Dye penetrant inspection
Dye penetrant inspection called liquid penetrate inspection or penetrant testing, is a applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials. The penetrant may be applied to all non-ferrous materials and ferrous materials, although for ferrous components magnetic-particle inspection is used instead for its subsurface detection capability. LPI is used to detect casting and welding surface defects such as hairline cracks, surface porosity, leaks in new products, fatigue cracks on in-service components; the oil and whiting method used in the railroad industry in the early 1900s was the first recognized use of the principles of penetrants to detect cracks. The oil and whiting method used an oil solvent for cleaning followed by the application of a whiting or chalk coating, which absorbed oil from the cracks revealing their locations. Soon a dye was added to the liquid. By the 1940s, fluorescent or visible dye was added to the oil used to penetrate test objects.
Experience showed that soak time were important. This started the practice of written instructions to provide uniform results; the use of written procedures has evolved, giving the ability for design engineers and manufacturers to get the high standard results from any properly trained and certified liquid penetrant testing technician. DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, brushing. After adequate penetration time has been allowed, the excess penetrant is removed and a developer is applied; the developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending on the type of dye used - fluorescent or nonfluorescent. Below are the main steps of Liquid Penetrant Inspection: 1. Pre-cleaning: The test surface is cleaned to remove any dirt, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications.
Cleaning methods may include alkaline cleaning steps, vapor degreasing, or media blasting. The end goal of this step is a clean surface where any defects present are open to the surface and free of contamination. Note that if media blasting is used, it may "work over" small discontinuities in the part, an etching bath is recommended as a post-blasting treatment. 2. Application of Penetrant: The penetrant is applied to the surface of the item being tested; the penetrant is a brilliant coloured mobile fluid with high wetting capability. The penetrant is allowed "dwell time" to soak into any flaws; the dwell time depends upon the penetrant being used, material being tested and the size of flaws sought. As expected, smaller flaws require a longer penetration time. Due to their incompatible nature one must be careful not to apply solvent-based penetrant to a surface, to be inspected with a water-washable penetrant. 3. Excess Penetrant Removal: The excess penetrant is removed from the surface; the removal method is controlled by the type of penetrant used.
Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are the common choices. Emulsifiers represent the highest sensitivity level, chemically interact with the oily penetrant to make it removable with a water spray; when using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because this can remove the penetrant from the flaws. If excess penetrant is not properly removed, once the developer is applied, it may leave a background in the developed area that can mask indications or defects. In addition, this may produce false indications hindering the ability to do a proper inspection; the removal of excessive penetrant is done towards one direction either vertically or horizontally as the case may be. 4. Application of Developer: After excess penetrant has been removed, a white developer is applied to the sample. Several developer types are available, including: non-aqueous wet developer, dry powder, water-suspendable, water-soluble.
Choice of developer is governed by penetrant compatibility, by inspection conditions. When using non-aqueous wet developer or dry powder, the sample must be dried prior to application, while soluble and suspendable developers are applied with the part still wet from the previous step. NAWD is commercially available in aerosol spray cans, may employ acetone, isopropyl alcohol, or a propellant, a combination of the two. Developer should form a semi-transparent coating on the surface; the developer draws penetrant from defects out onto the surface to form a visible indication known as bleed-out. Any areas that bleed out can indicate the location and possible types of defects on the surface. Interpreting the results and characterizing defects from the indications found may require some training and/or experience. 5. Inspection: The inspector will use visible light with adequate intensity for visible dye penetrant. Ultraviolet radiation of adequate intensity, along with low ambient light levels for fluorescent penetrant examinations.
Inspection of the test surface should take place after 10- to 30-minute development time
Kinase
In biochemistry, a kinase is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the substrate gains a phosphate group and the high-energy ATP molecule donates a phosphate group; this transesterification produces a phosphorylated substrate and ADP. Conversely, it is referred to as dephosphorylation when the phosphorylated substrate donates a phosphate group and ADP gains a phosphate group; these two processes and dephosphorylation, occur four times during glycolysis. Kinases are part of the larger family of phosphotransferases. Kinases should not be confused with phosphorylases, which catalyze the addition of inorganic phosphate groups to an acceptor, nor with phosphatases, which remove phosphate groups; the phosphorylation state of a molecule, whether it be a protein, lipid, or carbohydrate, can affect its activity and its ability to bind other molecules. Therefore, kinases are critical in metabolism, cell signalling, protein regulation, cellular transport, secretory processes, many other cellular pathways, which makes them important to human physiology.
