The region in the cell containing this genetic material is called a nucleoid remember that prokaryotes do not have a separate membrane-bound nucleus. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA.
Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange. The size of the genome in one of the most well-studied prokaryotes, E. So how does this fit inside a small bacterial cell?
The DNA is twisted by what is known as supercoiling. Supercoiled DNA is coiled more tightly than would be typically be found in a cell more than 10 nucleotides per twist of the helix. If you visualize twisting a rope until it twists back on itself, you have a pretty good visual of supercoiled DNA. This process allows the DNA to be compacted into the small space inside a bacteria. Eukaryotes have much more DNA than prokaryotes.
For example, an E. In eukaryotes such as humans and other animals, the genome consists of several double-stranded linear DNA molecules Figure 2 , which are located inside a membrane-bound nucleus. Each species of eukaryotes has a characteristic number of chromosomes in the nuclei plural of nucleus of its cells. A normal human gamete sperm or egg contains 23 chromosomes. A normal human body cell, or somatic cell, contains 46 chromosomes one set of 23 from the egg and one set of 23 from the sperm; Figure 2.
The letter n is used to represent a single set of chromosomes; therefore, a gamete sperm or egg is designated 1n , and is called a haploid cell. Somatic cells body cells are designated 2 n and are called diploid cells. Bettie J. Graham, Ph. Featured Content. Introduction to Genomics. Polygenic Risk Scores. PBCV-1 infection resembles infection by tailed bacteriophages because its genome must cross the cell wall and membrane of its host C.
The PBCV-1 spike first contacts the host cell wall [8] and the fibers aid in holding the virus to the wall. The spike is too thin to deliver DNA and so it probably serves to puncture the wall and is then jettisoned. Following expansion of the hole in the host wall by a virus-packaged enzyme s , the viral internal membrane presumably fuses with the host membrane, facilitating entry of the PBCV-1 DNA and virion-associated proteins into the cell, leaving an empty capsid attached to the surface [9].
An example of DNA packaging is the Its extended linear form of This value approaches the maximal theoretical density for DNA packaging and the DNA is almost at crystalline density inside the phage head [16]. DNA packaging density has implications for virus infection.
Experiments and theoretical calculations indicate that the high DNA packaging density in phages generates enormous internal pressure in the particles ranging up to 50 bars [19].
This pressure serves as a driving force for the rapid ejection of DNA from the virus particle. This pressure driven DNA ejection provides at least part of the energy required for transfer of the DNAs into their hosts [22].
Consequently, PBCV-1 may use similar mechanical forces to eject its genome into its host cell as phages [13]. In contrast, most NCLDVs are not faced with a cell wall and they initiate infection by either an endocytotic or an envelope fusion mechanism with the host plasma membrane; they then uncoat inside the cell.
In fact, when DNA is released from the vaccinia capsid it does not burst out but rather pours out like a thick fluid [17] , suggesting that forced ejection of vaccinia DNA is not important for its infection.
Phage DNA packaging depends primarily on two parameters, the function of motor proteins and cations. Evidence for charge neutralization of densely stored DNA in phages existed more than 50 years ago. While phages typically use cations to neutralize their DNA some phages use polyamines, such as putrescine and spermidine in addition to cations [24]. There is no evidence indicating basic proteins contribute much to neutralizing phage dsDNA genomes [25] , [26].
Polyomaviridae [28] and Papillomaviridae [29] for example can functionally co-opt host histone proteins. Other dsDNA viruses Adenoviridae , Asfarviridae , Baculoviridae express small arginine rich protamine-like proteins with putative DNA condensation functions [30] , [31].
Currently little information is available on the mode of DNA packaging in the large chloroviruses. This large number suggests that DNA binding proteins play a role in the organization and packaging of chlorovirus DNA genomes.
Phage DNA release is often triggered by an interaction between phage tails and host receptor. The host receptor for virus PBCV-1 is unknown, although circumstantial evidence suggests it is carbohydrate [33]. Under these conditions it occurs as if the DNA is released from the virus particle but not able to enter the host; as a consequence the particle is dynamically catapulted away from the host cell leaving an unraveled, quasi-linear DNA polymer tethered to the cell.
In some images it is possible to see the capsid at the end of the DNA thread projected away from the host data not shown. The reason for the release of DNA into the medium is not known. However we know from other studies that usually only one virus infects the host cell, while the remaining viruses are excluded [34]. The fact that DNA release into the medium is only apparent at very high m.
