1] It was later popularized by Scottish botanist Robert Brown in 1831. Brown was studying orchids microscopically when he observed an opaque area, which he called the areola or nucleus, in the cells of the flower's outer layer.[2]


Contents

Structure

The nucleus is the largest cellular organelle.[3] It varies in diameter from 11 to 22 μm and occupies about 10% of the total cell volume.[4] The viscous liquid within it is called nucleoplasm, and is similar to the cytoplasm found outside the nucleus.

Nuclear envelope and pores

Cell nucleus:A cross section of a nuclear pore on the surface of the nuclear envelope (1). Other diagram lables show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments.
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A cross section of a nuclear pore on the surface of the nuclear envelope (1). Other diagram lables show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments.
Main articles: Nuclear envelope and Nuclear pores

The nuclear envelope consists of two cellular membranes, an inner and an outer membrane, arranged parallel to one another and separated by 10 to 50 nm. One of the features that make the nuclear membranes unique are the large pores they contain. The nuclear envelope encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing freely between the nucleoplasm and the cytoplasm.[5]

The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (RER), and is similarly studded with ribosomes. The space between the membranes is called the perinuclear space and is continuous with the RER lumen.

Nuclear pores, which provide aqueous channels through the envelope, are composed of a number of different proteins, collectively referred to as nucleoporins. The pores are about 125 million daltons in molecular weight and consist of around 50 (in yeast) to 100 proteins (in vertebrates).[3] The pores are 100 nm in diameter; however, after the annulus and other regulatory gating system molecules are present, the space left for molecules to enter is reduced to 9 nm. This size allows the free passage of small water-soluble molecules whilst excluding larger structures, such as DNA or proteins. Large molecules can still enter the nucleus, but need to be transported. The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope.[6]

Most proteins, ribosomal subunits, and some RNAs are transported through the pore complexes in a process mediated by a family of transcription factors known as karyopherins. Those karyopherins that mediate movement into the nucleus are also called importins, while those that mediate movement out of the nucleus are called exportins. Most karyopherins interact directly with their cargo, although some use adaptor proteins.[7] Steroid hormones such as cortisol and aldosterone, as well as other small lipid-soluble molecules involved in intercellular signaling can diffuse through the cell membrane and into the cytoplasm, where they bind nuclear receptor proteins that are trafficked into the nucleus. There they serve as transcription factors when bound to their ligand; in the absence of ligand many such receptors function as histone deacetylases that repress gene expression.[3]

Cytoskeleton

Main article: Nuclear lamina

Two networks of intermediate filaments provide the nucleus with mechanical support: the nuclear lamina forms an organised meshwork on the nuclear face of the envelope; less organised support is provided on the cystolic face of the envelope. The mechanical functions provided include structural support for the nuclear envelope, as well as providing anchorage sites for chromosomes and nuclear pores.[4]

The nuclear lamina is mostly composed of lamin proteins. The lamin proteins are transported into the nucleus interior, where they are assembled, before being incorporated into the nuclear lamina.[8][9] In addition to their role in the lamina, lamin proteins are also found inside the nucleoplasm where they form another regular structure,[10] called the nucleoplasmic veil.

Like other intermediate filaments, the nuclear lamina monomer, the lamin, contains an alpha-helical region. This domain is used by two monomers to coil around each other, facing the same direction, and form a dimer structure called a coiled coil. Two of these dimer structures then join side by side, in an antiparallel arrangement, to form a tetramer. This tetramer, composed of four lamin proteins, is called a protofilament. Eight of these protofilaments form a lateral arrangement that is twisted to form a ropelike filament. These filaments can be assembled or disassembled in a dynamic manner, meaning that changes in length depend on the competing rates of filament addition and removal.[4]

Chromosomes

Main article: Chromosome

The cell nucleus contains the majority of the cell's genetic material, in the form of DNA molecules. During most of the cell cycle these are organized in a DNA-protein complex known as chromatin, and during cell division the chromatin can be seen to form well defined chromosomes.

There are two types of chromatin: euchromatin which is the less compact DNA form, and which contains genes that are frequently expressed by the cell;[11] heterochromatin which is the more compact form, and contains DNA that is not transcribed. It is further categorized into facultative heterochromatin, consisting of those non-expressed genes, and constitutive heterochromatin, which consists of DNA's structural components, telomeres and centromeres.

