The lytic cycle is one of the two cycles of viral reproduction, the other being the lysogenic cycle. These cycles should not, however, be seen as separate, but rather as somewhat interchangeable.[original research?] The lytic cycle is typically considered the main method of viral replication, since it results in the destruction of the infected cell.
The lytic cycle is a four-stage cycle.
Penetration
To infect a cell, a virus must first enter the cell through the plasma membrane and (if present) the cell wall. Viruses do so by either attaching to a receptor on the cell's surface or by simple mechanical force. The virus then releases its genetic material (either single- or double-stranded RNA or DNA) into the cell. In doing, the cell is infected and can also be targeted by the immune system.
Biosynthesis
The virus' nucleic acid uses the host cell’s machinery to make large amounts of viral components. In the case of DNA viruses, the DNA transcribes itself into messenger RNA (mRNA) molecules that are then used to direct the cell's ribosomes. One of the first polypeptides to be translated is one that destroys the hosts' DNA. In retroviruses (which inject an RNA strand), a unique enzyme called reverse transcriptase transcribes the viral RNA into DNA, which is then transcribed again into RNA.
Maturation and lysis
After many copies of viral components are made, they are assembled into complete viruses. The phage then directs production of an enzyme that breaks down the bacteria cell wall and allows fluid to enter. The cell eventually becomes filled with viruses (typically 100-200) and liquid, and bursts, or lyses; thus giving the lytic cycle its name. The new viruses are then free to infect other cells.
Lytic cycle without lysis
Some viruses escape the host cell without bursting the cell membrane, but rather bud off from it by taking a portion of the membrane with them. Because it otherwise is characteristic of the lytic cycle in other steps, it still belongs to this category. HIV, influenza and other viruses that infect eukaryotic organisms generally use this method.
Retrieved from "http://en.wikipedia.org/wiki/Lytic_cycle"
Monday, October 20, 2008
Lysogenic cycle
Lysogenic cycle
Lysogenic cycle, compared to lytic cycle
Lysogeny, or the lysogenic cycle, is one of two methods of viral reproduction
(the lytic cycle is the other). Lysogeny in prokaryotes is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome. The newly integrated genetic material, called a prophage can be transmitted to daughter cells at each subsequent cell division, and a later event (such as UV radiation) can release it, causing proliferation of new phages via the lytic cycle. Lysogenic cycles can also occur in eukaryotes, although the method of incorporation of DNA is not fully understood.
Mixed cycles
Following are some types of viruses that replicate by the lysogenic cycle, but also partly by the lytic cycle.
Bacteriophages
Some DNA phages, called temperate phages, only lyse a small fraction of bacterial cells; in the remaining majority of the bacteria, the phage DNA becomes integrated into the bacterial chromosome and replicates along with it. In this lysogenic state, the information contained in the viral nucleic acid is not expressed. The model organism for studying lysogeny is the lambda phage. Roughly 50-60 nucleotides are taken out of the lysogenic pathway and used.
Lysogenic conversion
In some interactions between lysogenic phages and bacteria, lysogenic conversion may occur. It is when a temperate phage induces a change in the phenotype of the bacteria infected that is not part of a usual phage cycle. Changes can often involve the external membrane of the cell by making it impervious to other phages or even by increasing the pathogenic capability of the bacteria for a host.
Examples:
Corynebacterium diphtheriae produces the toxin of diphtheria only when it is infected by the phage β. In this case, the gene that codes for the toxin is carried by the phage, not the bacteria.
Vibrio cholerae is a non-toxic strain that can become toxic, producing cholera toxin, when it is infected with the phage CTXφ.
Clostridium botulinum causes botulism.
Streptococcus pyogenes causes scarlet fever.
Shiga toxin
Tetanus
Extra genes present in prophage genomes which do not have a phage function but (may) act as fitness factors for the lysogen are termed "morons".[1]
Lysogenic cycle, compared to lytic cycle
Lysogeny, or the lysogenic cycle, is one of two methods of viral reproduction
(the lytic cycle is the other). Lysogeny in prokaryotes is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome. The newly integrated genetic material, called a prophage can be transmitted to daughter cells at each subsequent cell division, and a later event (such as UV radiation) can release it, causing proliferation of new phages via the lytic cycle. Lysogenic cycles can also occur in eukaryotes, although the method of incorporation of DNA is not fully understood.
Mixed cycles
Following are some types of viruses that replicate by the lysogenic cycle, but also partly by the lytic cycle.
