Monocytes are a type of leukocyte, or white blood cell. They are the largest type of leukocyte and can differentiate into macrophages and myeloid lineage dendritic cells; as a part of the vertebrate innate immune system monocytes influence the process of adaptive immunity. There are at least three subclasses of monocytes in human blood based on their phenotypic receptors. Monocytes are amoeboid in appearance, have a granulated cytoplasm. Containing unilobar nuclei, these cells are one of the types of mononuclear leukocytes which shelter azurophil granules; the archetypal geometry of the monocyte nucleus is ellipsoidal. Contrast to this classification occurs in polymorphonuclear leukocytes. Monocytes compose 2% to 10% of all leukocytes in the human body and serve multiple roles in immune function; such roles include: replenishing resident macrophages under normal conditions. In an adult human, half of the monocytes are stored in the spleen; these change into macrophages after entering into appropriate tissue spaces, can transform into foam cells in endothelium.
There are at least three types of monocytes in human blood: The classical monocyte is characterized by high level expression of the CD14 cell surface receptor The non-classical monocyte shows low level expression of CD14 and additional co-expression of the CD16 receptor. The intermediate monocyte with high level expression of CD14 and low level expression of CD16. While in humans the level of CD14 expression can be used to differentiate non-classical and intermediate monocytes, the slan cell surface marker was shown to give an unequivocal separation of the two cell types. Ghattas et al. state that the "intermediate" monocyte population is to be a unique subpopulation of monocytes, as opposed to a developmental step, due to their comparatively high expression of surface receptors involved in reparative processes as well as evidence that the "intermediate" subset is enriched in the bone marrow. After stimulation with microbial products the CD14+CD16++ monocytes produce high amounts of pro-inflammatory cytokines like tumor necrosis factor and interleukin-12.
Said et al. showed that activated monocytes express high levels of PD-1 which might explain the higher expression of PD-1 in CD14+CD16++ monocytes as compared to CD14++CD16- monocytes. Triggering monocytes-expressed PD-1 by its ligand PD-L1 induces IL-10 production which activates CD4 Th2 cells and inhibits CD4 Th1 cell function. In mice, monocytes can be divided in two subpopulations. Inflammatory monocytes, which are equivalent to human classical CD14++ CD16− monocytes and resident monocytes, which are equivalent to human non-classical CD14low CD16+ monocytes. Resident monocytes have the ability to patrol along the endothelium wall in the steady state and under inflammatory conditions. In man a monocyte crawling behavior, similar to the patrolling in mice, has been demonstrated both for the classical and the non-classical monocytes. Monocytes are produced by the bone marrow from precursors called monoblasts, bipotent cells that differentiated from hematopoietic stem cells. Monocytes circulate in the bloodstream for about one to three days and typically move into tissues throughout the body where they differentiate into macrophages and dendritic cells.
They constitute between eight percent of the leukocytes in the blood. About half of the body's monocytes are stored as a reserve in the spleen in clusters in the red pulp's Cords of Billroth. Moreover, monocytes are the largest corpuscle in blood. Monocytes which migrate from the bloodstream to other tissues will differentiate into tissue resident macrophages or dendritic cells. Macrophages are responsible for protecting tissues from foreign substances, but are suspected to be important in the formation of important organs like the heart and brain, they are cells that possess a large smooth nucleus, a large area of cytoplasm, many internal vesicles for processing foreign material. In vitro, monocytes can differentiate into dendritic cells by adding the cytokines granulocyte macrophage colony-stimulating factor and interleukin 4. Monocytes and their macrophage and dendritic-cell progeny serve three main functions in the immune system; these are phagocytosis, antigen presentation, cytokine production.
Phagocytosis is the process of uptake of microbes and particles followed by digestion and destruction of this material. Monocytes can perform phagocytosis using intermediary proteins such as antibodies or complement that coat the pathogen, as well as by binding to the microbe directly via pattern-recognition receptors that recognize pathogens. Monocytes are capable of killing infected host cells via antibody-dependent cell-mediated cytotoxicity. Vacuolization may be present in a cell that has phagocytized foreign matter. Many factors produced by other cells can regulate other functions of monocytes; these factors include most chemokines such as monocyte chemotactic protein-1 and monocyte chemotactic protein-3.