During interphase the chromatin organise themselves into discrete individual patches,[12] called chromosome territories.[13] Active genes, which are generally found in the euchromatic region of the chromosome, tend to be located towards the chromosome's territory boundary.[14]

Nucleolus

Cell nucleus:An electron micrograph of a cell nucleus, showing the darkly stained nucleolus.
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An electron micrograph of a cell nucleus, showing the darkly stained nucleolus.
Main article: Nucleolus

The nucleolus is a discrete densely-stained structure found in the nucleus. It is not surrounded by a membrane, and is sometimes called a suborganelle. It forms around tandem repeats of rDNA, DNA coding for ribosomal RNA (rRNA). These regions are called nucleolar organizer regions (NOR). The main roles of the nucleolus are to synthesize rRNA and assemble ribosomes. The structural cohesion of the nucleolus depends on its activity, as ribosomal assembly in the nucleolus results in the transient association of nucleolar components, facilitating further ribosomal assembly, and hence further association. This model is supported by observations that inactivation of rDNA results in intermingling of nucleolar structures.[15]

The first step in ribosomal assembly is transcription of the rDNA, by a protein called RNA polymerase I, forming a large pre-rRNA precursor. This is cleaved into the subunits 5.8S, 18S, and 28S rRNA.[16] The transcription, post-transcriptional processing, and assembly of rRNA occurs in the nucleolus, aided by small nucleolar RNA (snoRNA) molecules, some of which are derived from spliced introns from messenger RNAs encoding genes related to ribosomal function. The assembled ribosomal subunits are the largest structures passed through the nuclear pores.[3]

When observed under the electron microscope, the nucleolus can be seen to consist of three distinguishable regions: the inner most fibrillar centers (FCs), surrounded by the dense fibrillar component (DFC), which in turn is bordered by the granular component (GC). Transcription of the rDNA occurs either in the FC or at the FC-DFC boundary, and therefore when rDNA transcription in the cell is increased more FCs are detected. Most of the cleavage and modification of rRNAs occurs in the DFC, while the latter steps involving protein assembly onto the ribosomal subunits occurs in the GC.[16]

Other subnuclear bodies

Subnuclear structure sizes
Structure nameStructure diameter
Cajal bodies 0.2-2.0 μm[17]
PIKA 5 μm[18]
PML bodies 0.2-1.0 μm[19]
Speckles 20–25 nm[18]

Besides the nucleolus, the nucleus contains a number of other non-membrane delineated bodies. These include Cajal bodies, Gemini of coiled bodies, paraspeckles, polymorphic interphase karyosomal association (PIKA), PML bodies, and speckles. Although little is known about a number of these domains, they are significant in that they show that the nucleoplasm is not uniform mixture, but rather contains organised functional subdomains.[19]

Cajal bodies

A nucleus will contain between 1 and 10 compact structures called Cajal bodies (CB). The diameter of which measure between 0.2 μm and 2.0 μm depending on the cell type and species,[17] and when seen under an electron microscope, appear as balls of tangled thread.[18] They are involved in a number of different roles relating to RNA processing, specifically small nucleolar RNA (snoRNA) and small nuclear RNA (snRNA) maturation, and histone mRNA modification.[17]

Gemini of coiled bodies

Similar to Cajal bodies are Gemini of coiled bodies, also called Gems. The name is derived from the Gemini constellation, the twins, in reference to Gems' close association with CBs. They are similar in size and shape to CBs, but differ in composition. Unlike CBs, Gems don't contain snRNPs, but do contain a protein called survivor of motor neurons (SMN) whose function relates to snRNP biogenesis. Gems are believed to assist CBs in snRNP biogenesis.[20]

Nemaline rods

The presence of small intranuclear rods have been reported in some cases of nemaline myopathy. This condition typically results from mutations in actin, and the rods themsleves consist of mutant actin as well as other cytoskeletal proteins.[21]

PML bodies

Promyelocytic leukaemia bodies (PML bodies) are spherical bodies found scattered throughout the nucleoplasm, measuring around 0.2-1.0 μm. They are known by a number of other names, including nuclear domain 10 (ND10), Kremer bodies, and PML oncogenic domains. They are often seen in the nucleus in association with Cajal bodies and cleavage bodies. It has been suggested that they play a role in regulating transcription.[19]

Speckles

Sometimes referred to as interchromatin granule clusters, speckles are rich in splicing snRNPs and other splicing proteins necessary for pre-mRNA processing. Because of a cell's changing requirements, the composition and location of these bodies changes according to mRNA transcription and regulation via phosphorylation of specific proteins.[22]

Function

The main functions of the nucleus are providing a compartment separated from the rest of the cell and controlling transcription.

