Citation:Griswold,A.(2008)Genome packaging in prokaryotes: the circular chromosome of E. Education1(1):57





How carry out bacteria, doing not have a nucleus, organize and also pack your genome right into the cell? Supercoiling permits this but forces a different kind that transcription and also translation in prokaryotes.

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Most student learnat an early age the organisms have the right to be broadly divided right into two types: prokaryotesand eukaryotes. In main school, kids are taught that the main differencebetween this organisms is that eukaryotic bio cells contain membrane-boundorganelles, such as the nucleus, while prokaryotic cells execute not. Over there is muchmore to the story, however, particularly with regard to chromosomal structureand organization.

Much that what isknown around prokaryotic chromosome structure was derived from studies of Escherichia coli, a bacterium that livesin the human colon and is commonly used in laboratory cloning experiments. Inthe 1950s and also 1960s, this bacterium ended up being the model organism of selection for prokaryoticresearch once a team of scientists supplied phase-contrast microscopy andautoradiography to show that the vital genes the E. Coli space encoded on a solitary circular chromosome packaged withinthe cabinet nucleoid (Mason & Powelson, 1956; Cairns, 1963).

Prokaryotic cell donot save on computer nuclei or other membrane-bound organelles. In fact, the word "prokaryote"literally method "before the nucleus." The nucleoid is merely the area that aprokaryotic cabinet in i m sorry the chromosomal DNA is located. This setup isnot as simple as that sounds, however, especially considering the the E. Coli chromosome is number of orders ofmagnitude larger than the cell itself. So, if bacterial chromosomes space sohuge, how deserve to they to the right comfortably within a cell—much less in one tiny cornerof the cell?

The answer to this question lies in DNA packaging. Whereas standard scale wrap your DNA approximately proteins dubbed histones to help package the DNA right into smaller spaces, many prokaryotes do not have histones (with the exemption of those species in the domain Archaea). Thus, one method prokaryotes compress your DNA right into smaller spaces is through supercoiling (Figure 1). Imagine twisting a rubber tape so that it creates tiny coils. Now twist it also further, so the the original coils wrinkles over one another and form a condensed ball. Once this kind of twisting happens to a bacter genome, the is well-known as supercoiling. Genomes deserve to be negative supercoiled, meaning that the DNA is twisted in the opposite direction that the dual helix, or positively supercoiled, definition that the DNA is twisted in the very same direction together the dual helix. Many bacterial genomes room negatively supercoiled throughout normal growth.

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E. Coli.", "true", "Figure 1", "Supercoiled chromosome of E. Coli.", "", "All civil liberties reserved.", "623", "900", "");">Figure 1

During the 1980s and1990s, researchers discovered that multiple protein act with each other to wrinkles andcondense prokaryotic DNA. In particular, one protein dubbed HU, which is themost plentiful protein in the nucleoid, works through an enzyme referred to as topoisomeraseI to bind DNA and introduce sharp bends in the chromosome, generating thetension crucial for an unfavorable supercoiling. Current studies have additionally shownthat various other proteins, consisting of integration host aspect (IHF), can bind tospecific sequences within the genome and also introduce additional bends (Rice et al., 1996). The urgent DNA is thenorganized into a selection of conformations (Sinden & Pettijohn, 1981) thatare supercoiled and also wound approximately tetramers the the HU protein, much likeeukaryotic chromosomes space wrapped roughly histones (Murphy & Zimmerman,1997).

Once the prokaryoticgenome has actually been condensed, DNA topoisomerase I, DNA gyrase, and other proteinshelp preserve the supercoils. One of these maintain proteins, H-NS, dram anactive function in transcription by modulating the expression of the genes involvedin the response to environmental stimuli. Another maintenance protein, factorfor inversion stimulation (FIS), is abundant throughout exponential growth andregulates the expression of much more than 231 genes, including DNA topoisomerase I(Bradley et al., 2007).