Bacteriophages
Some DNA phages, called temperate phages, only lyse a small fraction of bacterial cells; in the remaining majority of the bacteria, the phage DNA becomes integrated into the bacterial chromosome and replicates along with it. In this lysogenic state, the information contained in the viral nucleic acid is not expressed. The model organism for studying lysogeny is the lambda phage. Roughly 50-60 nucleotides are taken out of the lysogenic pathway and used.
Lysogenic conversion
In some interactions between lysogenic phages and bacteria, lysogenic conversion may occur. It is when a temperate phage induces a change in the phenotype of the bacteria infected that is not part of a usual phage cycle. Changes can often involve the external membrane of the cell by making it impervious to other phages or even by increasing the pathogenic capability of the bacteria for a host.
Examples:
Corynebacterium diphtheriae produces the toxin of diphtheria only when it is infected by the phage β. In this case, the gene that codes for the toxin is carried by the phage, not the bacteria.
Vibrio cholerae is a non-toxic strain that can become toxic, producing cholera toxin, when it is infected with the phage CTXφ.
Clostridium botulinum causes botulism.
Streptococcus pyogenes causes scarlet fever.
Shiga toxin
Tetanus
Extra genes present in prophage genomes which do not have a phage function but (may) act as fitness factors for the lysogen are termed "morons".[1]
Friday, October 3, 2008
Any type of organism can be identified by examination of DNA sequences unique to that species. Identifying individuals within a species is less precise at this time, although when DNA sequencing technologies progress farther, direct comparison of very large DNA segments, and possibly even whole genomes, will become feasible and practical and will allow precise individual identification.To identify individuals, forensic scientists scan 13 DNA regions, or loci, that vary from person to person and use the data to create a DNA profile of that individual (sometimes called a DNA fingerprint). There is an extremely small chance that another person has the same DNA profile for a particular set of 13 regions.
Some Examples of DNA Uses for Forensic Identification
Identify potential suspects whose DNA may match evidence left at crime scenesExonerate persons wrongly accused of crimesIdentify crime and catastrophe victimsEstablish paternity and other family relationshipsIdentify endangered and protected species as an aid to wildlife officials (could be used for prosecuting poachers)Detect bacteria and other organisms that may pollute air, water, soil, and foodMatch organ donors with recipients in transplant programsDetermine pedigree for seed or livestock breedsAuthenticate consumables such as caviar and wineIs DNA effective in identifying persons?[answer provided by Daniel Drell of the U.S. DOE Human Genome Program]DNA identification can be quite effective if used intelligently. Portions of the DNA sequence that vary the most among humans must be used; also, portions must be large enough to overcome the fact that human mating is not absolutely random.Consider the scenario of a crime scene investigation . . .Assume that type O blood is found at the crime scene. Type O occurs in about 45% of Americans. If investigators type only for ABO, finding that the "suspect" in a crime is type O really doesn't reveal very much.If, in addition to being type O, the suspect is a blond, and blond hair is found at the crime scene, you now have two bits of evidence to suggest who really did it. However, there are a lot of Type O blonds out there.If you find that the crime scene has footprints from a pair of Nike Air Jordans (with a distinctive tread design) and the suspect, in addition to being type O and blond, is also wearing Air Jordans with the same tread design, you are much closer to linking the suspect with the crime scene.In this way, by accumulating bits of linking evidence in a chain, where each bit by itself isn't very strong but the set of all of them together is very strong, you can argue that your suspect really is the right person.With DNA, the same kind of thinking is used; you can look for matches (based on sequence or on numbers of small repeating units of DNA sequence) at many different locations on the person's genome; one or two (even three) aren't enough to be confident that the suspect is the right one, but thirteen sites are used. A match at all thirteen is rare enough that you (or a prosecutor or a jury) can be very confident ("beyond a reasonable doubt") that the right person is accused.See some recent articles about statistical analysis on this topic:NY Times Freakonomics Blog, Aug.19, 2008Los Angeles Times, July 20, 2008How is DNA typing done?Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. Scientists can use these variable regions to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products.In criminal cases, this generally involves obtaining samples from crime-scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific DNA regions (markers).Scientists find the markers in a DNA sample by designing small pieces of DNA (probes) that will each seek out and bind to a complementary DNA sequence in the sample. A series of probes bound to a DNA sample creates a distinctive pattern for an individual. Forensic scientists compare these DNA profiles to determine whether the suspect's sample matches the evidence sample. A marker by itself usually is not unique to an individual; if, however, two DNA samples are alike at four or five regions, odds are great that the samples are from the same person.If the sample profiles don't match, the person did not contribute the DNA at the crime scene.If the patterns match, the suspect may have contributed the evidence sample. While there is a chance that someone else has the same DNA profile for a particular probe set, the odds are exceedingly slim. The question is, How small do the odds have to be when conviction of the guilty or acquittal of the innocent lies in the balance? Many judges consider this a matter for a jury to take into consideration along with other evidence in the case. Experts point out that using DNA forensic technology is far superior to eyewitness accounts, where the odds for correct identification are about 50:50.The more probes used in DNA analysis, the greater the odds for a unique pattern and against a coincidental match, but each additional probe adds greatly to the time and expense of testing. Four to six probes are recommended. Testing with several more probes will become routine, observed John Hicks (Alabama State Department of Forensic Services). He predicted that DNA chip technology (in which thousands of short DNA sequences are embedded in a tiny chip) will enable much more rapid, inexpensive analyses using many more probes and raising the odds against coincidental matches.What are some of the DNA technologies used in forensic investigations?Restriction Fragment Length Polymorphism (RFLP)RFLP is a technique for analyzing the variable lengths of DNA fragments that result from digesting a DNA sample with a special kind of enzyme. This enzyme, a restriction endonuclease, cuts DNA at a specific sequence pattern know as a restriction endonuclease recognition site. The presence or absence of certain recognition sites in a DNA sample generates variable lengths of DNA fragments, which are separated using gel electrophoresis. They are then hybridized with DNA probes that bind to a complementary DNA sequence in the sample.RFLP was one of the first applications of DNA analysis to forensic investigation. With the development of newer, more efficient DNA-analysis techniques, RFLP is not used as much as it once was because it requires relatively large amounts of DNA. In addition, samples degraded by environmental factors, such as dirt or mold, do not work well with RFLP.PCR AnalysisPolymerase chain reaction (PCR) is used to make millions of exact copies of DNA from a biological sample. DNA amplification with PCR allows DNA analysis on biological samples as small as a few skin cells. With RFLP, DNA samples would have to be about the size of a quarter. The ability of PCR to amplify such tiny quantities of DNA enables even highly degraded samples to be analyzed. Great care, however, must be taken to prevent contamination with other biological materials during the identifying, collecting, and preserving of a sample.STR AnalysisShort tandem repeat (STR) technology is used to evaluate specific regions (loci) within nuclear DNA. Variability in STR regions can be used to distinguish one DNA profile from another. The Federal Bureau of Investigation (FBI) uses a standard set of 13 specific STR regions for CODIS. CODIS is a software program that operates local, state, and national databases of DNA profiles from convicted offenders, unsolved crime scene evidence, and missing persons. The odds that two individuals will have the same 13-loci DNA profile is about one in a billion.Mitochondrial DNA AnalysisMitochondrial DNA analysis (mtDNA) can be used to examine the DNA from samples that cannot be analyzed by RFLP or STR. Nuclear DNA must be extracted from samples for use in RFLP, PCR, and STR; however, mtDNA analysis uses DNA extracted from another cellular organelle called a mitochondrion. While older biological samples that lack nucleated cellular material, such as hair, bones, and teeth, cannot be analyzed with STR and RFLP, they can be analyzed with mtDNA. In the investigation of cases that have gone unsolved for many years, mtDNA is extremely valuable.All mothers have the same mitochondrial DNA as their daughters. This is because the mitochondria of each new embryo comes from the mother's egg cell. The father's sperm contributes only nuclear DNA. Comparing the mtDNA profile of unidentified remains with the profile of a potential maternal relative can be an important technique in missing-person investigations.Y-Chromosome AnalysisThe Y chromosome is passed directly from father to son, so analysis of genetic markers on the Y chromosome is especially useful for tracing relationships among males or for analyzing biological evidence involving multiple male contributors.The answer to this question is based on information from Using DNA to Solve Cold Cases - A special report from the National Institute of Justice (July 2002).Some Interesting Uses of DNA Forensic IdentificationIdentifying September 11th Victims Identifying the victims of the September 11, 2001, World Trade Center attack presented a unique forensic challenge because the number and identity of the victims were unknown and many victims were represented only by bone and tissue fragments. At the time of the