Calponin is a calcium binding protein. Calponin tonically inhibits the ATPase activity of myosin in smooth muscle. Phosphorylation of calponin by a protein kinase, dependent upon calcium binding to calmodulin, releases the calponin's inhibition of the smooth muscle ATPase. Calponin is made up of α-helices with hydrogen bond turns, it is made up of three domains. These domains in order of appearance are Calponin Homology, regulatory domain, Click-23, domain that contains the calponin repeats. At the CH domain calponin binds to actin within the RD domain. Calmodulin, when activated by calcium may bind weakly to the CH domain and inhibit calponin binding with α-actin. Calponin is responsible for binding many actin binding proteins and regulates the actin/myosin interaction. Calponin is thought to negatively affect the bone making process due to being expressed in high amounts in osteoblasts. Calponin at the US National Library of Medicine Medical Subject Headings
A coiled coil is a structural motif in proteins in which 2–7 alpha-helices are coiled together like the strands of a rope. Many coiled coil-type proteins are involved in important biological functions such as the regulation of gene expression, e.g. transcription factors. Notable examples are the oncoproteins c-jun, as well as the muscle protein tropomyosin; the possibility of coiled coils for α-keratin was somewhat controversial. Linus Pauling and Francis Crick independently came to the conclusion that this was possible at about the same time. In the summer of 1952, Pauling visited the laboratory in England. Pauling and Crick spoke about various topics. Upon returning to the United States, Pauling resumed research on the topic, he concluded that coiled coils exist, submitted a lengthy manuscript to the journal Nature in October. Pauling's son Peter Pauling worked at the same lab as Crick, mentioned the report to him. Crick believed that Pauling had stolen his idea, submitted a shorter note to Nature a few days after Pauling's manuscript arrived.
After some controversy and frequent correspondences, Crick's lab declared that the idea had been reached independently by both researchers, that no intellectual theft had occurred. In his note, Crick proposed the Coiled Coil and as well as mathematical methods for determining their structure. Remarkably, this was soon after the structure of the alpha helix was suggested in 1951 by Linus Pauling and coworkers; these studies were published in the absence of knowledge of a keratin sequence. The first keratin sequences were determined by Hanukoglu and Fuchs in 1982. Based on sequence and secondary structure prediction analyses identified the coiled-coil domains of keratins; these models have been confirmed by structural analyses of coiled-coil domains of keratins. Coiled coils contain a repeated pattern, hxxhcxc, of hydrophobic and charged amino-acid residues, referred to as a heptad repeat; the positions in the heptad repeat and are labeled abcdefg, where a and d are the hydrophobic positions being occupied by isoleucine, leucine, or valine.
Folding a sequence with this repeating pattern into an alpha-helical secondary structure causes the hydrophobic residues to be presented as a'stripe' that coils around the helix in left-handed fashion, forming an amphipathic structure. The most favorable way for two such helices to arrange themselves in the water-filled environment of the cytoplasm is to wrap the hydrophobic strands against each other sandwiched between the hydrophilic amino acids. Thus, it is the burial of hydrophobic surfaces that provides the thermodynamic driving force for the oligomerization; the packing in a coiled-coil interface is exceptionally tight, with complete van der Waals contact between the side-chains of the a and d residues. This tight packing was predicted by Francis Crick in 1952 and is referred to as Knobs into holes packing; the α-helices may be parallel or anti-parallel, adopt a left-handed super-coil. Although disfavored, a few right-handed coiled coils have been observed in nature and in designed proteins.
Viral entry into CD4-positive cells commences when three subunits of a glycoprotein 120 bind to CD4 receptor and a coreceptor. Glycoprotein gp120 is associated to a trimer of gp41 via van der Waals interactions. Upon binding of gp120 to the CD4 receptor and coreceptor, a number of conformational changes in the structure leads to the dissociation of gp120 and to the exposure of gp41 and at the same time to the anchoring of the gp41 N-terminal fusion peptide sequence into the host cell. A spring-loaded mechanism is responsible for bringing the viral and cell membranes in close enough proximity that they will fuse; the origin of the spring-loaded mechanism lies within the exposed gp41, which contains two consecutive heptad repeats following the fusion peptide at the N terminus of the protein. HR1 forms a parallel, trimeric coiled coil onto which HR2 region coils, forming the trimer-of-hairpins structure, thereby facilitating membrane fusion through bringing the membranes close to each other; the virus enters the cell and begins its replication.
