Nuclear dimorphism

Last updated May 10, 2022

Nuclear dimorphism is a term referred to the special characteristic of having two different kinds of nuclei in a cell. There are many differences between the types of nuclei. This feature is observed in protozoan ciliates, like Tetrahymena, and some foraminifera. Ciliates contain two nucleus types: a macronucleus that is primarily used to control metabolism, and a micronucleus which performs reproductive functions and generates the macronucleus. The compositions of the nuclear pore complexes help determine the properties of the macronucleus and micronucleus.^[1] Nuclear dimorphism is subject to complex epigenetic controls. Nuclear dimorphism is continuously being studied to understand exactly how the mechanism works and how it is beneficial to cells. Learning about nuclear dimorphism is beneficial to understanding old eukaryotic mechanisms that have been preserved within these unicellular organisms but did not evolve into multicellular eukaryotes.^[2]

Key components

The ciliated protozoan Tetrahymena is a useful research model for studying nuclear dimorphism; it maintains two distinct nuclear genomes, the micronucleus and the macronucleus. The macronucleus and micronucleus are located in the same cytoplasm, however, they are very different.^[1] The micronucleus genome contains five chromosomes that undergo mitosis during micronuclear division and meiosis during conjugation, which is the sexual division of the micronucleus. The macronuclear genome is broken down and catabolized once per life cycle during conjugation, allowing it to be site-specific, and a new macronucleus differentiates from a mitotic descendant of the conjugated micronucleus.^[3] The differences in division and overall processes show how functionally and structurally different the molecules are. These differences play an active role in the activities and functions of the cells in which they are located.

Macro vs. micronuclei

Macronuclei and micronuclei differ in their functions even though they are located within the same cell. The micronucleus is globally repressed during the vegetative state, and serves as the diploid germline nucleus, whereas all known vegetative gene expression happens in the macronucleus, which is a polyploid somatic nucleus.^[3] The micronucleus divides before micronucleus in the state of vegetative growth. The macronucleus is active in transcription. It also aids in the activity and control of the cytoplasm along with the nuclear events that happen within the cell. The micronucleus has chromatin that is densely packed as well as an absence of nucleoli.^[4] The micronucleus forms zygotic nuclei during meiosis during conjugation. These zygotic nuclei can follow a process and differentiate into macronucleus or micronucleus cells. Macronucleus cells, on the other hand, differentiate by changes to the DNA. This leads to macronucleus cells being huge compared to micronucleus cells, hence their naming of macro and micro.^[1]

Role of nuclear pore complex

Recent research has shown that the nuclear pore complexes in a binucleated ciliate may be distinct in their composition. This leads to the differences seen in the micronucleus and macronucleus. The nuclear pore complex is made up of nucleoporins, which are proteins. These nucleoporins, Nups, are specific for each type of nucleus. This leads to the structural differences seen between the two types. Since both nuclei are made of the same components, different amounts of the components are added in order to provide the structural differences that are necessary to the functions. The nuclear pore complex is involved with how molecules move across the nuclear envelope when trying to reach the nucleus or the cytoplasm in a process called nucleocytoplasmic trafficking.^[5] nuclear pore complexes have been found to be important in transport to the macronucleus and micronucleus since there are different processes happening in two very different nuclei at different times. These differences in the transport apparatuses between the two nuclei lead to the vast differences between micronucleus and macronucleus.^[1]

Research

As previously mentioned, research has been done involving Tetrahymena, a unicellular eukaryote. This eukaryote has very interesting mechanisms that impact their function. Research has been done to investigate these mechanisms has led to new discoveries of properties of this eukaryote and general properties of nuclear dimorphism.

Tetrahymena have two major parts of their life cycle. there is an asexual reproduction stage involving binary fission as well as a non-reproductive sexual stage called conjugation. During this conjugation stage, the micronucleus cell undergoes meiosis. During binary fission, the macronucleus divides amitotically, and the micronucleus cell divides mitotically. These differences play a role in the differences between macronucleus and micronucleus cells as well as provide difference between their vegetative genomes. During conjugation, some nuclei are selected. These nuclei are destroyed via a mechanism called programmed nuclear death. ^[2] Since conjugation is different for both steps, this leads to differences in micronucleus and macronucleus towards the end of conjugation. The changes remain throughout the cycle.^[1]

There are other unique biological and biochemical differences between micronucleus and macronucleus. There are three ways in which genetic information is distributed during nuclear division. These include meiosis in micronucleus cells, amitosis in micronucleus cells, and mitosis in micronucleus cells. Micronucleus cell meiosis involves stretching the genome outside the cell while macronucleus cell amitosis involves a random distribution of the genome.^[2]

Recent

Recent research has focused on the causes for the differences between the micronucleus and the macronucleus. Functional differences between micronucleus and macronucleus have been attributed to the selectivity of the transport across the nuclear membrane for some time, and it continues to be a topic of interest for research along with other continuing research. Which molecules can pass through depends on the nuclear pores of macronucleus and micronucleus. Macronucleus pores allow bigger molecules to enter compared to micronucleus pores. This difference is thought to be attributed to the makeup of proteins and nuclear pore complex arrangement between the two nuclei types.^[5]