Cell compartmentation

The nuclear envelope allows the nucleus to control its contents, and separate them from the rest of the cytoplasm where necessary. This is important for controlling processes on either side of the nuclear membrane. For example:

In some cases where a cytoplasmic process needs to be restricted, a key participant is removed to the nucleus. For example this occurs in the case of glycolysis, a cellular pathway for breaking down glucose to produce energy. Hexokinase is an enzyme responsible for the first the step of glycolysis, forming glucose-6-phosphate from glucose. At high concentrations of fructose-6-phosphate, a molecule made later from glucose-6-phosphate, a regulator protein removes hexokinase to the nucleus.[23]

In order to control which genes are being transcribed, the cell separates the proteins controlling gene expression from those genes. For example in the case of NF-κB genes, which are involved in most inflammatory responses, the genes are transcribed in response to a signal pathway. In one of the pathways involving these genes, TNF-α binds to a cell membrane receptor resulting in the recruitment of signalling proteins, and eventually freeing NF-κB. The nuclear localisation signal allows NF-κB to be transported through the nuclear pore and into the nucleus where it stimulates the transcription of the target genes.[4]

The compartmentation allows the cell to prevent translation of unspliced mRNA.[24] Eukaryotic mRNA contains introns that must be removed before being translated to produce functional proteins. The splicing is done inside the nucleus before the mRNA can be accessed by ribosomes for translation. Without the nucleus ribosomes would translate newly transcribed (unprocessed) mRNA resulting in misformed proteins.

Cell nucleus:A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. "Begin" indicates the 5' end of the DNA, where new RNA synthesis begins; "end" indicates the 3' end, where the primary transcripts are almost complete.
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A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. "Begin" indicates the 5' end of the DNA, where new RNA synthesis begins; "end" indicates the 3' end, where the primary transcripts are almost complete.

Gene expression

Gene expression first involves transcription, in which DNA is used as a template to produce RNA. In the case of genes encoding proteins, that RNA produced from this process is messenger RNA (mRNA), which then needs to be translated by ribosomes to form a protein. As ribosomes are located outside the nucleus, mRNA produced needs to be exported.

Since the nucleus is the site of transcription, it also contains a variety of proteins which either directly mediate transcription or are involved in regulating the process. These proteins include helicases that unwind the double-stranded DNA molecule to facilitate access to it, RNA polymerases that synthesize the growing RNA molecule, topoisomerases that restore the supercoiled state of DNA after it has been unwound, and a large variety of transcription factors that regulate expression.

Processing of pre-mRNA

Main article: Post-transcriptional modification

Newly synthesized mRNA molecules are known as primary transcripts or pre-mRNA. They must undergo post-transcriptional modification in the nucleus before being exported to the cytoplasm; mRNA that appears in the nucleus without these modifications is degraded rather than used for protein translation. The three main modifications are 5' capping, 3' polyadenylation, and RNA splicing. While in the nucleus, pre-mRNA is associated with a variety of proteins in complexes known as heterogeneous ribonucleoprotein particles (hnRNPs). Addition of the 5' cap occurs co-transcriptionally and is the first step in post-translational modification. The 3' poly-adenine tail is only added after transcription is complete.

RNA splicing, carried out by a complex called the spliceosome, is the process by which introns, or regions of DNA that do not code for protein, are removed from the pre-mRNA and the remaining exons connected to re-form a single continuous molecule. This process normally occurs after 5' capping and 3' polyadenylation but can begin before synthesis is complete in transcripts with many exons.[3] Many pre-mRNAs, including those encoding antibodies, can be spliced in multiple ways to produce different mature mRNAs that encode different protein sequences. This process is known as alternative splicing, and allows production of a large variety of proteins from a limited amount of DNA.

Dynamics and regulation

The nucleus is a dynamic structure that changes according the cell's requirements. In order to control the nuclear functions, the processes of entry and exit from the nucleus are regulated.