Inhibitors derived from HR2 such as Fuzeon bind to the HR1 region on gp41 have been developed. However, peptides derived from HR1 have little viral inhibition efficacy due to the propensity for these peptides to aggregate in solution. Chimeras of these HR1-derived peptides with GCN4 leucine zippers have been developed and have shown to be more active than Fuzeon, but these have not entered the clinic yet; because of their specific interaction coiled coils can be used as a "tags" to stabilize or enforce a specific oligomerization state. The general problem of deciding on the folded structure of a protein when given the amino acid sequence has not been solved. However, the coiled coil is one of a small number of folding motifs for which the relationships between the sequence and the final folded structure are comparatively well understood. Harbury et al. performed a landmark study using an archetypal coiled coil, GCN4, in which rules that govern the way that peptide sequence affects the oligomeric state were established.
The GCN4 coiled coil is a 31-amino-acid parallel, dimeric coiled coil and has a repeated isoleucine (or I
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to the cytoplasm of eukaryotic cells, some bacteria and some archaea. A microtubule can grow as long as 50 micrometres and are dynamic; the outer diameter of a microtubule is about 24 nm. They are formed by the polymerization of a dimer of two globular proteins and beta tubulin into protofilaments that can associate laterally to form a hollow tube, the microtubule; the most common form of a microtubule consists of 13 protofilaments in the tubular arrangement. Microtubules are important in a number of cellular processes, they are involved in maintaining the structure of the cell and, together with microfilaments and intermediate filaments, they form the cytoskeleton. They make up the internal structure of cilia and flagella, they provide platforms for intracellular transport and are involved in a variety of cellular processes, including the movement of secretory vesicles and intracellular macromolecular assemblies.
They are involved in cell division and are the major constituents of mitotic spindles, which are used to pull eukaryotic chromosomes apart. Microtubules are nucleated and organized by microtubule organizing centers, such as the centrosome found in the center of many animal cells or the basal bodies found in cilia and flagella, or the spindle pole bodies found in most fungi. There are many proteins that bind to microtubules, including the motor proteins kinesin and dynein, severing proteins like katanin, other proteins important for regulating microtubule dynamics. An actin-like protein has been found in a gram-positive bacterium Bacillus thuringiensis, which forms a microtubule-like structure and is involved in plasmid segregation. Tubulin and microtubule-mediated processes, like cell locomotion, were seen by early microscopists, like Leeuwenhoek. However, the fibrous nature of flagella and other structures were discovered two centuries with improved light microscopes, confirmed in the 20th century with the electron microscope and biochemical studies.
Microtubule in vitro assays for motor proteins such as dynein and kinesin are researched by fluorescently tagging a microtubule and fixing either the microtubule or motor proteins to a microscope slide visualizing the slide with video-enhanced microscopy to record the travel of the microtubule motor proteins. This allows the movement of the motor proteins along the microtubule or the microtubule moving across the motor proteins; some microtubule processes can be determined by kymograph. In eukaryotes, microtubules are long, hollow cylinders made up of polymerised α- and β-tubulin dimers; the inner space of the hollow microtubule cylinders is referred to as the lumen. The α and β-tubulin subunits are 50% identical at the amino acid level, each have a molecular weight of 50 kDa; these α/β-tubulin dimers polymerize end-to-end into linear protofilaments that associate laterally to form a single microtubule, which can be extended by the addition of more α/β-tubulin dimers. Microtubules are formed by the parallel association of thirteen protofilaments, although microtubules composed of fewer or more protofilaments have been observed in vitro.