Kinases mediate the transfer of a phosphate moiety from a high energy molecule to their substrate molecule, as seen in the figure below. Kinases are needed to stabilize this reaction because the phosphoanhydride bond contains a high level of energy. Kinases properly orient their substrate and the phosphoryl group within their active sites, which increases the rate of the reaction. Additionally, they use positively charged amino acid residues, which electrostatically stabilize the transition state by interacting with the negatively charged phosphate groups. Alternatively, some kinases utilize bound metal cofactors in their active sites to coordinate the phosphate groups. Protein kinases can be classed as catalytically active or as pseudokinases, reflecting the evolutionary loss of one or more of the catalytic amino acids that position or hydrolyse ATP. However, in terms of signalling outputs and disease relevance, both kinases and pseudokinases are important signalling modulators in human cells, making kinases important drug targets.
Kinases are used extensively to regulate complex processes in cells. Phosphorylation of molecules can enhance or inhibit their activity and modulate their ability to interact with other molecules; the addition and removal of phosphoryl groups provides the cell with a means of control because various kinases can respond to different conditions or signals. Mutations in kinases that lead to a loss-of-function or gain-of-function can cause cancer and disease in humans, including certain types of leukemia and neuroblastomas, spinocerebellar ataxia, forms of agammaglobulinaemia, many others; the first protein to be recognized as catalyzing the phosphorylation of another protein using ATP was observed in 1954 by Gene Kennedy at which time he described a liver enzyme that catalyzed the phosphorylation of casein. In 1956, Edmond H. Fischer and Edwin G. Krebs discovered that the interconversion between phosphorylase a and phosphorylase b was mediated by phosophorylation and dephosphorylation; the kinase that transferred a phosphoryl group to Phosphorylase b, converting it to Phosphorylase a, was named Phosphorylase Kinase.
Years the first example of a kinase cascade was identified, whereby Protein Kinase A phosphorylates Phosphorylase Kinase. At the same time, it was found that PKA inhibited glycogen synthase, the first example of a phosphorylation event that resulted in inhibition. In 1969, Lester Reed discovered that pyruvate dehydrogenase was inactivated by phosphorylation, this discovery was the first clue that phosphorylation might serve as a means of regulation in other metabolic pathways besides glycogen metabolism. In the same year, Tom Langan discovered that PKA phosphorylates histone H1, which suggested phosphorylation might regulate nonenzymatic proteins; the 1970s included the discovery of calmodulin-dependent protein kinases and the finding that proteins can be phosphorylated on more than one amino acid residue. The 1990s may be described as the "decade of protein kinase cascades". During this time, the MAPK/ERK pathway, the JAK kinases, the PIP3-dependent kinase cascade were discovered. Kinases are classified into broad groups by the substrate they act upon: protein kinases, lipid kinases, carbohydrate kinases.
Kinases can be found from bacteria to mold to worms to mammals. More than five hundred different kinases have been identified in humans, their diversity and their role in signaling makes them an interesting object of study. Various other kinases act on small molecules such as lipids, amino acids, nucleotides, either for signaling or to prime them for metabolic pathways. Specific kinases are named after their substrates. Protein kinases have multiple substrates, proteins can serve as substrates for more than one specific kinase. For this reason protein kinases are named based on. Sometimes they are further subdivided into categories. For example, type I and type II cyclic-AMP dependent protein kinases have identical catalytic subunits but different regulatory subunits that bind cyclic AMP. Protein kinases act on proteins, by phosphorylating them on their serine, tyrosine, or histidine residues. Phosphorylation can modify the function of a protein in many ways, it can increase or decrease a protein's activity, stabilize
Escherichia virus T4
Escherichia virus T4 is a species of bacteriophages that infect Escherichia coli bacteria. It is a member of virus subfamily Tevenvirinae and includes among other strains Enterobacteria phage T2, Enterobacteria phage T4 and Enterobacteria phage T6. T4 is capable of undergoing only a lytic lifecycle and not the lysogenic lifecycle; the T4 virus's double-stranded DNA genome encodes 289 proteins. The T4 genome is terminally redundant and is first replicated as a unit several genomic units are recombined end-to-end to form a concatemer; when packaged, the concatemer is cut at unspecific positions of the same length, leading to several genomes that represent circular permutations of the original. The T4 genome bears eukaryote-like intron sequences; the Shine-Dalgarno sequence GAGG dominates in virus T4 early genes, whereas the sequence GGAG is a target for the T4 endonuclease RegB that initiates the early mRNA degradation. T4 is a large virus, at 90 nm wide and 200 nm long; the DNA genome is held in an icosahedral head known as a capsid.