Nuclear transport

Cell nucleus:Macromolecules, such as RNA and proteins, are actively transported across the nuclear membrane in a process called the Ran-GTP nuclear transport cycle.
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Macromolecules, such as RNA and proteins, are actively transported across the nuclear membrane in a process called the Ran-GTP nuclear transport cycle.
Main article: Nuclear transport

The entry and exit of large molecules from the nucleus is tightly controlled by the nuclear pore complexes. Although small molecules can enter the nucleus without regulation,[25] macromolecules such as RNA and proteins require association karyopherins called importins to enter the nucleus and exportins to exit. "Cargo" proteins that must be translocated from the cytoplasm to the nucleus contain short amino acid sequences known as nuclear localization signals which are bound by importins, while those transported from the nucleus to the cytoplasm carry nuclear export signals bound by exportins. The ability of importins and exportins to transport their cargo is regulated by GTPases, enzymes that hydrolyze the molecule guanosine triphosphate to release energy. The key GTPase in nuclear transport is Ran, which can bind either GTP or GDP (guanosine diphosphate) depending on whether it is located in the nucleus or the cytoplasm. Whereas importins depend on RanGTP to dissociate from their cargo, exportins require RanGTP in order to bind to their cargo.[7]

Nuclear import depends on the importin binding its cargo in the cytoplasm, carrying it through the nuclear pore into the nucleus. Inside the nucleus RanGTP acts to separate the cargo from the importin, allowing the importin to exit the nucleus and be reused. Nuclear export is similar, as the exportin binds the cargo inside the nucleus, leaves through the nuclear pore, and by interacting with RanGDP separates.

Specialized export proteins exist for translocation of mature mRNA and tRNA to the cytoplasm after post-transcriptional modification is complete. This quality-control mechanism is important due to the these molecules' central role in protein translation; mis-expression of a protein due to incomplete excision of exons or mis-incorporation of amino acids could have negative consequences for the cell; thus incompletely modified RNA that reaches the cytoplasm is degraded rather than used in translation.[3]

Assembly and disassembly

During its lifetime a nucleus may be broken down, a process which depending on the circumstances may eventually be followed by its being reconstructed. During these events, the main components whose break down needs to be controlled are the structural ones, namely the nuclear envelope and the nuclear lamina.

During the cell cycle

Cell nucleus:An image of a newt lung cell stained with fluorescent dyes during anaphase. The mitotic spindles can be seen, stained green, attached to the two sets of chromosomes, stained light blue. Each complete set of chromosomes is being pulled by the spindle towards its own centrosomes, also stained green.
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An image of a newt lung cell stained with fluorescent dyes during anaphase. The mitotic spindles can be seen, stained green, attached to the two sets of chromosomes, stained light blue. Each complete set of chromosomes is being pulled by the spindle towards its own centrosomes, also stained green.

During the cell cycle the cell divides to form two cells. In order for this process to be possible, each of the new daughter cells must have a full set of genes, a process requiring replication of the chromosomes as well as segregation of the separate sets. This requires that the replicated chromosomes, the sister chromatids, be attached to microtubules, which in turn are attached to different centrosomes. The sister chromatids can then be pulled to separate locations in the cell. However, in many cells the centrosome is located in the cytoplasm, outside the nucleus, the microtubles would be unable to attach to the chromatids in the presence of the nuclear envelope.[26] Therefore the early stages in the cell cycle, beginning in prophase and until around prometaphase, the nuclear membrane is dismantled.[10] Likewise, during the same period, the nuclear lamina is also dissembled, a process regulated by phosphoyrlation of the lamins.

Towards the end of the cell cycle, the nuclear membrane is reformed, and around the same time, the nuclear lamina reassembled by dephosphorylating the lamins.[10]

During apoptosis

Apoptosis is a controlled process resulting in death of the cell. Various of the changes directly affect the nucleus and its contents, especially condensation of the chromatin, disintegration of the nuclear envelope and lamina. The progressive organisation of the nuclear lamina throughout apoptosis is used to initiate and regulate the various phases of apoptosis.[10] The breakdown of the lamina is controlled by a group of proteins called caspases that cleave the individual lamins.

During viral infection

The nuclear envelope acts as a barrier that prevents both DNA and RNA viruses from entering the nucleus. Some viruses require access to proteins inside the nucleus in order to replicate and/or assemble. DNA viruses, such as herpesvirus replicate and assemble in the cell nucleus, and exit by budding through the inner nuclear membrane. This process is accompanied by disassembly of the lamina on the nuclear face of the inner membrane.[10]

Anucleated and polynucleated cells

Cell nucleus:Human red blood cells, like those of other mammals, lack nuclei. This occurs as a normal part of the cells' development.
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Human red blood cells, like those of other mammals, lack nuclei. This occurs as a normal part of the cells' development.

