Where is prokaryotic dna stored

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This arrangement is not as simple as it sounds, however, especially considering that the E. So, if bacterial chromosomes are so huge, how can they fit comfortably inside a cell—much less in one small corner of the cell?

The answer to this question lies in DNA packaging. Whereas eukaryotes wrap their DNA around proteins called histones to help package the DNA into smaller spaces, most prokaryotes do not have histones with the exception of those species in the domain Archaea. Thus, one way prokaryotes compress their DNA into smaller spaces is through supercoiling Figure 1. Imagine twisting a rubber band so that it forms tiny coils. Now twist it even further, so that the original coils fold over one another and form a condensed ball.

When this type of twisting happens to a bacterial genome , it is known as supercoiling. Genomes can be negatively supercoiled, meaning that the DNA is twisted in the opposite direction of the double helix , or positively supercoiled, meaning that the DNA is twisted in the same direction as the double helix.

Most bacterial genomes are negatively supercoiled during normal growth. During the s and s, researchers discovered that multiple proteins act together to fold and condense prokaryotic DNA. In particular, one protein called HU, which is the most abundant protein in the nucleoid, works with an enzyme called topoisomerase I to bind DNA and introduce sharp bends in the chromosome, generating the tension necessary for negative supercoiling.

Recent studies have also shown that other proteins, including integration host factor IHF , can bind to specific sequences within the genome and introduce additional bends Rice et al. One of these maintenance proteins, H-NS, plays an active role in transcription by modulating the expression of the genes involved in the response to environmental stimuli. Another maintenance protein, factor for inversion stimulation FIS , is abundant during exponential growth and regulates the expression of more than genes, including DNA topoisomerase I Bradley et al.

Supercoiling explains how chromosomes fit into a small corner of the cell, but how do the proteins involved in replication and transcription access the thousands of genes in prokaryotic chromosomes when everything is packaged together so tightly? It has been determined that prokaryotic DNA replication occurs at a rate of 1, nucleotides per second, and prokaryotic transcription occurs at a rate of about 40 nucleotides per second Lewin, , so bacteria must have highly efficient methods of accessing their DNA strands.

But how? Researchers have noted that the nucleoid usually appears as an irregularly shaped mass within the prokaryotic cell, but it becomes spherical when the cell is treated with chemicals to inhibit transcription or translation.

Moreover, during transcription, small regions of the chromosome can be seen to project from the nucleoid into the cytoplasm i. These projections are thought to explain the mysterious shape of nucleoids during active growth. When transcription is inhibited, however, the projections retreat into the nucleoid, forming the aforementioned spherical shape.

Because there is no nuclear membrane to separate prokaryotic DNA from the ribosomes within the cytoplasm, transcription and translation occur simultaneously in these organisms. This is strikingly different from eukaryotic chromosomes, which are confined to the membrane-bound nucleus during most of the cell cycle. In eukaryotes, transcription must be completed in the nucleus before the newly synthesized mRNA molecules can be transported to the cytoplasm to undergo translation into proteins.

Recently, it has become apparent that one size does not fit all when it comes to prokaryotic chromosome structure. While most prokaryotes, like E. For example, Vibrio cholerae , the bacteria that causes cholera, contains two circular chromosomes. One of these chromosomes contains the genes involved in metabolism and virulence , while the other contains the remaining essential genes Trucksis et al. An even more extreme example is provided by Borrelia burgdorferi , the bacterium that causes Lyme disease.

Unlike E. Other organisms, such as Bacillus subtilis , form nucleoids that closely resemble those of E. Furthermore, the DNA molecules of Archaea, a taxonomic domain composed of single-celled, nonbacterial prokaryotes that share many similarities with eukaryotes, can be negatively supercoiled, positively supercoiled, or not supercoiled at all. It is important to note that archaeans are the only group of prokaryotes that use eukaryote-like histones, rather than the architectural proteins described above, to condense their DNA molecules Sandman et al.

The acquisition of histones by archaeans is thought to have paved the way for the evolution of larger and more complex eukaryotic cells Minsky et al. Most prokaryotes reproduce asexually and are haploid , meaning that only a single copy of each gene is present. This makes it relatively easy to generate mutations in the lab and study the resulting phenotypes.

