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https://en.wikipedia.org/wiki/Habitat_destruction
https://prezi.com/iuzvykk0n17v/human-overpopulation-and-habitat-destruction/
A living fossil is an extant taxon that cosmetically resemble ancestral species known only from the fossil record. To be considered a living fossil, the fossil species must be old relative to the time of origin of the extant clade. Living fossils commonly are of species-poor lineages, but they need not be. While the body plan of a living fossil remains superficially similar, it is never the same species as the remote ancestors it resembles, because genetic drift would inevitably change its chromosomal structure.
Living fossils exhibit stasis (also called "bradytely") over geologically long time scales. Popular literature may wrongly claim that a "living fossil" has undergone no significant evolution since fossil times, with practically no molecular evolution or morphological changes. Scientific investigations have repeatedly discredited such claims.[1][2][3]
The minimal superficial changes to living fossils are mistakenly declared as an absence of evolution, but they are examples of stabilizing selection, which is an evolutionary process—and perhaps the dominant process of morphological evolution.[4]
https://en.wikipedia.org/wiki/Living_fossil
The Voluntary Human Extinction Movement (VHEMT[A]) is an environmental movement that calls for all people to abstain from reproduction in order to cause the gradual voluntary extinction of humankind. VHEMT supports human extinction primarily because, in the group's view, it would prevent environmental degradation. The group states that a decrease in the human population would prevent a significant amount of human-caused suffering. The extinctions of non-human species and the scarcity of resources required by humans are frequently cited by the group as evidence of the harm caused by human overpopulation.
VHEMT was founded in 1991 by Les U. Knight, an American activist who became involved in the American environmental movement in the 1970s and thereafter concluded that human extinction was the best solution to the problems facing the Earth's biosphere and humanity. Knight publishes the group's newsletter and serves as its spokesman. Although the group is promoted by a website and represented at some environmental events, it relies heavily on coverage from outside media to spread its message. Many commentators view its platform as unacceptably extreme, while endorsing the logic of reducing the rate of human reproduction. In response to VHEMT, some journalists and academics have argued that humans can develop sustainable lifestyles or can reduce their population to sustainable levels. Others maintain that, whatever the merits of the idea, the human reproductive drive will prevent humankind from ever voluntarily seeking extinction.
https://en.wikipedia.org/wiki/Voluntary_Human_Extinction_Movement
Human extinction is the hypothetical end of the human species due to either natural causes such as an asteroid impact or large-scale volcanism, or anthropogenic (human) causes, also known as omnicide. For the latter, some of the many possible contributors include climate change, global nuclear annihilation, biological warfare and ecological collapse. Other scenarios center on emerging technologies, such as advanced artificial intelligence, biotechnology, or self-replicating nanobots. Scientists say there is relatively low risk of near term human extinction due to natural causes.[2] The likelihood of human extinction through our own activities, however, is a current area of research and debate.
https://en.wikipedia.org/wiki/Human_extinction
Since the 1980s, decreases in amphibian populations, including population decline and localized mass extinctions, have been observed in locations all over the world. These declines are known as one of the most critical threats to global biodiversity.
Recent (2007) research[1][2] indicates the reemergence of varieties of chrytid fungi may account for a substantial fraction of the overall decline. A more recent (2018) paper[3] published in Science confirms this.
Several secondary causes may be involved, including other diseases, habitat destruction and modification, exploitation, pollution, pesticide use, introduced species, and ultraviolet-B radiation (UV-B). However, many of the causes of amphibian declines are still poorly understood, and the topic is currently a subject of much ongoing research. Calculations based on extinction rates suggest that the current extinction rate of amphibians could be 211 times greater than the background extinction rate and the estimate goes up to 25,000–45,000 times if endangered species are also included in the computation.[4]
Although scientists began observing reduced populations of several European amphibian species already in the 1950s, awareness of the phenomenon as a global problem and its subsequent classification as a modern-day mass extinction only dates from the 1980s. By 1993, more than 500 species of frogs and salamanders present on all five continents were in decline.
https://en.wikipedia.org/wiki/Decline_in_amphibian_populations
Several studies report a substantial decline in insect populations. Most commonly, the declines involve reductions in abundance, though in some cases entire species are going extinct. The declines are far from uniform. In some localities, there have been reports of increases in overall insect population, and some types of insects appear to be increasing in abundance across the world.