The T4’s tail is hollow so that it can pass its nucleic acid into the cell it is infecting after attachment. The tail attaches to a host cell with the help of tail fibres; the tail fibres are important in recognizing host cell surface receptors, so they determine if a bacterium is within the virus's host range. The structure of the 6 megadalton T4 baseplate that comprises 127 polypeptide chains of 13 different proteins has been described in atomic detail. An atomic model of the proximal region of the tail tube formed by gp54 and the main tube protein gp19 have been created; the tape measure protein gp29 is present in the baseplate-tail tube complexes, but it could not be modeled. The T4 virus initiates an Escherichia coli infection by binding OmpC porin proteins and lipopolysaccharide on the surface of E. coli cells with its long tail fibers. A recognition signal is sent through the LTFs to the baseplate; this unravels the short tail fibers. The baseplate changes conformation and the tail sheath contracts, causing GP5 at the end of the tail tube to puncture the outer membrane of the cell.
The lysozyme domain of GP5 degrades the periplasmic peptidoglycan layer. The remaining part of the membrane is degraded and DNA from the head of the virus can travel through the tail tube and enter the E. coli cell. The lytic lifecycle takes 30 minutes and consists of: Adsorption and penetration Arrest of host gene expression Enzyme synthesis DNA replication Formation of new virus particles After the life cycle is complete, the host cell bursts open and ejects the newly built viruses into the environment, destroying the host cell. T4 has a burst size of 100-150 viral particles per infected host. Complementation and recombination tests can be used to map out the rII gene locus by using T4; these Escherichia viruses infect a host cell with their information and blow up the host cell, thereby propagating themselves. Virus T4 genome is synthesized within the host cell using Rolling Circle Replication; the time it takes for DNA replication in a living cell was measured as the rate of virus T4 DNA elongation in virus-infected E. coli.
During the period of exponential DNA increase at 37 °C, the rate was 749 nucleotides per second. The mutation rate per base pair per replication during virus T4 DNA synthesis is 1.7 per 10−8, a accurate DNA copying mechanism, with only 1 error in 300 copies. The virus codes for unique DNA repair mechanisms; the T4 DNA packaging motor has been found to load DNA into virus capsids at a rate up to 2000 base pairs per second. The power involved, if scaled up in size, would be equivalent to that of an average automobile engine. Multiplicity reactivation is the process by which two or more virus genomes, each containing inactivating genome damage, can interact within an infected cell to form a viable virus genome. Salvador Luria, while studying UV irradiated virus T4 in 1946, discovered MR and proposed that the observed reactivation of damaged virus occurs by a recombination mechanism; this preceded the confirmation of DNA as the genetic material in 1952 in related virus T2 by the Hershey–Chase experiment.
As remembered by Luria the discovery of reactivation of irradiated virus started a flurry of activity in the study of repair of radiation damage within the early phage group. It turned out that the repair of damaged virus by mutual help that Luria had discovered was only one special case of DNA repair. Cells of all types, not just and their viruses, but all organisms studied, including humans, are now known to have complex biochemical processes for repairing DNA damages. DNA repair processes are now recognized as playing critical roles in protecting against aging and infertility. MR is represented by "survival curves" where survival of plaque forming ability of multiply infected cells is plotted against dose of genome damaging agent. For comparison, the survival of virus plaque forming ability of singly infected cells is plotted against dose of genome damaging agent; the top figure shows the survival curves for v
Subventricular zone
The subventricular zone is both embryonic and adult neural tissues in the vertebrate central nervous system. In embryonic life, the SVZ refers to a secondary proliferative zone containing neural progenitor cells, which divide to produce neurons in the process of neurogenesis; the primary neural stem cells of the brain and spinal cord, termed radial glial cells, reside in the ventricular zone. In the developing cerebral cortex, which resides in the dorsal telencephalon, the SVZ and VZ are transient tissues that do not exist in the adult. However, the SVZ of the ventral telencephalon persists throughout life; the adult SVZ is a paired brain structure situated throughout the lateral walls of the lateral ventricles. It is composed of four distinct layers of variable thickness and cell density, as well as cellular composition. Along with the dentate gyrus of the hippocampus, the SVZ is one of two places where neurogenesis has been found to occur in the adult mammalian brain; the innermost layer contains a single layer of ependymal cells lining the ventricular cavity.