Supercoilingexplains how chromosomes fit into a little corner the the cell, yet how do theproteins associated in replication and transcription access the thousands ofgenes in prokaryotic chromosomes when whatever is packaged together sotightly? It has actually been established that prokaryotes DNA replication wake up at arate that 1,000 nucleotides per second, and prokaryotic warrior occurs in ~ arate of around 40 nucleotides per second (Lewin, 2007), therefore bacteria must havehighly effective methods the accessing your DNA strands. Yet how?

Researchers havenoted the the nucleoid usually appears as one irregularly shame mass withinthe prokaryotes cell, yet it becomes spherical when the cabinet is treated withchemicals to inhibit warrior or translation. Moreover, duringtranscription, small regions the the chromosome have the right to be viewed to task from thenucleoid into the cytoplasm (i.e., the interior of the cell), whereby they unwindand associate with ribosomes, thus enabling easy accessibility by varioustranscriptional proteins (Dürrenberger et al., 1988). These projectionsare thought to define the mysterious shape of nucleoids during energetic growth.When transcription is inhibited, however, the projections retreat right into thenucleoid, creating the abovementioned spherical shape.

Because there is nonuclear membrane to different prokaryotic DNA from the ribosomes within thecytoplasm, transcriptionand translation take place simultaneously in these organisms. This is strikinglydifferent from eukaryotic bio chromosomes, which are confined come the membrane-boundnucleus during most the the cell cycle. In eukaryotes, transcription need to becompleted in the nucleus before the freshly synthesized mRNA molecules can be transportedto the cytoplasm to undergo translation into proteins.

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Ixodes sp.) carries the bacter (Borrelia sp.) that causes Lyme disease.", "true", "Some rights reserved.", "881", "677", "");">Figure 2:Deer tick.
Ixodes sp.) carries the bacterium (Borrelia sp.) that causes Lyme disease.", "true", "Some civil liberties reserved.", "881", "677", "");">This black-legged tick (Ixodes sp.) dead the bacter (Borrelia sp.) that reasons Lyme disease.

Recently, that hasbecome noticeable that one size does not fit all once it pertains to prokaryoticchromosome structure. While many prokaryotes, prefer E. Coli, contain a solitary circular DNA molecule that makes up theirentire genome, current studies have actually indicated that some prokaryotes save on computer asmany as 4 linear or one chromosomes. For example, Vibrio cholerae, the bacteria that causes cholera, has twocircular chromosomes. Among these chromosomes contains the genes involved inmetabolism and also virulence, when the other contains the continuing to be essentialgenes (Trucksis et al., 1998). Aneven more extreme instance is listed by Borreliaburgdorferi, the bacter that causes Lyme disease. This organism istransmitted with the bite that deer ticks (Figure 2), and also it contains up to 11 duplicates of asingle linear chromosome (Ferdows &Barbour, 1989). Unlike E. Coli,Borrelia can not supercoil that is linearchromosomes into a tight ball within the nucleoid; rather, these strands arediffused transparent the cell (Hinnebusch& Bendich, 1997).

Other organisms,such together Bacillus subtilis, formnucleoids that very closely resemble those of E.coli, but they use various architectural proteins to execute so. Furthermore,the DNA molecule of Archaea, a taxonomic domain composed of single-celled,nonbacterial prokaryotes the share plenty of similarities v eukaryotes, deserve to benegatively supercoiled, positively supercoiled, or not supercoiled at all. It isimportant to keep in mind that archaeans space the only team of prokaryotes that useeukaryote-like histones, quite than the architectural protein describedabove, to condense your DNA molecule (Sandman et al., 1990). The salvation of histones by archaeans is thoughtto have actually paved the way for the evolution of larger and also more complicated eukaryoticcells (Minsky et al., 1997).