Some of the insects most affected include bees, butterflies, moths, beetles, dragonflies and damselflies. Anecdotal evidence has been offered of much greater apparent abundance of insects in the 20th century; recollections of the windscreen phenomenon are an example.[2]
Possible causes of the decline have been identified as habitat destruction, including intensive agriculture, the use of pesticides (particularly insecticides), urbanization, and industrialization; introduced species; and climate change.[3] Not all insect orders are affected in the same way; many groups are the subject of limited research, and comparative figures from earlier decades are often not available.
In response to the reported declines, increased insect related conservation measures have been launched. In 2018 the German government initiated an "Action Programme for Insect Protection",[4][5] and in 2019 a group of 27 British entomologists and ecologists wrote an open letter calling on the research establishment in the UK "to enable intensive investigation of the real threat of ecological disruption caused by insect declines without delay".[6]
https://en.wikipedia.org/wiki/Decline_in_insect_populations
Genetic pollution is a controversial[1][2] term for uncontrolled[3][4] gene flow into wild populations. It is defined as "the dispersal of contaminated altered genes from genetically engineered organisms to natural organisms, esp. by cross-pollination",[5] but has come to be used in some broader ways. It is related to the population genetics concept of gene flow, and genetic rescue, which is genetic material intentionally introduced to increase the fitness of a population.[6] It is called genetic pollution when it negatively impacts on the fitness of a population, such as through outbreeding depression and the introduction of unwanted phenotypes which can lead to extinction.
Conservation biologists and conservationists have used the term to describe gene flow from domestic, feral, and non-native species into wild indigenous species, which they consider undesirable. They promote awareness of the effects of introduced invasive species that may "hybridize with native species, causing genetic pollution". In the fields of agriculture, agroforestry and animal husbandry, genetic pollution is used to describe gene flows between genetically engineered species and wild relatives. The use of the word "pollution" is meant to convey the idea that mixing genetic information is bad for the environment, but because the mixing of genetic information can lead to a variety of outcomes, "pollution" may not always be the most accurate descriptor.
https://en.wikipedia.org/wiki/Genetic_pollution#cite_note-heartland-2
Genetic erosion (also known as genetic depletion) is a process where the limited gene pool of an endangered species diminishes even more when reproductive individuals die off before reproducing with others in their endangered low population. The term is sometimes used in a narrow sense, such as when describing the loss of particular alleles or genes, as well as being used more broadly, as when referring to the loss of a phenotype or whole species.
Genetic erosion occurs because each individual organism has many unique genes which get lost when it dies without getting a chance to breed. Low genetic diversity in a population of wild animals and plants leads to a further diminishing gene pool – inbreeding and a weakening immune system can then "fast-track" that species towards eventual extinction.
By definition, endangered species suffer varying degrees of genetic erosion. Many species benefit from a human-assisted breeding program to keep their population viable,[citation needed] thereby avoiding extinction over long time-frames. Small populations are more susceptible to genetic erosion than larger populations.
Genetic erosion gets compounded and accelerated by habitat loss and habitat fragmentation – many endangered species are threatened by habitat loss and (fragmentation) habitat. Fragmented habitat create barriers in gene flow between populations.
The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see inbreeding and population bottlenecks) can cause reduced biological fitness and increase the chance of extinction of that species or population.
Genetic erosion in agricultural and livestock is the loss of biological genetic diversity – including the loss of individual genes, and the loss of particular recombinants of genes (or gene complexes) – such as those manifested in locally adapted landraces of domesticated animals or plants that have become adapted to the natural environment in which they originated.
The major driving forces behind genetic erosion in crops are variety replacement, land clearing, overexploitation of species, population pressure, environmental degradation, overgrazing, governmental policy, and changing agricultural systems. The main factor, however, is the replacement of local varieties of domestic plants and animals by other varieties or species that are non-local. A large number of varieties can also often be dramatically reduced when commercial varieties are introduced into traditional farming systems. Many researchers believe that the main problem related to agro-ecosystem management is the general tendency towards genetic and ecological uniformity imposed by the development of modern agriculture.