Another recently experimentally tested difference between micronucleus and macronucleus is the specificity that comes from the specific proteins in each. The different nucleoporins in each contributes to structural differences between the two nuclei which in turn, causes functional differences.^[5]

Tetrahymena continue to be explored and researched in order to understand how they work and how they manage their complex biological processes. Ciliates and eukaryotes similar to them helps explain old eukaryotic mechanisms that were conserved with them. Since unicellular ciliates represent the last common ancestor of the eukaryotes, it also helps to explain the mechanisms and peaks an interest in why these mechanisms were preserved then disappeared through evolution.^[2] While much has been researched and discovered about nuclear dimorphism, there is still room for more research to enhance the current knowledge by enhancing previous studies.

Related Research Articles

Cell (biology) Basic unit of all known organisms

The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids.

In cell biology, the nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm; and the nuclear matrix, a network within the nucleus that adds mechanical support, much like the cytoskeleton supports the cell as a whole.

A nuclear pore is a part of a large complex of proteins, known as a nuclear pore complex that spans the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. There are approximately 1,000 nuclear pore complexes (NPCs) in the nuclear envelope of a vertebrate cell, but this number varies depending on cell type and the stage in the life cycle. The human nuclear pore complex (hNPC) is a 110 megadalton (MDa) structure. The proteins that make up the nuclear pore complex are known as nucleoporins; each NPC contains at least 456 individual protein molecules and is composed of 34 distinct nucleoporin proteins. About half of the nucleoporins typically contain solenoid protein domains—either an alpha solenoid or a beta-propeller fold, or in some cases both as separate structural domains. The other half show structural characteristics typical of "natively unfolded" or intrinsically disordered proteins, i.e. they are highly flexible proteins that lack ordered tertiary structure. These disordered proteins are the FG nucleoporins, so called because their amino-acid sequence contains many phenylalanine—glycine repeats.

Tetrahymena, a unicellular eukaryote, is a genus of free-living ciliates. The genus Tetrahymena is the most widely studied member of its phylum. It can produce, store and react with different types of hormones. Tetrahymena cells can recognize both related and hostile cells.

<i>Paramecium</i> Genus of unicellular ciliates, commonly studied as a representative of the ciliate group

Paramecium is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. Paramecia are widespread in freshwater, brackish, and marine environments and are often very abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to conjugate and divide, it has been widely used in classrooms and laboratories to study biological processes. Its usefulness as a model organism has caused one ciliate researcher to characterize it as the "white rat" of the phylum Ciliophora.

A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. All prokaryotes are unicellular and are classified into bacteria and archaea. Many eukaryotes are multicellular, but some are unicellular such as protozoa, unicellular algae, and unicellular fungi. Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.8–4.0 billion years ago.

The nucleoplasm is a type of protoplasm that makes up the cell nucleus, the most prominent organelle of the eukaryotic cell. It is enclosed by the nuclear envelope, also known as the nuclear membrane. The nucleoplasm resembles the cytoplasm of a eukaryotic cell in that it is a gel-like substance found within a membrane, although the nucleoplasm only fills out the space in the nucleus and has its own unique functions. The nucleoplasm suspends structures within the nucleus that are not membrane-bound and is responsible for maintaining the shape of the nucleus. The structures suspended in the nucleoplasm include chromosomes, nuclear bodies, nucleoporins, and nuclear speckles.

A heterokaryon is a multinucleate cell that contains genetically different nuclei. Heterokaryotic and heterokaryosis are derived terms. This is a special type of syncytium. This can occur naturally, such as in the mycelium of fungi during sexual reproduction, or artificially as formed by the experimental fusion of two genetically different cells, as e.g., in hybridoma technology.

The theory of cellularization, also known as the syncytial theory or ciliate-acoel theory, is a theory to explain the origin of Metazoa. The idea was proposed by Hadži and Hanson.

A macronucleus is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, such as metabolism. During conjugation, the macronucleus disintegrates, and a new macronucleus is formed by karyogamy of the micronuclei.

Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal instability. Micronuclei are commonly seen in cancerous cells and may indicate genomic damage events that can increase the risk of developmental or degenerative diseases. Micronuclei form during anaphase from lagging acentric chromosome or chromatid fragments caused by incorrectly repaired or unrepaired DNA breaks or by nondisjunction of chromosomes. This incorrect segregation of chromosomes may result from hypomethylation of repeat sequences present in pericentromeric DNA, irregularities in kinetochore proteins or their assembly, dysfunctional spindle apparatus, or flawed anaphase checkpoint genes. Micronuclei can contribute to genome instability by promoting a catastrophic mutational event called chromothripsis. Many micronucleus assays have been developed to test for the presence of these structures and determine their frequency in cells exposed to certain chemicals or subjected to stressful conditions.

Paramecium caudatum is a species of unicellular protist in the phylum Ciliophora. They can reach 0.33 mm in length and are covered with minute hair-like organelles called cilia. The cilia are used in locomotion and feeding. The species is very common, and widespread in marine, brackish and freshwater environments.