Most prokaryotesreproduce asexually and are haploid, meaning that just a solitary copy that eachgene is present. This provides it reasonably easy to create mutations in the laband research the resulting phenotypes. By contrast, eukaryotes that reproducesexually normally contain lot of chromosomes and also are said to be diploid,because two copies of every gene exist—with one copy comes from every of one organism"sparents.

Yet anotherdifference in between prokaryotes and eukaryotes is that prokaryotes cells oftencontain one or much more plasmids (i.e., extrachromosomal DNA molecules that areeither direct or circular). These piece of DNA different from chromosomes in thatthey are typically smaller and also encode nonessential genes, such as those thataid growth in particular conditions or encode antibiotic resistance. Borrelia, because that instance, includes morethan 20 circular and also linear plasmids the encode gene responsible forinfecting ticks and also humans (Fraser et al.,1997). Plasmids are regularly much smaller sized than chromosomes (i.e., much less than 1,500kilobases), and also they replicate independently of the rest of the genome.However, part plasmids are qualified of integrating into chromosomes or movingfrom cell to cell.

Perhaps due to thespace limit of pack so countless essential genes onto a solitary chromosome,prokaryotes have the right to be highly efficient in terms of genomic organization. Verylittle an are is left between prokaryotic genes. As a result, noncodingsequences account for an typical of 12% of the prokaryotic genome, as opposedto upwards the 98% that the genetic material in standard scale (Ahnert et al., 2008). Furthermore, unlikeeukaryotic chromosomes, many prokaryotic genomes are organized into polycistronicoperons, or clusters of an ext than one coding an ar attached to a singlepromoter,separated by just a few base pairs. The proteins encoded by every operon oftencollaborate ~ above a solitary task, such together the metabolism of a sugar into by-productsthat deserve to be provided for energy (Figure 3).

Three structure genes code for proteins connected in lactose import and metabolism in bacteria. The genes are organized together in a cluster called the lac operon.
lac operon.", "Figure 3", "Three structural genes password for proteins associated in lactose import and metabolism in bacteria. The genes are arranged together in a cluster dubbed the lac operon.", "627","", "The lac operon in bacteria consists of a promoter, an operator, and three structural genes. This regions occur in a details arrangement, and, in the diagram, the operon is gift in order indigenous left to right. The adhering to regions room shown: first the promoter in red, then the operator in yellow, and also finally the three structural genes, which include beta-galactosidase in blue, beta-galactoside permease in dark pink, and also beta-galactoside transacetylase in orange. Added regulatory sequences are situated to the left of the promoter in the region, upstream the the lac operon.")" class="inlineLinks"> Figure detail

The organization ofprokaryotic DNA therefore differs from the of standard scale in numerous importantways. The many notable difference is the condensation process that prokaryoticDNA molecules experience in order come fit inside reasonably small cells. Otherdifferences, while no as dramatic, space summarized in Table 1.

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Table 1: prokaryotic versusEukaryotic Chromosomes

Prokaryotic ChromosomesEukaryotic Chromosomes many prokaryotes save a solitary circular chromosome. prokaryotes chromosomes room condensed in the nucleoid via DNA supercoiling and also the binding of miscellaneous architectural proteins. due to the fact that prokaryotic DNA can connect with the cytoplasm, transcription and translation take place simultaneously. most prokaryotes contain only one copy of every gene (i.e., they space haploid). Nonessential prokaryotic genes are frequently encoded on extrachromosomal plasmids. prokaryotic genomes room efficient and also compact, containing little repetitive DNA.

eukaryotes contain multiple linear chromosomes. eukaryotic chromosomes room condensed in a membrane-bound cell core via histones. In eukaryotes, warrior occurs in the nucleus, and also translation occurs in the cytoplasm. most eukaryotes save two copies of every gene (i.e., they space diploid). part eukaryotic genomes room organized into operons, but most space not. Extrachromosomal plasmids room not generally present in eukaryotes. standard scale contain huge amounts that noncoding and also repetitive DNA.

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