In the case of Animal Genetic Resources for Food and Agriculture, major causes of genetic erosion are reported to include indiscriminate cross-breeding, increased use of exotic breeds, weak policies and institutions in animal genetic resources management, neglect of certain breeds because of a lack of profitability or competitiveness, the intensification of production systems, the effects of diseases and disease management, loss of pastures or other elements of the production environment, and poor control of inbreeding.[2]
Costly (and sometimes controversial) ex-situ conservation techniques aim to increase the genetic biodiversity on our planet, as well as the diversity in local gene pools. by guarding against genetic erosion. Modern concepts like seedbanks, sperm banks, and tissue banks have become much more commonplace and valuable. Sperm, eggs, and embryos can now be frozen and kept in banks, which are sometimes called "Modern Noah's Arks" or "Frozen Zoos". Cryopreservation techniques are used to freeze these living materials and keep them alive in perpetuity by storing them submerged in liquid nitrogen tanks at very low temperatures. Thus, preserved materials can then be used for artificial insemination, in vitro fertilization, embryo transfer, and cloning methodologies to protect diversity in the gene pool of critically endangered species.
It can be possible to save an endangered species from extinction by preserving only parts of specimens, such as tissues, sperm, eggs, etc. – even after the death of a critically endangered animal, or collected from one found freshly dead, in captivity or from the wild. A new specimen can then be "resurrected" with the help of cloning, so as to give it another chance to breed its genes into the living population of the respective threatened species. Resurrection of dead critically endangered wildlife specimens with the help of cloning is still being perfected, and is still too expensive to be practical, but with time and further advancements in science and methodology it may well become a routine procedure not to far into the future.
Recently, strategies for finding an integrated approach to in situ and ex situ conservation techniques have been given considerable attention, and progress is being made.[3]
See also[edit]
- Center of origin
- Conservation biology
- Crop wild relative
- Gene bank
- Genetic pollution
- Genetics
- Mutational meltdown
- Neglected and underutilized crops
- Population genetics
https://en.wikipedia.org/wiki/Genetic_erosion
Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay. Causes of habitat fragmentation include geological processes that slowly alter the layout of the physical environment[1](suspected of being one of the major causes of speciation[1]), and human activity such as land conversion, which can alter the environment much faster and causes the extinction of many species. More specifically, habitat fragmentation is a process by which large and contiguous habitats get divided into smaller, isolated patches of habitats.[2][3]
https://en.wikipedia.org/wiki/Habitat_fragmentation
Mutational meltdown (not to be confused with the concept of an error catastrophe[1]) is the accumulation of harmful mutations in a small population, which leads to loss of fitness and decline of the population size, which may lead to further accumulation of deleterious mutations due to fixation by genetic drift.
A population experiencing mutational meltdown is trapped in a downward spiral and will go extinct if the phenomenon lasts for some time. Usually, the deleterious mutations would simply be selected away, but during mutational meltdown, the number of individuals thus suffering an early death is too large relative to overall population size so that mortality exceeds the birth rate.
The accumulation of mutations in small populations can be divided into three phases. In the second phase a population starts in mutation/selection equilibrium, mutations are fixed at a constant rate through time, and the population size is constant because the fecundity exceeds mortality. However, after a sufficient number of mutations have been fixed in the population, the birth rate is slightly less than the death rate, and the population size begins to decrease. The smaller population size allows for a more rapid fixation of deleterious mutations, and a more rapid decline of population size, etc.
See also[edit]
Gene banks are a type of biorepository that preserves genetic material. For plants, this is done by in vitro storage, freezing cuttings from the plant, or stocking the seeds (e.g. in a seedbank). For animals, this is done by the freezing of sperm and eggs in zoological freezers until further need. With corals, fragments are taken and stored in water tanks under controlled conditions.[1] Genetic material in a 'gene bank' is preserved in a variety of ways, such as freezing at -196° Celsius in liquid nitrogen, being placed in artificial ecosystems, and put in controlled nutrient mediums.
Accession is the common term given to an individual sample in a gene bank, such as a distinct species or variety.
In plants, it is possible to unfreeze the material and propagate it. However, in animals, a living female is required for artificial insemination. While it is often difficult to use frozen animal sperm and eggs, there are many examples of it being done successfully.