Microtubules have a distinct polarity, critical for their biological function. Tubulin polymerizes end to end, with the β-subunits of one tubulin dimer contacting the α-subunits of the next dimer. Therefore, in a protofilament, one end will have the α-subunits exposed while the other end will have the β-subunits exposed; these ends are designated ends, respectively. The protofilaments bundle parallel to one another with the same polarity, so, in a microtubule, there is one end, the end, with only β-subunits exposed, while the other end, the end, has only α-subunits exposed. While microtubule elongation can occur at both the and ends, it is more rapid at the end; the lateral association of the protofilaments generates a pseudo-helical structure, with one turn of the helix containing 13 tubulin dimers, each from a different protofilament. In the most common "13-3" architecture, the 13th tubulin dimer interacts with the next tubulin dimer with a vertical offset of 3 tubulin monomers due to the helicity of the turn.
There are other alternative architectures, such as 11-3, 12-3, 14-3, 15-4, or 16-4, that have been detected at a much lower occurrence. Microtubules can morph into other forms such as helical filaments, which are observed in protist organisms like foraminifera. There are two distinct types of interactions that can occur between the subunits of lateral protofilaments within the microtubule called the A-type and B-type lattices. In the A-type lattice, the lateral associations of protofilaments occur between adjacent α and β-tubulin subunits. In the B-type lattice, the α and β-tubulin subunits from one protofilament interact with the α and β-tubulin subunits from an adjacent protofilament, respectively. Experimental studies have shown that the B-type lattice is the primary arrangement within microtubules. However, in most microtubules there is a seam in which tubulin subunits interact α-β; some species of Prosthecobacter contain microtubules. The structure of these bacterial microtubules is similar to that of eukaryotic microtubules, consisting of a hollow tube of protofilaments assembled from heterodimers of bacterial tubulin A and bacterial tubulin B.
Both BtubA and BtubB share features of both α- and β-tubulin. Unlike eukar
A base pair is a unit consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA. Dictated by specific hydrogen bonding patterns, Watson–Crick base pairs allow the DNA helix to maintain a regular helical structure, subtly dependent on its nucleotide sequence; the complementary nature of this based-paired structure provides a redundant copy of the genetic information encoded within each strand of DNA. The regular structure and data redundancy provided by the DNA double helix make DNA well suited to the storage of genetic information, while base-pairing between DNA and incoming nucleotides provides the mechanism through which DNA polymerase replicates DNA and RNA polymerase transcribes DNA into RNA. Many DNA-binding proteins can recognize specific base-pairing patterns that identify particular regulatory regions of genes. Intramolecular base pairs can occur within single-stranded nucleic acids.
This is important in RNA molecules, where Watson–Crick base pairs permit the formation of short double-stranded helices, a wide variety of non-Watson–Crick interactions allow RNAs to fold into a vast range of specific three-dimensional structures. In addition, base-pairing between transfer RNA and messenger RNA forms the basis for the molecular recognition events that result in the nucleotide sequence of mRNA becoming translated into the amino acid sequence of proteins via the genetic code; the size of an individual gene or an organism's entire genome is measured in base pairs because DNA is double-stranded. Hence, the number of total base pairs is equal to the number of nucleotides in one of the strands; the haploid human genome is estimated to be about 3.2 billion bases long and to contain 20,000–25,000 distinct protein-coding genes. A kilobase is a unit of measurement in molecular biology equal to 1000 base pairs of DNA or RNA; the total amount of related DNA base pairs on Earth is estimated at 5.0×1037 and weighs 50 billion tonnes.
In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC. Hydrogen bonding is the chemical interaction. Appropriate geometrical correspondence of hydrogen bond donors and acceptors allows only the "right" pairs to form stably. DNA with high GC-content is more stable than DNA with low GC-content. But, contrary to popular belief, the hydrogen bonds do not stabilize the DNA significantly; the larger nucleobases and guanine, are members of a class of double-ringed chemical structures called purines. Purines are complementary only with pyrimidines: pyrimidine-pyrimidine pairings are energetically unfavorable because the molecules are too far apart for hydrogen bonding to be established. Purine-pyrimidine base-pairing of AT or GC or UA results in proper duplex structure; the only other purine-pyrimidine pairings would be AC and GT and UG. The GU pairing, with two hydrogen bonds, does occur often in RNA. Paired DNA and RNA molecules are comparatively stable at room temperature, but the two nucleotide strands will separate above a melting point, determined by the length of the molecules, the extent of mispairing, the GC content.