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
Neutrophils are the most abundant type of granulocytes and the most abundant type of white blood cells in most mammals. They form an essential part of the innate immune system, their functions vary in different animals. They are formed from stem cells in the bone marrow and differentiated into subpopulations of neutrophil-killers and neutrophil-cagers, they are short-lived and motile, or mobile, as they can enter parts of tissue where other cells/molecules cannot. Neutrophils may be banded neutrophils, they form part of the polymorphonuclear cells family together with eosinophils. The name neutrophil derives from staining characteristics on hematoxylin and eosin histological or cytological preparations. Whereas basophilic white blood cells stain dark blue and eosinophilic white blood cells stain bright red, neutrophils stain a neutral pink. Neutrophils contain a nucleus divided into 2–5 lobes. Neutrophils are a type of phagocyte and are found in the bloodstream. During the beginning phase of inflammation as a result of bacterial infection, environmental exposure, some cancers, neutrophils are one of the first-responders of inflammatory cells to migrate towards the site of inflammation.
They migrate through the blood vessels through interstitial tissue, following chemical signals such as Interleukin-8, C5a, fMLP, Leukotriene B4 and H2O2 in a process called chemotaxis. They are the predominant cells in pus, accounting for its whitish/yellowish appearance. Neutrophils are recruited to the site of injury within minutes following trauma and are the hallmark of acute inflammation; when adhered to a surface, neutrophil granulocytes have an average diameter of 12–15 micrometers in peripheral blood smears. In suspension, human neutrophils have an average diameter of 8.85 µm. With the eosinophil and the basophil, they form the class of polymorphonuclear cells, named for the nucleus' multilobulated shape; the nucleus has the separate lobes connected by chromatin. The nucleolus disappears as the neutrophil matures, something that happens in only a few other types of nucleated cells. In the cytoplasm, the Golgi apparatus is small and ribosomes are sparse, the rough endoplasmic reticulum is absent.
The cytoplasm contains about 200 granules, of which a third are azurophilic. Neutrophils will show increasing segmentation. A normal neutrophil should have 3–5 segments. Hypersegmentation occurs in some disorders, most notably vitamin B12 deficiency; this is noted in a manual review of the blood smear and is positive when most or all of the neutrophils have 5 or more segments. Neutrophils are the most abundant white blood cells in humans; the stated normal range for human blood counts varies between laboratories, but a neutrophil count of 2.5–7.5 x 109/L is a standard normal range. People of African and Middle Eastern descent may have lower counts. A report may divide neutrophils into segmented bands; when circulating in the bloodstream and inactivated, neutrophils are spherical. Once activated, they change shape and become more amorphous or amoeba-like and can extend pseudopods as they hunt for antigens. Neutrophils have a preference to engulf refined carbohydrates over bacteria. In 1973 Sanchez et al. found that the neutrophil phagocytic capacity to engulf bacteria is affected when simple sugars are digested, that fasting strengthens the neutrophils' phagocytic capacity to engulf bacteria.
However, the digestion of normal starches has no effect. It was concluded that the function, not the number, of phagocytes in engulfing bacteria was altered by the ingestion of sugars. In 2007 researchers at the Whitehead Institute of Biomedical Research found that given a selection of sugars, neutrophils engulf some types of sugar preferentially; the average lifespan of inactivated human neutrophils in the circulation has been reported by different approaches to be between 5 and 90 hours. Upon activation, they marginate and undergo selectin-dependent capture followed by integrin-dependent adhesion in most cases, after which they migrate into tissues, where they survive for 1–2 days. Neutrophils are much more numerous than the longer-lived monocyte/macrophage phagocytes. A pathogen is to first encounter a neutrophil; some experts hypothesize. The short lifetime of neutrophils minimizes propagation of those pathogens that parasitize phagocytes because the more time such parasites spend outside a host cell, the more they will be destroyed by some component of the body's defenses.
Because neutrophil antimicrobial products can damage host tissues, their short life limits damage to the host during inflammation. Neutrophils will be removed after phagocytosis of pathogens by macrophages. PECAM-1 and phosphatidylserine on the cell surface are involved in this process. Neutrophils undergo a process called chemotaxis via amoeboid movement, which allows them to migrate toward sites of infection or inflammation. Cell surface receptors allow neutrophils to detect chemical gr
Chromosome 16 is one of the 23 pairs of chromosomes in humans. People have two copies of this chromosome. Chromosome 16 spans about 90 million base pairs and represents just under 3% of the total DNA in cells; the following are some of the gene count estimates of human chromosome 16. Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies. Among various projects, the collaborative consensus coding sequence project takes an conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes; the following is a partial list of genes on human chromosome 16. For complete list, see the link in the infobox on the right. Attention deficit hyperactivity disorder Asperger syndrome Autism spectrum disorder Autosomal dominant polycystic kidney disease Batten disease Familial Mediterranean fever Synesthesia Thalassemia Trisomy 16 Red hair National Institutes of Health.