Although most cells have a single nucleus, some cell types can have none or many nuclei. This can be a normal process, as in the maturation of mammalian red blood cells, or an abnormal product of faulty cell division.

Anucleated cells contain no nucleus and are therefore incapable of dividing to produce daughter cells. The best-known anucleated cell is the mammalian red blood cell, or erythrocyte, which also lacks other organelles such as mitochondria and serves primarily as a transport vessel to ferry oxygen from the lungs to the body's tissues. Erythrocytes mature via erythropoiesis in the bone marrow, where they lose their nuclei, organelles, and ribosomes. The nucleus is expelled during the process of differentiation from an erythroblast to a reticulocyte, the immediate precursor of the mature erythrocyte.[27] The presence of mutagens may induce the release of some immature "micronucleated" erythrocytes into the bloodstream.[28][29] Anucleated cells can also arise from flawed cell division in which one daughter lacks a nucleus and the other is binucleate.

Polynucleated cells contain multiple nuclei. Most Acantharean species of protozoa[30] and some fungi in mycorrhizae[31] have naturally polynucleated cells. Cells arising from the fusion of monocytes and macrophages, known as giant multinucleated cells, sometimes accompany inflammation[32] and are also implicated in tumor formation.[33]

Evolution

There are four contending theories of the origin of the nucleus in eukaryotic cells.[34]

One theory proposed by Purificacion Lopez-Garcia and David Moreira is the syntrophic model. This model holds that a symbiotic relationship between the archaea and bacteria created the nucleus containing eukaryotic cell. It is believed that archaea, similar to modern methanogenic archaea, entered bacteria, similar to modern myxobacteria, and developed a symbiosis which eventually fused into a chimeric new organism the eukarya. An archaeal origin of the nucleus is supported by observations that archaea and eukarya have similar genes for certain proteins, including histones. Observations that myxobacteria are motile, can form multicelluar complexes, and possess kinases and G proteins similar to eukarya, supports a bacterial origin for the eukaryotic cell.[35]

Another model by John Fuerst proposes that eukaryotic like cells existed at the same time as archaea and bacteria were splitting off on their own lineages. This model is based on the existence of modern planctomycetes bacteria that possess a nuclear structure with primitive pores and other compartmentalized membrane structures.[36] A similar proposal states that a eukarya like cell, the chronocyte, evolved first and phagocytosed archaea and bacteria to generate the nucleus and the eukaryotic cell.[37]

The third and most controversial model, known as viral eukaryogenesis, posits that the membrane-bound nucleus, along with other eukaryotic features, originated from the infection of a prokaryote by a virus. The suggestion is based on similarities between eukaryotes and viruses such as linear DNA strands, mRNA capping, and tight binding to proteins (analogizing histones to viral envelopes). One version of the proposal suggests that the nucleus evolved in concert with phagocytosis to form an early cellular "predator".[38] Another variant proposes that eukaryotes originated from early archaea infected by poxviruses, on the basis of observed similarity between the DNA polymerases in modern poxviruses and eukaryotes.[39][40] It has been suggested that the as-yet-unresolved question of the evolution of sex could be related to the viral eukaryogenesis hypothesis.[41]

The fourth model proposed by Albert de Roos is based on the self assembly of membranes by lipid-protein interactions. The model is based on a Darwinist approach of the evolution of compartmentalization of function by interaction of laminar proteins, intrinsic proteins, and cytoskeletal proteins with lipid vesicles to form the nucleus, and exomembranes to form the endoplasmic reticulum and plasma membrane.[42]

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Further reading

A review article about nuclear lamins, explaining their structure and various roles
A review article about nuclear transport, explains the principles of the mechanism, and the various transport pathways
A review article about the nucleus, explaining the structure of chromosomes within the organelle, and describing the nucleolus and other subnuclear bodies
A review article about the evolution of the nucleus, explaining a number of different theories
A university level textbook focusing on cell biology. Contains information on nucleus structure and function, including nuclear transport, and subnuclear domains
Organelles of the cell
Acrosome | Cell wall | Cell membrane | Chloroplast | Cilium/Flagellum | Centrosome | Cytoplasm | Endoplasmic reticulum | Endosome | Golgi apparatus | Lysosome | Melanosome | Mitochondrion | Myofibril | Nucleus | Nucleolus (sub-organelle, found within the nucleus) | Parenthesome | Peroxisome | Plastid | Ribosome | Vacuole | Vesicle

Categories


Organelles | Medical terms

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