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 Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histori ...

ciliates, like ''Tetrahymena'', and some

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

. Ciliates contain two nucleus types: a macronucleus that is primarily used to control

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...

, and a

micronucleus 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 i ...

which performs

reproductive The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are al ...

functions and generates the macronucleus. The compositions of the nuclear pore complexes help determine the properties of the macronucleus and micronucleus. Nuclear dimorphism is subject to complex

epigenetic In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are " ...

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. Ciliate

Key components

The ciliated protozoan ''

Tetrahymena ''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 recogn ...

'' is a useful research model for studying nuclear dimorphism; it maintains two distinct nuclear

genomes In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gen ...

, the micronucleus and the macronucleus. The macronucleus and micronucleus are located in the same cytoplasm, however, they are very different. The micronucleus genome contains five

chromosomes A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

that undergo

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maint ...

during micronuclear division and

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...

during

conjugation Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics *Complex conjugation, the change ...

, which is the sexual division of the micronucleus. The macronuclear genome is broken down and catabolized once per

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia * Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesi ...

during conjugation, allowing it to be site-specific, and a new macronucleus differentiates from a mitotic descendant of the conjugated micronucleus. 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 A persistent vegetative state (PVS) or post-coma unresponsiveness (PCU) is a disorder of consciousness in which patients with severe brain damage are in a state of partial arousal rather than true awareness. After four weeks in a vegetative stat ...

, and serves as the

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respecti ...

germline In biology and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. They ...

nucleus, whereas all known vegetative gene expression happens in the macronucleus, which is a

polyploid Polyploidy is a condition in which the cells of an organism have more than one pair of (homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contain ...

somatic nucleus. 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. 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.

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

nucleoporin 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 membr ...

s, 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. 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.

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 Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...

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. 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. 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

in micronucleus cells, amitosis in micronucleus cells, and

in micronucleus cells. Micronucleus cell meiosis involves stretching the genome outside the cell while macronucleus cell amitosis involves a random distribution of the genome.

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 pore 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 complex ...

s 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. Another recently experimentally tested difference between micronucleus and macronucleus is the specificity that comes from the specific proteins in each. The different

s in each contributes to structural differences between the two nuclei which in turn, causes functional differences. ''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. 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.

References