"Chromosome 16". Genetics Home Reference. Retrieved 2017-05-06. "Chromosome 16". Human Genome Project Information Archive 1990–2003. Retrieved 2017-05-06
Integrins are transmembrane receptors that facilitate cell-extracellular matrix adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, organization of the intracellular cytoskeleton, movement of new receptors to the cell membrane; the presence of integrins allows flexible responses to events at the cell surface. Several types of integrins exist, one cell may have multiple different types on its surface. Integrins are found in all animals. Integrins work alongside other receptors such as cadherins, the immunoglobulin superfamily cell adhesion molecules and syndecans, to mediate cell–cell and cell–matrix interaction. Ligands for integrins include fibronectin, vitronectin and laminin. Integrins are obligate heterodimers, meaning that they have two subunits: α and β. Integrins in mammals have twenty-four α and nine β subunits, in Drosophila five α and two β subunits, in Caenorhabditis nematodes two α subunits and one β subunit.
The α and β subunits each possess several cytoplasmic domains. Variants of some subunits are formed by differential RNA splicing. Through different combinations of the α and β subunits, around 24 unique integrins are generated. Integrin subunits have short cytoplasmic domains of 40 -- 70 amino acids; the exception is the beta-4 subunit, which has a cytoplasmic domain of 1,088 amino acids, one of the largest of any membrane protein. Outside the cell membrane, the α and β chains lie close together along a length of about 23 nm, they have been compared to lobster claws, although they don't "pinch" their ligand, they chemically interact with it at the insides of the "tips" of their "pinchers". The molecular mass of the integrin subunits can vary from 90 kDa to 160 kDa. Beta subunits have four cysteine-rich repeated sequences. Both α and β subunits bind several divalent cations; the role of divalent cations in the α subunit is unknown, but may stabilize the folds of the protein. The cations in the β subunits are more interesting: they are directly involved in coordinating at least some of the ligands that integrins bind.
Integrins can be categorized in multiple ways. For example, some α chains have an additional structural element inserted toward the N-terminal, the alpha-A domain. Integrins carrying this domain either bind to collagens, or act as cell-cell adhesion molecules; this α-I domain is the binding site for ligands of such integrins. Those integrins that don't carry this inserted domain have an A-domain in their ligand binding site, but this A-domain is found on the β subunit. In both cases, the A-domains carry up to three divalent cation binding sites. One is permanently occupied in physiological concentrations of divalent cations, carries either a calcium or magnesium ion, the principal divalent cations in blood at median concentrations of 1.4 mM and 0.8 mM. The other two sites become occupied by cations when ligands bind—at least for those ligands involving an acidic amino acid in their interaction sites. An acidic amino acid features in the integrin-interaction site of many ECM proteins, for example as part of the amino acid sequence Arginine-Glycine-Aspartic acid.
Despite many years of effort, discovering the high-resolution structure of integrins proved to be challenging, as membrane proteins are classically difficult to purify, as integrins are large and linked to many sugar trees. Low-resolution images of detergent extracts of intact integrin GPIIbIIIa, obtained using electron microscopy, data from indirect techniques that investigate the solution properties of integrins using ultracentrifugation and light scattering, were combined with fragmentary high-resolution crystallographic or NMR data from single or paired domains of single integrin chains, molecular models postulated for the rest of the chains; the X-ray crystal structure obtained for the complete extracellular region of one integrin, αvβ3, shows the molecule to be folded into an inverted V-shape that brings the ligand-binding sites close to the cell membrane. More the crystal structure was obtained for the same integrin bound to a small ligand containing the RGD-sequence, the drug cilengitide.