Higher GC content results in higher melting temperatures. On the converse, regions of a genome that need to separate — for example, the promoter regions for often-transcribed genes — are comparatively GC-poor. GC content and melting temperature must be taken into account when designing primers for PCR reactions; the following DNA sequences illustrate pair double-stranded patterns. By convention, the top strand is written from the 5' end to the 3' end. A base-paired DNA sequence: ATCGATTGAGCTCTAGCG TAGCTAACTCGAGATCGCThe corresponding RNA sequence, in which uracil is substituted for thymine in the RNA strand: AUCGAUUGAGCUCUAGCG UAGCUAACUCGAGAUCGC Chemical analogs of nucleotides can take the place of proper nucleotides and establish non-canonical base-pairing, leading to errors in DNA replication and DNA transcription; this is due to their isosteric chemistry. One common mutagenic base analog is 5-bromouracil, which resembles thymine but can base-pair to guanine in its enol form. Other chemicals, known as DNA intercalators, fit into the gap between adjacent bases on a single strand and induce frameshift mutations by "masquerading" as a base, causing the DNA replication machinery to skip or insert additional nucleotides at the intercalated site.
Most intercalators are known or suspected carcinogens. Examples include ethidium acridine. An unnatural base pair is a designed subunit of DNA, created in a laboratory and does not occur in nature. DNA sequences have been described which use newly created nucleobases to form a third base pair, in addition to the two ba
Vimentin is a structural protein that in humans is encoded by the VIM gene. Vimentin is a type III intermediate filament protein, expressed in mesenchymal cells. IF proteins are found in all animal cells as well as bacteria. IF, along with tubulin-based microtubules and actin-based microfilaments, comprises the cytoskeleton. All IF proteins are expressed in a developmentally-regulated fashion; because of this, vimentin is used as a marker of mesenchymally-derived cells or cells undergoing an epithelial-to-mesenchymal transition during both normal development and metastatic progression. A vimentin monomer, like all other intermediate filaments, has a central α-helical domain, capped on each end by non-helical amino and carboxyl domains. Two monomers are co-translationally expressed in a way that facilitates their formation of a coiled-coil dimer, the basic subunit of vimentin assembly; the α-helical sequences contain a pattern of hydrophobic amino acids that contribute to forming a "hydrophobic seal" on the surface of the helix.
In addition, there is a periodic distribution of acidic and basic amino acids that seems to play an important role in stabilizing coiled-coil dimers. The spacing of the charged residues is optimal for ionic salt bridges, which allows for the stabilization of the α-helix structure. While this type of stabilization is intuitive for intrachain interactions, rather than interchain interactions, scientists have proposed that the switch from intrachain salt bridges formed by acidic and basic residues to the interchain ionic associations contributes to the assembly of the filament. Vimentin plays a significant role in supporting and anchoring the position of the organelles in the cytosol. Vimentin is attached to the nucleus, endoplasmic reticulum, mitochondria, either laterally or terminally; the dynamic nature of vimentin is important. Scientists found that vimentin provided cells with a resilience absent from the microtubule or actin filament networks, when under mechanical stress in vivo. Therefore, in general, it is accepted that vimentin is the cytoskeletal component responsible for maintaining cell integrity..
Transgenic mice that lack vimentin did not show functional differences. It is possible that the microtubule network may have compensated for the absence of the intermediate network; this result supports an intimate interactions between microtubules and vimentin. Moreover, when microtubule depolymerizers were present, vimentin reorganization occurred, once again implying a relationship between the two systems. On the other hand, wounded mice that lack the vimentin gene heal slower than their wild type counterparts. In essence, vimentin is responsible for maintaining cell shape, integrity of the cytoplasm, stabilizing cytoskeletal interactions. Vimentin has been shown to eliminate toxic proteins in JUNQ and IPOD inclusion bodies in asymmetric division of mammalian cell lines. Vimentin is found to control the transport of low-density lipoprotein, LDL, -derived cholesterol from a lysosome to the site of esterification. With the blocking of transport of LDL-derived cholesterol inside the cell, cells were found to store a much lower percentage of the lipoprotein than normal cells with vimentin.