This release of DNA into the medium must be fast because it is possible to detect isolated DNA molecules already within 5 min post infection. Figure 1A shows a fluorescent image with a host cell and the unfolded DNA polymer from a virus particle. The DNA dye DAPI produces bright staining of the nucleus of the chlorella cell in the lower right part of the figure; in the upper left part of the figure the unraveled DNA from a virus capsid protrudes as a nearly linear structure from the cell.
In the case of phages it has been argued that the release of the genome can be explained on the basis of Brownian motion [35]. Brownian movement would not generate the sort of straight lines and would also be much slower [22]. Hence the present data stress the importance of an osmotic pressure in the dense environment of the capsid, which creates a driving force for DNA ejection. A: Fluorescence images of C. The incubation medium contained C.
The image shows a chlorella cell cc and the viral DNA molecule, which is propelled away from the alga cell. B: Magnification of the area indicated by the box in A. Inset: same area as in B with conventional light microscopy and phase contrast. C: same as in A but with two DNA bands projecting away from a chlorella cell cc.
E: Electron micrograph of viral DNA projecting away from host cell wall. From this hole two linear structures project towards the left side. The part marked in E is magnified in F and presented in artificial colors in order to highlight the linear structures projecting away from the cell wall hole. G: fluorescence intensity profile along DNA molecule between arrows in B. H: Histogram of distances between individual fluorescence maxima as in E from 30 ejected DNA molecules.
Frequently we observed two DNA strands under the same conditions, which projected away from the host at a common point of origin Fig.
Since the surface of a Chlorella cell is ca. Hence it is more likely that the two DNA polymers were not from separate viruses but from a single virus. This interpretation is supported by the electron microscopic images depicted in Figs.
These images show a C. From this location two linear structures project away from the host in an angular fashion. The combination of electron microscopic and fluorescent images suggests that PBCV-1 DNA might not in all cases enter its host initially by either of its termini. This scenario would suggest that packaging of the DNA in the virion differs from ejection because it is unlikely that DNA packaging begins in the middle of the genome.
Images of ejected DNA at higher magnifications indicate that the fluorescence associated with the DNA exhibits distinct maxima. The fluorescence signal alternates between high and low fluorescence intensity along an imaginary line Fig.
The locations of the intensity maxima coincide with structures, which occasionally can be seen with phase contrast in a light microscope inset Fig. This observation implies that the non-uniform fluorescence of DAPI staining is not caused by a preference of the dye to interact with A-T rich regions in the DNA but instead the intensity maxima are due to local concentrations of DNA.
Advances in live cell culture and microscopy in the twentieth century eventually allowed scientists to identify viruses. Advances in genetics dramatically improved the identification process. Capsid - The capsid is the protein shell that encloses the nucleic acid; with its enclosed nucleic acid, it is called the nucleocapsid. This shell is composed of protein organized in subunits known as capsomers. They are closely associated with the nucleic acid and reflect its configuration, either a rod-shaped helix or a polygon-shaped sphere.
The capsid has three functions: 1 it protects the nucleic acid from digestion by enzymes, 2 contains special sites on its surface that allow the virion to attach to a host cell, and 3 provides proteins that enable the virion to penetrate the host cell membrane and, in some cases, to inject the infectious nucleic acid into the cell's cytoplasm. Under the right conditions, viral RNA in a liquid suspension of protein molecules will self-assemble a capsid to become a functional and infectious virus.
Envelope - Many types of virus have a glycoprotein envelope surrounding the nucleocapsid. The envelope is composed of two lipid layers interspersed with protein molecules lipoprotein bilayer and may contain material from the membrane of a host cell as well as that of viral origin. The virus obtains the lipid molecules from the cell membrane during the viral budding process.
However, the virus replaces the proteins in the cell membrane with its own proteins, creating a hybrid structure of cell-derived lipids and virus-derived proteins. Many viruses also develop spikes made of glycoprotein on their envelopes that help them to attach to specific cell surfaces. Nucleic Acid - Just as in cells, the nucleic acid of each virus encodes the genetic information for the synthesis of all proteins. While the double-stranded DNA is responsible for this in prokaryotic and eukaryotic cells, only a few groups of viruses use DNA.
Most viruses maintain all their genetic information with the single-stranded RNA. There are two types of RNA-based viruses. In most, the genomic RNA is termed a plus strand because it acts as messenger RNA for direct synthesis translation of viral protein. A few, however, have negative strands of RNA.
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