As detailed above, this revealed why divalent cations are critical for RGD-ligand binding to integrins. The interaction of such sequences with integrins is believed to be a primary switch by which ECM exerts its effects on cell behaviour; the structure poses many questions regarding ligand binding and signal transduction. The ligand binding site is directed towards the C-terminal of the integrin, the region where the molecule emerges from the cell membrane. If it emerges orthogonally from the membrane, the ligand binding site would be obstructed as integrin ligands are massive and well cross-linked components of the ECM. In fact, little is known about the angle that membrane proteins subtend to the plane of the membrane; the default assumption is that they emerge rather like little lollipops, but the evidence for thi
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are proteins, but in non-protein coding genes such as transfer RNA or small nuclear RNA genes, the product is a functional RNA; the process of gene expression is used by all known life—eukaryotes and utilized by viruses—to generate the macromolecular machinery for life. Several steps in the gene expression process may be modulated, including the transcription, RNA splicing and post-translational modification of a protein. Gene regulation gives the cell control over structure and function, is the basis for cellular differentiation and the versatility and adaptability of any organism. Gene regulation may serve as a substrate for evolutionary change, since control of the timing and amount of gene expression can have a profound effect on the functions of the gene in a cell or in a multicellular organism. In genetics, gene expression is the most fundamental level at which the genotype gives rise to the phenotype, i.e. observable trait.
The genetic code stored in DNA is "interpreted" by gene expression, the properties of the expression give rise to the organism's phenotype. Such phenotypes are expressed by the synthesis of proteins that control the organism's shape, or that act as enzymes catalysing specific metabolic pathways characterising the organism. Regulation of gene expression is thus critical to an organism's development. A gene is a stretch of DNA. Genomic DNA consists of two antiparallel and reverse complementary strands, each having 5' and 3' ends. With respect to a gene, the two strands may be labeled the "template strand," which serves as a blueprint for the production of an RNA transcript, the "coding strand," which includes the DNA version of the transcript sequence.. The production of the RNA copy of the DNA is called transcription, is performed in the nucleus by RNA polymerase, which adds one RNA nucleotide at a time to a growing RNA strand as per the complementarity law of the bases; this RNA is complementary to the template 3' → 5' DNA strand, itself complementary to the coding 5' → 3' DNA strand.
Therefore, the resulting 5' → 3' RNA strand is identical to the coding DNA strand with the exception that Thymines are replaced with uracils in the RNA. A coding DNA strand reading "ATG" is indirectly transcribed through the “TAC” in the non-coding template strand as "AUG" in the mRNA. In prokaryotes, transcription is carried out by a single type of RNA polymerase, which needs a DNA sequence called a Pribnow box as well as a sigma factor to start transcription. In eukaryotes, transcription is performed by three types of RNA polymerases, each of which needs a special DNA sequence called the promoter and a set of DNA-binding proteins—transcription factors—to initiate the process. RNA polymerase. RNA polymerase II transcribes all protein-coding genes but some non-coding RNAs. Pol II includes a C-terminal domain, rich in serine residues; when these residues are phosphorylated, the CTD binds to various protein factors that promote transcript maturation and modification. RNA polymerase III transcribes 5S rRNA, transfer RNA genes, some small non-coding RNAs.
Transcription ends. While transcription of prokaryotic protein-coding genes creates messenger RNA, ready for translation into protein, transcription of eukaryotic genes leaves a primary transcript of RNA, which first has to undergo a series of modifications to become a mature mRNA; these include 5' capping, set of enzymatic reactions that add 7-methylguanosine to the 5' end of pre-mRNA and thus protect the RNA from degradation by exonucleases. The m7G cap is bound by cap binding complex heterodimer, which aids in mRNA export to cytoplasm and protect the RNA from decapping. Another modification is 3' polyadenylation, they occur if polyadenylation signal sequence is present in pre-mRNA, between protein-coding sequence and terminator. The pre-mRNA is first cleaved and a series of ~200 adenines are added to form poly tail, which protects the RNA from degradation. Poly tail is bound by multiple poly-binding proteins necessary for mRNA export and translation re-initiation. A important modification of eukaryotic pre-mRNA is RNA splicing.
The majority of eukaryotic pre-mRNAs consist of alternating segments called introns. During the process of splicing, an RNA-protein catalytical complex known as spliceosome catalyzes two transesterification reactions, which remove an intron and release it in form of lariat structure, splice neighbouring exons together. In certain cases, some introns or exons can be either removed or retained in mature mRNA; this so-called alternative splicing creates series of different transcripts originating from a single gene. Because these transcripts can be translated into different proteins, splicing extends the complexity of eukaryotic gene expression. Extensive RNA processing may be an evolutionary advantage made possible by the nucleus of eukaryotes. In prokaryotes and translation happen together, whilst in eukaryotes, the nuclear membrane separates the two processes, giving time for RNA processing to
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.