These expansions may interact intimately with the astrocytic processes that are interconnected with the hypocellular layer. The secondary layer provides for a hypocellular gap abutting the former and has been shown to contain a network of functionally correlated Glial Fibrillary Acid Protein -positive astrocytic processes that are linked to junctional complexes, yet lack cell bodies except for the rare neuronal somata. While the function of this layer is yet unknown in humans, it has been hypothesized that the astrocytic and ependymal interconnections of Layer I and II may act to regulate neuronal functions, establish metabolic homeostasis, and/or control neuronal stem cell proliferation and differentiation during development; such characteristics of the layer may act as a remainder of early developmental life or pathway for cellular migration given similarity to a homologous layer in bovine SVZ shown to have migratory cells common only to higher order mammals. The third layer forms a ribbon of astrocyte cell bodies that are believed to maintain a subpopulation of astrocytes able to proliferate in vivo and form multipotent neurospheres with self-renewal abilities in vitro.
While some oligodendrocytes and ependymal cells have been found within the ribbon, they not only serve an unknown function, they are uncommon by comparison to the population of astrocytes that reside in the layer. The astrocytes present in Layer III can be divided into three populations through electron microscopy, with no unique functions yet recognizable; the fourth and final layer serves as a transition zone between Layer III with its ribbon of astrocytes and the brain parenchyma. It is identified by a high presence of myelin in the region. Four cell types are described in the SVZ:1. Ciliated Ependymal Cells: are positioned facing the lumen of the ventricle, function to circulate the cerebrospinal fluid. 2. Proliferating Neuroblasts: express PSA-NCAM, Tuj1, Hu, migrate in line order to the Olfactory Bulb 3. Slow Proliferating Cells: express Nestin and GFAP, function to ensheathe migrating Type A Neuroblasts4. Proliferating Cells or Transit Amplifying Progenitors: express Nestin, form clusters interspaced among chains throughout region The SVZ is a known site of neurogenesis and self-renewing neurons in the adult brain, serving as such due to the interacting cell types, extracellular molecules, localized epigenetic regulation promoting such cellular proliferation.
Along with the subgranular zone of the dentate gyrus, the subventricular zone serves as a source of neural stem cells in the process of adult neurogenesis. It harbors the largest population of proliferating cells in the adult brain of rodents and humans. In 2010, it was shown that the balance between neural stem cells and neural progenitor cells is maintained by an interaction between the epidermal growth factor receptor signaling pathway and the Notch signaling pathway. While it has yet to have been studied in-depth in the human brain, the SVZ function in the rodent brain has been, to a certain extent and defined for its abilities. With such research, it has been found that the dual-functioning astrocyte is the dominant cell in the rodent SVZ; this function is induced by microglia and endothelial cells that interact cooperatively with neuronal stem cells to promote neurogenesis in vitro, as well as extracellular matrix components such as tenascin-C and Lewis X. The human SVZ is different, from the rodent SVZ in two distinct ways.
Operation Crossroads
Operation Crossroads was a pair of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. They were the first nuclear weapon tests since Trinity in July 1945, the first detonations of nuclear devices since the atomic bombing of Nagasaki on August 9, 1945; the purpose of the tests was to investigate the effect of nuclear weapons on warships. The Crossroads tests were the first of many nuclear tests held in the Marshall Islands, the first to be publicly announced beforehand and observed by an invited audience, including a large press corps, they were conducted by Joint Army/Navy Task Force One, headed by Vice Admiral William H. P. Blandy rather than by the Manhattan Project, which had developed nuclear weapons during World War II. A fleet of 95 target ships was assembled in Bikini Lagoon and hit with two detonations of Fat Man plutonium implosion-type nuclear weapons of the kind dropped on Nagasaki, each with a yield of 23 kilotons of TNT; the first test was Able. The bomb was named Gilda after Rita Hayworth's character in the 1946 film Gilda, was dropped from the B-29 Superfortress Dave's Dream of the 509th Bombardment Group on July 1, 1946.