In an effort to conserve agricultural biodiversity, gene banks are used to store and conserve the plant genetic resources of major crop plants and their crop wild relatives. There are many gene banks all over the world, with the Svalbard Global Seed Vault being considered the most famous one.[2]
The database of the largest gene banks in the world can be queried via a common website, Genesys. A number of global gene banks are coordinated by the CGIAR Genebank Platform
https://en.wikipedia.org/wiki/Gene_bank
A center of origin (or center of diversity) is a geographical area where a group of organisms, either domesticated or wild, first developed its distinctive properties.[2] They are also considered centers of diversity. Centers of origin were first identified in 1924 by Nikolai Vavilov.
https://en.wikipedia.org/wiki/Vavilov_center
Dysgenics (also known as cacogenics)[1] is the study of factors producing the accumulation and perpetuation of defective or disadvantageous genes and traits in offspring of a particular population or species.[2][3]
The adjective "dysgenic" is the antonym of "eugenic". It was first used c. 1915 by David Starr Jordan, describing the supposed dysgenic effects of World War I.[4] Jordan believed that healthy men were as likely to die in modern warfare as anyone else and that war killed only the physically healthy men of the populace whilst preserving the disabled at home.[5]
In the context of human genetics, a dysgenic effect is the projected or observed tendency of a reduction in selection pressures and decreased infant mortality since the Industrial Revolutionresulting in the increased propagation of deleterious traits and genetic disorders. Richard Lynn in his Dysgenics: Genetic Deterioration in Modern Populations (1996) identified three main concerns: deterioration in health, in intelligence, and in conscientiousness.
https://en.wikipedia.org/wiki/Dysgenics
Defaunation is the global, local or functional extinction of animal populations or species from ecological communities.[1] The growth of the human population, combined with advances in harvesting technologies, has led to more intense and efficient exploitation of the environment.[2] This has resulted in the depletion of large vertebrates from ecological communities, creating what has been termed "empty forest".[3][2][4] Defaunation differs from extinction; it includes both the disappearance of species and declines in abundance.[5] Defaunation effects were first implied at the Symposium of Plant-Animal Interactions at the University of Campinas, Brazil in 1988 in the context of Neotropical forests.[6] Since then, the term has gained broader usage in conservation biology as a global phenomenon.[1][6]
It is estimated that more than 50 percent of all wildlife has been lost in the last 40 years.[7] In 2016, it was estimated that by 2020, 68% of the world's wildlife would be lost.[8] In South America, there is believed to be a 70 percent loss.[9] A 2021 study found that only around 3% of the planet's terrestrial surface is ecologically and faunally intact, with healthy populations of native animal species and little to no human footprint.[10][11]
In November 2017, over 15,000 scientists around the world issued a second warning to humanity, which, among other things, urged for the development and implementation of policies to halt "defaunation, the poaching crisis, and the exploitation and trade of threatened species."[12]
https://en.wikipedia.org/wiki/Defaunation
Biotechnology risk is a form of existential risk that could come from biological sources, such as genetically engineered biological agents.[1][2] The origin of such a high-consequence pathogen could be a deliberate release (in the form of bioterrorism or biological weapons), an accidental release, or a naturally occurring event.
A chapter on biotechnology and biosecurity was published in Nick Bostrom's 2008 anthology Global Catastrophic Risks, which covered risks including as viral agents.[3] Since then, new technologies like CRISPR and gene drives have been introduced.
While the ability to deliberately engineer pathogens has been constrained to high-end labs run by top researchers, the technology to achieve this (and other astonishing feats of bioengineering) is rapidly becoming cheaper and more widespread. Such examples include the diminishing cost of sequencing the human genome (from $10 million to $1,000), the accumulation of large datasets of genetic information, the discovery of gene drives, and the discovery of CRISPR.[4] Biotechnology risk is therefore a credible explanation for the Fermi paradox.[5]
Gain-of-function mutations[edit]
Research[edit]
Pathogens may be intentionally or unintentionally genetically modified to change their characteristics, including virulence or toxicity.[2] When intentional, these mutations can serve to adapt the pathogen to a laboratory setting, understand the mechanism of transmission or pathogenesis, or in the development of therapeutics. Such mutations have also been used in the development of biological weapons, and dual-use risk continues to be a concern in the research of pathogens.[6] The greatest concern is frequently associated with gain-of-function mutations, which confer novel or increased functionality, and the risk of their release. Gain-of-function research on viruses has been occurring since the 1970s, and came to notoriety after influenza vaccines were serially passed through animal hosts.[citation needed]
Mousepox[edit]
A group of Australian researchers unintentionally changed characteristics of the mousepox virus while trying to develop a virus to sterilize rodents as a means of biological pest control.[2][7][8] The modified virus became highly lethal even in vaccinated and naturally resistant mice.[9]
Influenza[edit]
In 2011, two laboratories published reports of mutational screens of avian influenza viruses, identifying variant which become transmissible through the air between ferrets. These viruses seem to overcome an obstacle which limits the global impact of natural H5N1.[10][11] In 2012, scientists further screened point mutations of the H5N1 virus genome to identify mutations which allowed airborne spread.[12][13] While the stated goal of this research was to improve surveillance and prepare for influenza viruses which are of particular risk in causing a pandemic,[14] there was significant concern that the laboratory strains themselves could escape.[15] Marc Lipsitch and Alison P. Galvani coauthored a paper in PLoS Medicine arguing that experiments in which scientists manipulate bird influenza viruses to make them transmissible in mammals deserve more intense scrutiny as to whether or not their risks outweigh their benefits.[16] Lipsitch also described influenza as the most frightening "potential pandemic pathogen".[17]
Regulation[edit]
In 2014, the United States instituted a moratorium on gain-of-function research into influenza, MERS, and SARS.[18] This was in response to the particular risks these airborne pathogens pose. However, many scientists opposed the moratorium, arguing that this limited their ability to develop antiviral therapies.[19] The scientists argued gain-of-function mutations were necessary, such as adapting MERS to laboratory mice so it could be studied.