This dependence seems to be the first process of a biochemical function in any cell that depends on a cellular intermediate filament network. This type of dependence has ramifications on the adrenal cells, which rely on cholesteryl esters derived from LDL. Vimentin plays a role in aggresome formation, where it forms a cage surrounding a core of aggregated protein, it has been used as a sarcoma tumor marker to identify mesenchyme. Methylation of the vimentin gene has been established as a biomarker of colon cancer and this is being utilized in the development of fecal tests for colon cancer. Statistically significant levels of vimentin gene methylation have been observed in certain upper gastrointestinal pathologies such as Barrett's esophagus, esophageal adenocarcinoma, intestinal type gastric cancer. High levels of DNA methylation in the promotor region have been associated with markedly decreased survival in hormone positive breast cancers. Downregulation of vimentin was identified in cystic variant of papillary thyroid carcinoma using a proteomic approach.
See Anti-citrullinated protein antibody for its use in diagnosis of rheumatoid arthritis. Vimentin has been shown to interact with: DSP MEN1 MYST2 PLEC PKN1 SPTAN1 UPP1 YWHAZ PRKCI The 3' UTR of Vimentin mRNA has been found to bind a 46kDa protein. Vimentin Images
A chromosome is a deoxyribonucleic acid molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind to and condense the DNA molecule to prevent it from becoming an unmanageable tangle. Chromosomes are visible under a light microscope only when the cell is undergoing the metaphase of cell division. Before this happens, every chromosome is copied once, the copy is joined to the original by a centromere, resulting either in an X-shaped structure if the centromere is located in the middle of the chromosome or a two-arm structure if the centromere is located near one of the ends; the original chromosome and the copy are now called sister chromatids. During metaphase the X-shape structure is called a metaphase chromosome. In this condensed form chromosomes are easiest to distinguish and study. In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.
Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe; this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer. Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation; the word chromosome comes from the Greek χρῶμα and σῶμα, describing their strong staining by particular dyes. The term was coined by von Waldeyer-Hartz, referring to the term chromatin, introduced by Walther Flemming; some of the early karyological terms have become outdated.
For example and Chromosom, both ascribe color to a non-colored state. The German scientists Schleiden, Virchow and Bütschli were among the first scientists who recognized the structures now familiar as chromosomes. In a series of experiments beginning in the mid-1880s, Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity, it is the second of these principles, so original. Wilhelm Roux suggested. Boveri was able to confirm this hypothesis. Aided by the rediscovery at the start of the 1900s of Gregor Mendel's earlier work, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes. Boveri influenced two generations of American cytologists: Edmund Beecher Wilson, Nettie Stevens, Walter Sutton and Theophilus Painter were all influenced by Boveri. In his famous textbook The Cell in Development and Heredity, Wilson linked together the independent work of Boveri and Sutton by naming the chromosome theory of inheritance the Boveri–Sutton chromosome theory.
Ernst Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T. H. Morgan, all of a rather dogmatic turn of mind. Complete proof came from chromosome maps in Morgan's own lab; the number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscope, he counted 24 pairs, his error was copied by others and it was not until 1956 that the true number, 46, was determined by Indonesia-born cytogeneticist Joe Hin Tjio. The prokaryotes – bacteria and archaea – have a single circular chromosome, but many variations exist; the chromosomes of most bacteria, which some authors prefer to call genophores, can range in size from only 130,000 base pairs in the endosymbiotic bacteria Candidatus Hodgkinia cicadicola and Candidatus Tremblaya princeps, to more than 14,000,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum. Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome.
Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria have a one-point from which replication starts, whereas some archaea contain multiple replication origins; the genes in prokaryotes are organized in operons, do not contain introns, unlike eukaryotes. Prokaryotes do not possess nuclei. Instead, their DNA is organized into a structure called the nucleoid; the nucleoid occupies a defined region of the bacterial cell. This structure is, dynamic and is maintained and remodeled by the actions of a range of histone-like proteins, which associate with the bacterial chromosome. In archaea, the DNA in chromosomes is more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes. Certain bacteria contain plasmids or other extrachromosomal DNA; these are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. In prokaryotes and viruses, the DNA is densely packed and organized.