Chromosome consists of double—stranded circular DNA. Prokaryotes do not contain nucleus or other membrane bounded organelles. The term "prokaryotes" actually means "before nucleus". Chromosome is stored in a special area called nucleoid. The genome of prokaryotes is often significantly larger than the cell itself. How is it possible that the genetic information does fit into the cell? When a prokaryotic cell with a plasmid divides, the daughter cells each receive a copy of the plasmid, along with its regular chromosome.
See more from our free eBook library. An article from Khan Academy about binary fission in bacteria. Eukaryotic Chromosomes. Eukaryotic vs. Prokaryotic Chromosomes. DNA Structure. When you select "Subscribe" you will start receiving our email newsletter. 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. Prokaryotic and eukaryotic chromosomes: What's the difference? Bioessays 22 , — Bradley, M. Effects of Fis on Escherichia coli gene expression during different growth stages. Microbiology , — Cairns, J. The chromosome of Escherichia coli.
Intracellular location of the histonelike protein HU in Escherichia coli. Journal of Bacteriology , — Endy, D. Modelling cellular behaviour. As the RNA is being transcribed, ribosomes can begin the translation process of stringing together amino acids. In contrast, in eukaryotic cells, transcription always occurs first, and it takes place within the nucleus.
The RNA molecule needs to undergo editing before it leaves the nucleus. Then, translation is conducted by a ribosome in the cytoplasm.
In general, eukaryotic cells contain a lot more genetic material than prokaryotic cells. For example, each human cell has around 2m, or 3 billion base pairs, of DNA that must be compacted to fit within the nucleus. In eukaryotic cells, chromatin consists of all the DNA within the nucleus and its associated proteins, called histones.
Eukaryotic chromosomes are composed of chromatin, and each consists of two complementary strands of DNA coiled tightly around histones. The structure of chromatin is scaffolded, with three distinct levels. Then, the linked nucleosomes undergo supercoiling. Like beads on a string, the connected nucleosomes loop around each other to form nm solenoid fiber.
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