The National Science Advisory Board for Biosecurity also has instituted rules for research proposals using gain-of-function research of concern.[20] The rules outline how experiments to be evaluated for risks, safety measures, and potential benefits; prior to funding.
In order to limit access to minimize the risk of easy access to genetic material from pathogens, including viruses, the members of the International Gene Synthesis Consortium screen orders for regulated pathogen and other dangerous sequences.[21] Orders for pathogenic or dangerous DNA are verified for customer identity, barring customers on governmental watch lists, and only to institutions "demonstrably engaged in legitimate research".
CRISPR[edit]
Following surprisingly fast advances in CRISPR editing, an international summit proclaimed in December 2015 that it was "irresponsible" to proceed with human gene editing until issues in safety and efficacy were addressed.[22] One of the mechanisms that CRISPR can cause existential risk is through gene drives, which are said to have potential to "revolutionize" ecosystem management.[23] Gene drives are a novel technology that have potential to make genes spread through wild populations like wildfire. They have the potential to quickly spread resistance genes against malaria in order to rebuff the malaria parasite P. falciparum.[24] These gene drives were originally engineered in January 2015 by Ethan Bier and Valentino Gantz – this editing was spurred by the discovery of CRISPR-Cas9. In late 2015, DARPA started to study approaches that could halt gene drives if they went out of control and threatened biological species.[25]
See also[edit]
https://en.wikipedia.org/wiki/Biotechnology_risk
https://en.wikipedia.org/wiki/Fermi_paradox
A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts or human activities such as specicide, widespread violence or intentional culling, and human population planning. Such events can reduce the variation in the gene pool of a population; thereafter, a smaller population, with a smaller genetic diversity, remains to pass on genes to future generations of offspring through sexual reproduction. Genetic diversity remains lower, increasing only when gene flow from another population occurs or very slowly increasing with time as random mutations occur.[1][self-published source] This results in a reduction in the robustness of the population and in its ability to adapt to and survive selecting environmental changes, such as climate change or a shift in available resources.[2] Alternatively, if survivors of the bottleneck are the individuals with the greatest genetic fitness, the frequency of the fitter genes within the gene pool is increased, while the pool itself is reduced.
The genetic drift caused by a population bottleneck can change the proportional random distribution of alleles and even lead to loss of alleles. The chances of inbreeding and genetic homogeneity can increase, possibly leading to inbreeding depression. Smaller population size can also cause deleterious mutations to accumulate.[3]
Population bottlenecks play an important role in conservation biology (see minimum viable population size) and in the context of agriculture (biological and pest control).[4]
Scientists have witnessed population bottlenecks in American bison, greater prairie chickens, northern elephant seals, golden hamsters, and cheetahs. The New Zealand black robins experienced a bottleneck of five individuals, all descendants of a single female. Geneticists have found evidence for past bottlenecks in pandas, golden snub-nosed monkeys, and humans.
https://en.wikipedia.org/wiki/Population_bottleneck
https://en.wikipedia.org/wiki/Genetic_erosion
A genetically modified virus is a virus that has been altered or generated using biotechnology methods, and remains capable of infection. Genetic modification involves the directed insertion, deletion, artificial synthesis or change of nucleotide bases in viral genomes. Genetically modified viruses are mostly generated by the insertion of foreign genes intro viral genomes for the purposes of biomedical, agricultural, bio-control, or technological objectives. The terms genetically modified virus and genetically engineered virus are used synonymously.