It detonated 520 feet above the target fleet and caused less than the expected amount of ship damage because it missed its aim point by 2,130 feet. The second test was Baker; the bomb was known as Helen of Bikini and was detonated 90 feet underwater on July 25, 1946. Radioactive sea spray caused extensive contamination. A third deep-water test named Charlie was planned for 1947 but was canceled because of the United States Navy's inability to decontaminate the target ships after the Baker test. Only nine target ships were able to be scrapped rather than scuttled. Charlie was rescheduled as Operation Wigwam, a deep-water shot conducted in 1955 off the California coast. Bikini's native residents agreed to evacuate the island, were evacuated on board the LST-861, with most moving to the Rongerik Atoll. In the 1950s, a series of large thermonuclear tests rendered Bikini unfit for subsistence farming and fishing because of radioactive contamination. Bikini remains uninhabited as of 2015, though it is visited by sport divers.
Planners attempted to protect participants in the Operation Crossroads tests against radiation sickness, but one study showed that the life expectancy of participants was reduced by an average of three months. The Baker test's radioactive contamination of all the target ships was the first case of immediate, concentrated radioactive fallout from a nuclear explosion. Chemist Glenn T. Seaborg, the longest-serving chairman of the Atomic Energy Commission, called Baker "the world's first nuclear disaster." The first proposal to test nuclear weapons against naval warships was made on August 16, 1945, by Lewis Strauss, future chairman of the Atomic Energy Commission. In an internal memo to Secretary of the Navy James Forrestal, Strauss argued, "If such a test is not made, there will be loose talk to the effect that the fleet is obsolete in the face of this new weapon and this will militate against appropriations to preserve a postwar Navy of the size now planned." With few bombs available, he suggested a large number of targets dispersed over a large area.
A quarter century earlier, in 1921, the Navy had suffered a public relations disaster when General Billy Mitchell's bombers sank every target ship the Navy provided for the Project B ship-versus-bomb tests. The Strauss test would be designed to demonstrate ship survivability. Nine days Senator Brien McMahon, who within a year would write the Atomic Energy Act and organize and chair the Congressional Joint Committee on Atomic Energy, made the first public proposal for such a test, but one designed to demonstrate the vulnerability, rather than survivability, of ships, he proposed dropping an atomic bomb on captured Japanese ships and suggested, "The resulting explosion should prove to us just how effective the atomic bomb is when used against the giant naval ships." On September 19, the Chief of the United States Army Air Forces, General of the Army Henry H. Arnold, asked the Navy to set aside ten of the thirty-eight captured Japanese ships for use in the test proposed by McMahon. Meanwhile, the Navy proceeded with its own plan, revealed at a press conference on October 27 by the Commander in Chief, United States Fleet, Fleet Admiral Ernest King.
It involved between 80 and 100 target ships, most of them surplus U. S. ships. As the Army and the Navy maneuvered for control of the tests, Assistant Secretary of War Howard C. Peterson observed, "To the public, the test looms as one in which the future of the Navy is at stake... if the Navy withstands better than the public imagines it will, in the public mind the Navy will have'won.'"The Army's candidate to direct the tests, Major General Leslie Groves, head of the Manhattan Project which built the bombs, did not get the job. The Joint Chiefs of Staff decided that because the Navy was contributing the most men and materiel, the test should be headed by a naval officer. Commodore William S. "Deak" Parsons was a naval officer who had worked on the Manhattan Project and participated in the bombing of Hiroshima. He was now the assistant to the Deputy Chief of Naval Operations for Special Weapons, Vice Admiral William H. P. Blandy, whom he proposed for the role; this recommendation was accepted, on January 11, 1946, President Harry S. Truman appointed Blandy as head of Army/Navy Joint Task Force One, created to conduct the tests.