Nucleoporins are a family of proteins which are the constituent building blocks of the nuclear pore complex (NPC). The nuclear pore complex is a massive structure embedded in the nuclear envelope at sites where the inner and outer nuclear membranes fuse, forming a gateway that regulates the flow of macromolecules between the cell nucleus and the cytoplasm. Nuclear pores enable the passive and facilitated transport of molecules across the nuclear envelope. Nucleoporins, a family of around 30 proteins, are the main components of the nuclear pore complex in eukaryotic cells. Nucleoporin 62 is the most abundant member of this family. Nucleoporins are able to transport molecules across the nuclear envelope at a very high rate. A single NPC is able to transport 60,000 protein molecules across the nuclear envelope every minute.

Karyorelictea is a class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, Loxodes, is found in freshwater.

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group and are variously used in swimming, crawling, attachment, feeding, and sensation.

Chilodonella uncinata is a single-celled organism of the ciliate class of alveoles. As a ciliate, C. uncinata has cilia covering its body and a dual nuclear structure, the micronucleus and macronucleus. Unlike some other ciliates, C. uncinata contains millions of minichromosomes in its macronucleus while its micronucleus is estimated to contain 3 chromosomes. Childonella uncinata is the causative agent of Chilodonelloza, a disease that affects the gills and skin of fresh water fish, and may act as a facultative parasite of mosquito larva.

Colpidium colpoda are free-living ciliates commonly found in many freshwater environments including streams, rivers, lakes and ponds across the world. Colpidium colpoda is also frequently found inhabiting wastewater treatment plants. This species is used as an indicator of water quality and waste treatment plant performance.

Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species of protists have also been observed using autogamy as a means of reproduction. Flowering plants engage in autogamy regularly, while the protists that engage in autogamy only do so in stressful environments.

The genome of most cells of eukaryotes remains mainly constant during life. However, there are cases of genome being altered in specific cells or in different life cycle stages during development. For example, not every human cell has the same genetic content as red blood cells which are devoid of nucleus. One of the best known groups in respect of changes in somatic genome are ciliates. The process resulting in a variation of somatic genome that differs from germline genome is called somatic genome processing.

Tetrahymena thermophila is a species of Ciliophora in the family Tetrahymenidae. It is a free living protozoa and occurs in fresh water.

References

1 2 3 4 5 Goldfarb DS, Gorovsky MA (June 2009). "Nuclear dimorphism: two peas in a pod". Current Biology. 19 (11): R449-52. doi: 10.1016/j.cub.2009.04.023 . PMID 19515351.
1 2 3 4 Orias E, Cervantes MD, Hamilton EP (2011). "Tetrahymena thermophila, a unicellular eukaryote with separate germline and somatic genomes". Research in Microbiology. 162 (6): 578–86. doi:10.1016/j.resmic.2011.05.001. PMC 3132220 . PMID 21624459.
1 2 Orias E (2000). "Toward sequencing the Tetrahymena genome: exploiting the gift of nuclear dimorphism". The Journal of Eukaryotic Microbiology. Journal of Eukaryotic Microbiology. 47 (4): 328–33. doi:10.1111/j.1550-7408.2000.tb00057.x. PMID 11140445.
↑ Görtz H (1988). Paramecium. Berlin, Heidelberg: Springer Berlin Heidelberg. ISBN 9783642730863. OCLC 851763096.
1 2 3 Iwamoto M, Osakada H, Mori C, Fukuda Y, Nagao K, Obuse C, Hiraoka Y, Haraguchi T (May 2017). "Tetrahymena". Journal of Cell Science. 130 (10): 1822–1834. doi:10.1242/jcs.199398. PMC 5450191 . PMID 28386019.

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[:0-1] 1 2 3 4 5 Goldfarb DS, Gorovsky MA (June 2009). "Nuclear dimorphism: two peas in a pod". Current Biology. 19 (11): R449-52. doi: 10.1016/j.cub.2009.04.023 . PMID 19515351.

[:2-2] 1 2 3 4 Orias E, Cervantes MD, Hamilton EP (2011). "Tetrahymena thermophila, a unicellular eukaryote with separate germline and somatic genomes". Research in Microbiology. 162 (6): 578–86. doi:10.1016/j.resmic.2011.05.001. PMC 3132220 . PMID 21624459.

[:1-3] 1 2 Orias E (2000). "Toward sequencing the Tetrahymena genome: exploiting the gift of nuclear dimorphism". The Journal of Eukaryotic Microbiology. Journal of Eukaryotic Microbiology. 47 (4): 328–33. doi:10.1111/j.1550-7408.2000.tb00057.x. PMID 11140445.

[4] Görtz H (1988). Paramecium. Berlin, Heidelberg: Springer Berlin Heidelberg. ISBN 9783642730863. OCLC 851763096.

[:3-5] 1 2 3 Iwamoto M, Osakada H, Mori C, Fukuda Y, Nagao K, Obuse C, Hiraoka Y, Haraguchi T (May 2017). "Tetrahymena". Journal of Cell Science. 130 (10): 1822–1834. doi:10.1242/jcs.199398. PMC 5450191 . PMID 28386019.

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