attack, no systems were in place for rapidly identifying victims in disasters with more than 500 fatalities. The National Institutes of Justice assembled a panel of experts from the National Institutes of Health and other institutions to develop processes to identify victims using DNA collected at the site. Panel members produced forms and kits needed to enable the medical examiner’s office to collect reference DNA from victims’ previously stored medical specimens. These specimens were collected and entered into a database. The medical examiner's office also received about 20,000 pieces of human remains from the World Trade Center site, and a database of the victims’ DNA profiles was created. New information technology infrastructure was developed for data transfer between the state police and medical examiner’s office and to interconnect the databases and analytical tools used by panel members. In 2005 the search was declared at an end because many of the unidentified remains were too small or too damaged to be identified by the DNA extraction methods available at that time. Remains of only 1585, of the 2792 people known to have died had been identified. In 2007, the medical examiner's office reopened the search after the Bode Technology Group developed a new methodology of DNA extraction that required much less sample material than previously necessary. The victim DNA database and the new methods have allowed more victims to be identified, and further identifications will be possible as forensic DNA technology improves.The DNA Shoah ProjectThe DNA Shoah Project is a genetic database of people who lost family during the Holocaust. The database will serve to reunite families separated during wartime and aid in identifying victims who remain buried anonymously throughout Europe.Disappeared Children in ArgentinaNumerous people (known as "the Disappeared") were kidnapped and murdered in Argentina in the 1970s. Many were pregnant. Their children were taken at birth and, along with other kidnapped children, were raised by their kidnappers. The grandparents of these children have been looking for them for many years. Read an article about a DNA researcher who has been helping them.Tomb of the UnknownsSon of Louis XVI and Marie AntionettePARIS, Apr 19, 2000 (Reuters) -- Scientists cracked one of the great mysteries of European history by using DNA tests to prove that the son of executed French King Louis XVI and Marie-Antoinette died in prison as a child. Royalists have argued for 205 years over whether Louis-Charles de France perished in 1795 in a grim Paris prison or managed to escape the clutches of the French Revolution. In December 1999, the presumed heart of the child king was removed from its resting place to enable scientists to compare its DNA makeup with samples from living and dead members of the royal family -- including a lock of his mother Marie-Antoinette's hair.The Murdered Nicholas Romanov, the Last Czar of Russia, and His FamilyPeruvian Ice MaidenThe Ice Maiden was a 12-to-14-year old girl sacrificed by Inca priests 500 years ago to satisfy the mountain gods of the Inca people. She was discovered in 1995 by climbers on Mt. Ampato in the Peruvian Andes. She is perhaps the best preserved mummy found in the Andes because she was in a frozen state. Analysis of the Ice Maiden's DNA offers a wonderful opportunity for understanding her genetic origin. If we could extract mitochondrial DNA from the Ice Maiden's tissue and successfully amplify and sequence it, then we could begin to trace her maternal line of descent and possibly locate past and current relatives.African Lemba Tribesmen In southern Africa, a people known as the Lemba heed the call of the shofar. They have believed for generations that they are Jews, direct descendants of the biblical patriarchs Abraham, Isaac, and Jacob. However unlikely the Lemba's claims may seem, modern science is finding ways to test them. The ever-growing understanding of human genetics is revealing connections between peoples that have never been seen before.Super Bowl XXXIV Footballs and 2000 Summer Olympic SouvenirsThe NFL used DNA technology to tag all the Super Bowl XXXIV balls, ensuring their authenticity for years to come and helping to combat the growing epidemic of sports memorabilia fraud. The footballs were marked with an invisible, yet permanent, strand of synthetic DNA. The DNA strand is unique and is verifiable any time in the future using a specially calibrated laser.A section of human genetic code taken from several unnamed Australian athletes was added to ink used to mark all official goods — everything from caps to socks — from the 2000 Summer Olympic Games. The technology is used as a way to mark artwork or one-of-a-kind sports souvenirs.Migration PatternsEvolutionarily stable mitochondrial DNA and Y chromosomes have allowed bioanthropologists to begin to trace human migration patterns around the world and identify family lineageSee Genetic Anthropology, Ancestry, and Ancient Human MigrationsWine Heritage Using DNA fingerprinting techniques akin to those used to solve crimes and settle paternity suits, scientists at the University of California, Davis, have discovered that 18 of the world's most renowned grapevine varieties, or cultivars are close relatives. These include varieties long grown in northeastern France such as