By contrast, eukaryotes that reproduce sexually generally contain multiple chromosomes and are said to be diploid , because two copies of each gene exist—with one copy coming from each of an organism's parents. Yet another difference between prokaryotes and eukaryotes is that prokaryotic cells often contain one or more plasmids i. These pieces of DNA differ from chromosomes in that they are typically smaller and encode nonessential genes, such as those that aid growth in specific conditions or encode antibiotic resistance.

Borrelia , for instance, contains more than 20 circular and linear plasmids that encode genes responsible for infecting ticks and humans Fraser et al. Plasmids are often much smaller than chromosomes i. However, some plasmids are capable of integrating into chromosomes or moving from cell to cell. Perhaps due to the space constraints of packing so many essential genes onto a single chromosome, prokaryotes can be highly efficient in terms of genomic organization.

Very little space is left between prokaryotic genes. Furthermore, unlike eukaryotic chromosomes, most prokaryotic genomes are organized into polycistronic operons, or clusters of more than one coding region attached to a single promoter , separated by only a few base pairs. The proteins encoded by each operon often collaborate on a single task, such as the metabolism of a sugar into by-products that can be used for energy Figure 3.

The organization of prokaryotic DNA therefore differs from that of eukaryotes in several important ways. The most notable difference is the condensation process that prokaryotic DNA molecules undergo in order to fit inside relatively small cells.

Other differences, while not as dramatic, are summarized in Table 1. Abbott, A. Lyme disease: Uphill struggle. Nature , — doi Ahnert, S. How much non-coding DNA do eukaryotes require? Journal of Theoretical Biology , — Bendich, A. Eukaryotes, having probably evolved from prokaryotes, have more complex traits in both cell and DNA organization.

Prokaryotic cells are known to be much less complex than eukaryotic cells since eukaryotic cells are considered to be present at a later point of evolution. It is probable that eukaryotic cells evolved from prokaryotic cells.

Differences in complexity can be seen at the cellular level. The single characteristic that is both necessary and sufficient to define an organism as a eukaryote is a nucleus surrounded by a nuclear envelope with nuclear pores. The composition of the cell wall differs significantly between the domains Bacteria and Archaea, the two domains of life into which prokaryotes are divided.

The composition of their cell walls also differs from the eukaryotic cell walls found in plants cellulose or fungi and insects chitin. Some bacteria have a capsule outside the cell wall. Other structures are present in some prokaryotic species, but not in others. For example, the capsule found in some species enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and increases its resistance to our immune responses.

Some species also have flagella used for locomotion and pili used for attachment to surfaces. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea. Domains of life : Bacteria and Archaea are both prokaryotes, but differ enough to be placed in separate domains. An ancestor of modern Archaea is believed to have given rise to Eukarya, the third domain of life.

Archaeal and bacterial phyla are shown; the evolutionary relationship between these phyla is still open to debate. The plasma membrane is a thin lipid bilayer 6 to 8 nanometers that completely surrounds the cell and separates the inside from the outside. Its selectively-permeable nature keeps ions, proteins, and other molecules within the cell, preventing them from diffusing into the extracellular environment, while other molecules may move through the membrane.

The general structure of a cell membrane is a phospholipid bilayer composed of two layers of lipid molecules. In archaeal cell membranes, isoprene phytanyl chains linked to glycerol replace the fatty acids linked to glycerol in bacterial membranes. Some archaeal membranes are lipid monolayers instead of bilayers. Plasma membrane structure : Archaeal phospholipids differ from those found in Bacteria and Eukarya in two ways.

First, they have branched phytanyl sidechains instead of linear ones. Second, an ether bond instead of an ester bond connects the lipid to the glycerol. The cytoplasm of prokaryotic cells has a high concentration of dissolved solutes. Therefore, the osmotic pressure within the cell is relatively high. The cell wall is a protective layer that surrounds some cells and gives them shape and rigidity. It is located outside the cell membrane and prevents osmotic lysis bursting due to increasing volume.