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https://en.wikipedia.org/wiki/Genetically_modified_virus
Transcription activator-like effector nucleases (TALEN) are restriction enzymes that can be engineered to cut specific sequences of DNA. They are made by fusing a TAL effector DNA-binding domain to a DNA cleavage domain (a nuclease which cuts DNA strands). Transcription activator-like effectors (TALEs) can be engineered to bind to practically any desired DNA sequence, so when combined with a nuclease, DNA can be cut at specific locations.[1] The restriction enzymes can be introduced into cells, for use in gene editing or for genome editing in situ, a technique known as genome editing with engineered nucleases. Alongside zinc finger nucleases and CRISPR/Cas9, TALEN is a prominent tool in the field of genome editing.
https://en.wikipedia.org/wiki/Transcription_activator-like_effector_nuclease
CRISPR (/ˈkrɪspər/) (which is an acronym for clustered regularly interspaced short palindromic repeats) is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea.[2] These sequences are derived from DNA fragments of bacteriophages that had previously infected the prokaryote. They are used to detect and destroy DNA from similar bacteriophages during subsequent infections. Hence these sequences play a key role in the antiviral (i.e. anti-phage) defense system of prokaryotes and provide a form of acquired immunity.[2][3][4][5] CRISPR are found in approximately 50% of sequenced bacterial genomes and nearly 90% of sequenced archaea.[6]
Cas9 (or "CRISPR-associated protein 9") is an enzyme that uses CRISPR sequences as a guide to recognize and cleave specific strands of DNA that are complementary to the CRISPR sequence. Cas9 enzymes together with CRISPR sequences form the basis of a technology known as CRISPR-Cas9that can be used to edit genes within organisms.[8][9] This editing process has a wide variety of applications including basic biological research, development of biotechnological products, and treatment of diseases.[10][11] The development of the CRISPR-Cas9 genome editing technique was recognized by the Nobel Prize in Chemistry in 2020 which was awarded to Emmanuelle Charpentier and Jennifer Doudna.[12][13]
https://en.wikipedia.org/wiki/CRISPR
Transduction is the process by which foreign DNA is introduced into a cell by a virus or viral vector.[1] An example is the viral transfer of DNA from one bacterium to another and hence an example of horizontal gene transfer.[2]Transduction does not require physical contact between the cell donating the DNA and the cell receiving the DNA (which occurs in conjugation), and it is DNase resistant (transformation is susceptible to DNase). Transduction is a common tool used by molecular biologists to stably introduce a foreign gene into a host cell's genome (both bacterial and mammalian cells).
Discovery (bacterial transduction)[edit]
Transduction was discovered by Norton Zinder and Joshua Lederberg at the University of Wisconsin–Madison in 1952 in Salmonella.[3][4]
In the lytic and lysogenic cycles[edit]
Transduction happens through either the lytic cycle or the lysogenic cycle. When bacteriophages (viruses that infect bacteria) that are lytic infect bacterial cells, they harness the replicational, transcriptional, and translation machinery of the host bacterial cell to make new viral particles (virions). The new phage particles are then released by lysis of the host. In the lysogenic cycle, the phage chromosome is integrated as a prophage into the bacterial chromosome, where it can stay dormant for extended periods of time. If the prophage is induced (by UV light for example), the phage genome is excised from the bacterial chromosome and initiates the lytic cycle, which culminates in lysis of the cell and the release of phage particles. Generalized transduction (see below) occurs in both cycles during the lytic stage, while specialized transduction (see below) occurs when a prophage is excised in the lysogenic cycle.
As a method for transferring genetic material[edit]
Transduction by bacteriophages[edit]
The packaging of bacteriophage DNA into phage capsids has low fidelity. Small pieces of bacterial DNA may be packaged into the bacteriophage particles. There are two ways that this can lead to transduction.