Chardonnay, the "king of whites," and reds such as Pinot and Gamay noir, are close relatives.DNA Banks for Endangered Animal SpeciesPoached AnimalsDeclining Grizzly Bear PopulationSnowball the Cat A woman was murdered in Prince Edward Island, Canada. Her estranged husband was implicated because a snowy white cat hair was found in a jacket near the scene of the crime, and DNA fragments from the hair matched DNA fragments from Snowball, the cat belonging to the husband's parents. See M. Menotti-Raymond et al., "Pet cat hair implicates murder suspect," Nature, 386, 774, 1997. Also see Holmes, Judy, Feline Forensics, Syracuse University Magazine, Summer 2001.Angiosperm Witness for the Prosecution The first case in which a murderer was convicted on plant DNA evidence was described in the PBS TV series, "Scientific American Frontiers." A young woman was murdered in Phoenix, Arizona, and a pager found at the scene of the crime led the police to a prime suspect. He admitted picking up the victim but claimed she had robbed him of his wallet and pager. The forensic squad examined the suspect's pickup truck and collected pods later identified as the fruits of the palo verde tree (Cercidium spp.). One detective went back to the murder scene and found several Palo Verde trees, one of which showed damage that could have been caused by a vehicle. The detective's superior officer innocently suggested the possibility of linking the fruits and the tree by using DNA comparison, not realizing that this had never been done before. Several researchers were contacted before a geneticist at the University of Arizona in Tucson agreed to take on the case. Of course, it was crucial to establish evidence that would stand up in court on whether individual plants (especially Palo Verde trees) have unique patterns of DNA. A preliminary study on samples from different trees at the murder scene and elsewhere quickly established that each Palo Verde tree is unique in its DNA pattern. It was then a simple matter to link the pods from the suspect's truck to the damaged tree at the murder scene and obtain a conviction. [WNED-TV (PBS - Buffalo, N.Y.)]DNA Forensics DatabasesNational DNA Databank: CODISThe COmbined DNA Index System, CODIS, blends computer and DNA technologies into a tool for fighting violent crime. The current version of CODIS uses two indexes to generate investigative leads in crimes where biological evidence is recovered from the crime scene. The Convicted Offender Index contains DNA profiles of individuals convicted of felony sex offenses (and other violent crimes). The Forensic Index contains DNA profiles developed from crime scene evidence. All DNA profiles stored in CODIS are generated using STR (short tandem repeat) analysis.CODIS utilizes computer software to automatically search its two indexes for matching DNA profiles. Law enforcement agencies at federal, state, and local levels take DNA from biological evidence (e.g., blood and saliva) gathered in crimes that have no suspect and compare it to the DNA in the profiles stored in the CODIS systems. If a match is made between a sample and a stored profile, CODIS can identify the perpetrator.This technology is authorized by the DNA Identification Act of 1994. All 50 states have laws requiring that DNA profiles of certain offenders be sent to CODIS. As of August 2007, the database contained over 5 million DNA profiles in its Convicted Offender Index and about 188,000 DNA profiles collected from crime scenes but not connected to a particular offender. (source http://www.fbi.gov/hq/lab/codis/clickmap.htm).As more offender DNA samples are collected and law enforcement officers become better trained and equipped to collect DNA samples at crime scenes, the backlog of samples awaiting testing throughout the criminal justice system is increasing dramatically. In March 2003 President Bush proposed $1 billion in funding over 5 years to reduce the DNA testing backlog, build crime lab capacity, stimulate research and development, support training, protect the innocent, and identify missing persons. For more information, see the U.S. Department of Justice's Advancing Justice Through DNA Technology.More on CODISCODIS: Combined DNA Index System - Information from the FBI.The FBI Laboratory's Combined DNA Index System Program - Enter regional information to learn more about CODIS in your area. From Promega Corporation, a major supplier of reagents and other materials to support molecular biology research.National Commission on the Future of DNA Evidence.Postconviction DNA Testing: Recommendations for Handling Requests - Report from the National Commission on the Future of DNA Evidence.What Every Law Enforcement Officer Should Know About DNA Evidence (September 1999) - Report from the National Commission on the Future of DNA Evidence.Slide Show: Forensic DNA Legislation 2002 - A look at states' CODIS legislation.Ethics of State DNA Collection (2004 meeting presentations and handouts from National Conference of State Legislatures' Criminal Justice Program, Genetic Technologies Project, and Center for Ethics in Government)Ethical, Legal, and Social Concerns about DNA DatabankingThe primary concern is privacy. DNA profiles are different from fingerprints, which are useful only for identification. DNA can provide insights into many intimate aspects of people and their families