Parsons became Deputy Task Force Commander for Technical Direction. USAAF Major General William E. Kepner was Deputy Task Force Commander for Aviation. Blandy codenamed the tests Operation Crossroads. Under pressure from the Army, Blandy agreed to cro
Escherichia coli
Escherichia coli known as E. coli, is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus Escherichia, found in the lower intestine of warm-blooded organisms. Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in their hosts, are responsible for product recalls due to food contamination. The harmless strains are part of the normal microbiota of the gut, can benefit their hosts by producing vitamin K2, preventing colonization of the intestine with pathogenic bacteria, having a symbiotic relationship. E. coli is expelled into the environment within fecal matter. The bacterium grows massively in fresh fecal matter under aerobic conditions for 3 days, but its numbers decline afterwards. E. Coli and other facultative anaerobes constitute about 0.1% of gut microbiota, fecal–oral transmission is the major route through which pathogenic strains of the bacterium cause disease. Cells are able to survive outside the body for a limited amount of time, which makes them potential indicator organisms to test environmental samples for fecal contamination.
A growing body of research, has examined environmentally persistent E. coli which can survive for extended periods outside a host. The bacterium can be grown and cultured and inexpensively in a laboratory setting, has been intensively investigated for over 60 years. E. coli is a chemoheterotroph whose chemically defined medium must include a source of carbon and energy. E. coli is the most studied prokaryotic model organism, an important species in the fields of biotechnology and microbiology, where it has served as the host organism for the majority of work with recombinant DNA. Under favorable conditions, it takes up to 20 minutes to reproduce. E. coli is a facultative anaerobic and nonsporulating bacterium. Cells are rod-shaped, are about 2.0 μm long and 0.25–1.0 μm in diameter, with a cell volume of 0.6–0.7 μm3. E. Coli stains Gram-negative because its cell wall is composed of a thin peptidoglycan layer and an outer membrane. During the staining process, E. coli picks up the color of the counterstain safranin and stains pink.
The outer membrane surrounding the cell wall provides a barrier to certain antibiotics such that E. coli is not damaged by penicillin. Strains that possess flagella are motile; the flagella have a peritrichous arrangement. It attaches and effaces to the microvilli of the intestines via an adhesion molecule known as intimin. E. coli can live on a wide variety of substrates and uses mixed-acid fermentation in anaerobic conditions, producing lactate, ethanol and carbon dioxide. Since many pathways in mixed-acid fermentation produce hydrogen gas, these pathways require the levels of hydrogen to be low, as is the case when E. coli lives together with hydrogen-consuming organisms, such as methanogens or sulphate-reducing bacteria. Optimum growth of E. coli occurs at 37 °C, but some laboratory strains can multiply at temperatures up to 49 °C. E. coli grows in a variety of defined laboratory media, such as lysogeny broth, or any medium that contains glucose, ammonium phosphate monobasic, sodium chloride, magnesium sulfate, potassium phosphate dibasic, water.
Growth can be driven by aerobic or anaerobic respiration, using a large variety of redox pairs, including the oxidation of pyruvic acid, formic acid and amino acids, the reduction of substrates such as oxygen, fumarate, dimethyl sulfoxide, trimethylamine N-oxide. E. coli is classified as a facultative anaerobe. It uses oxygen when it is available, it can, continue to grow in the absence of oxygen using fermentation or anaerobic respiration. The ability to continue growing in the absence of oxygen is an advantage to bacteria because their survival is increased in environments where water predominates; the bacterial cell cycle is divided into three stages. The B period occurs between the beginning of DNA replication; the C period encompasses the time it takes to replicate the chromosomal DNA. The D period refers to the stage between the conclusion of DNA replication and the end of cell division; the doubling rate of E. coli is higher. However, the length of the C and D periods do not change when the doubling time becomes less than the sum of the C and D periods.
At the fastest growth rates, replication begins before the previous round of replication has completed, resulting in multiple replication forks along the DNA and overlapping cell cycles. E. coli and related bacteria possess the ability to transfer DNA via bacterial conjugation or transduction, which allows genetic material to spread horizontally through an existing population. The process of transduction, which uses the bacterial virus called a bacteriophage, is where the spread of the gene encoding for the Shiga toxin from the Shigella bacteria to E. coli helped produce E. coli O157:H7, the Shiga toxin-producing strain of E. coli. E. coli encompasses an enormous population of bacteria that exhibit a high degree of both genetic and phenotypic diversity. Genome sequencing of a large number of isolates of E. coli and related bacteria shows that a taxonomic reclassification would be desirable. However, this has not been done due to its medical importance, E. coli remains one of the most diverse bacterial species: only 20% of the genes in a typical E. coli genome is shared among all strains.
In fact, from the evolutionary point of view, the members of genus Shigella (S. dysenteriae, S. fle