Generalized transduction[edit]
Generalized transduction occurs when random pieces of bacterial DNA are packaged into a phage. It happens when a phage is in the lytic stage, at the moment that the viral DNA is packaged into phage heads. If the virus replicates using 'headful packaging', it attempts to fill the head with genetic material. If the viral genome results in spare capacity, viral packaging mechanisms may incorporate bacterial genetic material into the new virion. Alternatively, generalized transduction may occur via recombination. Generalized transduction is a rare event and occurs on the order of 1 phage in 11,000.[3]
The new virus capsule that contains part bacterial DNA then infects another bacterial cell. When the bacterial DNA packaged into the virus is inserted into the recipient cell three things can happen to it:
- The DNA is recycled for spare parts.
- If the DNA was originally a plasmid, it will re-circularize inside the new cell and become a plasmid again.
- If the new DNA matches with a homologous region of the recipient cell's chromosome, it will exchange DNA material similar to the actions in bacterial recombination.
Specialized transduction[edit]
Specialized transduction is the process by which a restricted set of bacterial genes is transferred to another bacterium. The genes that get transferred (donor genes) flank where the prophage is located on the chromosome. Specialized transduction occurs when a prophage excises imprecisely from the chromosome so that bacterial genes lying adjacent to it are included in the excised DNA. The excised DNA is then packaged into a new virus particle, which then delivers the DNA to a new bacterium. Here, the donor genes can be inserted into the recipient chromosome or remain in the cytoplasm, depending on the nature of the bacteriophage.
When the partially encapsulated phage material infects another cell and becomes a prophage, the partially coded prophage DNA is called a "heterogenote".
An example of specialized transduction is λ phage in Escherichia coli.[5]
Lateral transduction[edit]
Lateral transduction is the process by which very long fragments of bacterial DNA are transferred to another bacterium. So far, this form of transduction has been only described in Staphylococcus aureus, but it can transfer more genes and at higher frequencies than generalized and specialized transduction. In lateral transduction, the prophage starts its replication in situ before excision in a process that leads to replication of the adjacent bacterial DNA. After which, packaging of the replicated phage from its pac site (located around the middle of the phage genome) and adjacent bacterial genes occurs in situ, to 105% of a phage genome size. Successive packaging after initiation from the original pac site leads to several kilobases of bacterial genes being packaged into new viral particles that are transferred to new bacterial strains. If the transferred genetic material in these transducing particles provides sufficient DNA for homologous recombination, the genetic material will be inserted into the recipient chromosome. Because multiple copies of the phage genome are produced during in situ replication, some of these replicated prophages excise normally (instead of being packaged in situ), producing normal infectious phages.[6]
Mammalian cell transduction with viral vectors[edit]
Transduction with viral vectors can be used to insert or modify genes in mammalian cells. It is often used as a tool in basic research and is actively researched as a potential means for gene therapy.
Process[edit]
In these cases, a plasmid is constructed in which the genes to be transferred are flanked by viral sequences that are used by viral proteins to recognize and package the viral genome into viral particles. This plasmid is inserted (usually by transfection) into a producer cell together with other plasmids (DNA constructs) that carry the viral genes required for formation of infectious virions. In these producer cells, the viral proteins expressed by these packaging constructs bind the sequences on the DNA/RNA (depending on the type of viral vector) to be transferred and insert it into viral particles. For safety, none of the plasmids used contains all the sequences required for virus formation, so that simultaneous transfection of multiple plasmids is required to get infectious virions. Moreover, only the plasmid carrying the sequences to be transferred contains signals that allow the genetic materials to be packaged in virions, so that none of the genes encoding viral proteins are packaged. Viruses collected from these cells are then applied to the cells to be altered. The initial stages of these infections mimic infection with natural viruses and lead to expression of the genes transferred and (in the case of lentivirus/retrovirus vectors) insertion of the DNA to be transferred into the cellular genome. However, since the transferred genetic material does not encode any of the viral genes, these infections do not generate new viruses (the viruses are "replication-deficient").
Some enhancers have been used to improve transduction efficiency such as polybrene, protamine sulfate, retronectin, and DEAE Dextran.[7]
Medical applications[edit]
- Gene therapy: Correcting genetic diseases by direct modification of genetic error.
See also[edit]
- Electroporation – use of an electrical field to increase cell membrane permeability.
- Phage therapy – therapeutic use of bacteriophages.
- Signal transduction
- Transfection – means of inserting DNA into a cell.
- Transformation (genetics) – means of inserting DNA into a cell.
- Viral vector – commonly used tool to deliver genetic material into cells.
In molecular biology and genetics, transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material from its surroundings through the cell membrane.
https://en.wikipedia.org/wiki/Transduction_(genetics)
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