including susceptibility to particular diseases, legitimacy of birth, and perhaps predispositions to certain behaviors and sexual orientation. This information increases the potential for genetic discrimination by government, insurers, employers, schools, banks, and others.Collected samples are stored, and many state laws do not require the destruction of a DNA record or sample after a conviction has been overturned. So there is a chance that a person's entire genome may be available —regardless of whether they were convicted or not. Although the DNA used is considered "junk DNA", single tandem repeated DNA bases (STRs), which are not known to code for proteins, in the future this information may be found to reveal personal information such as susceptibilities to disease and certain behaviors.Practicality is a concern for DNA sampling and storage. An enormous backlog of over half a million DNA samples waits to be entered into the CODIS system. The statute of limitations has expired in many cases in which the evidence would have been useful for conviction.Who is chosen for sampling also is a concern. In the United Kingdom, for example, all suspects can be forced to provide a DNA sample. Likewise, all arrestees --regardless of the degree of the charge and the possibility that they may not be convicted--can be compelled to comply. This empowers police officers, rather than judges and juries, to provide the state with intimate evidence that could lead to "investigative arrests."In the United States each state legislature independently decides whether DNA can be sampled from arrestees or convicts. In 2006, the New Mexico state legislature passed Katie's Bill, a law that requires the police to take DNA samples from suspects in most felony arrests. Previous New Mexico laws required DNA to be sampled only from convicted felons. The bill is named for Katie Sepich, whose 2003 murder went unsolved until her killer's DNA entered the database in 2005 when he was convinced of another felony. Her killer had been arrested, but not convicted, for burglary prior to 2005.Opponents of the law assert that it infringes on the privacy and rights of the innocent. While Katie’s Law does allow cleared suspects to petition to have their DNA samples purged from the state database, the purging happens only after the arrest. Civil liberties advocates say that Katie's Bill still raises the question of Fourth Amendment violations against unreasonable search and seizure and stress that the law could be abused to justify arrests made on less than probable cause just to obtain DNA evidence.As of September 2007, all 50 states have laws that require convicted sex offenders to submit DNA, 44 states have laws that require convicted felons to submit DNA, 9 states require DNA samples from those convicted of certain misdemeanors, and 11 states—including Alaska, Arizona, California, Kansas, Louisiana, Minnesota, New Mexico, North Dakota, Tennessee, Texas, and Virginia—have laws authorizing arrestee DNA sampling.Potential Advantages and Disadvantages of Banking Arrestee DNAAdvantagesMajor crimes often involve people who also have committed other offenses. Having DNA banked potentially could make it easier to identify suspects, just as fingerprint databases do.Innocent people currently are incarcerated for crimes they did not commit; if DNA samples had been taken at the time of arrest, these individuals could have been proven innocent and thereby avoided incarceration..Banking arrestees' DNA instead of banking only that of convicted criminals could result in financial savings in investigation, prosecution, and incarceration.DisadvantagesArrestees often are found innocent of crimes. The retention of innocent people's DNA raises significant ethical and social issues.If people’s DNA is in police databases, they might be identified as matches or partial matches to DNA found at crime scenes. This occurs even with innocent people, for instance, if an individual had been at a crime scene earlier or had a similar DNA profile to the actual criminal.Sensitive genetic information, such as family relationships and disease susceptibility, can be obtained from DNA samples. Police, forensic science services, and researchers using the database have access to people’s DNA without their consent. This can be seen as an intrusion of personal privacy and a violation of civil liberties.Studies of the United Kingdom’s criminal database, which retains the DNA samples of all suspects, show that ethnic minorities are over represented in the population of arrestees and are, therefore, overrepresented in the criminal DNA database. This raises the concern of an institutionalized ethnic bias in the criminal justice system.Even the most secure database has a chance of being compromised.DNA Forensics LinksAdvancing Justice Through DNA Technology (2004 Report from the Department of Justice)Master Index: An Information Center in Forensic Science, Law, and Public Policy for Lawyers, Forensic Scientists, Educators, and Public OfficialsThe Forensic Use of Bioinformation: Ethical Issues a report from the Nuffield Council on Bioethics (2006).National Institute of Justice PublicationsDNA: A Prosecutor's Practice Notebook (2007).Can Jury Trial Innovations Improve Juror Understanding of DNA Evidence? (November 2006).Lessons Learned From 9/11: DNA Identification in Mass Fatality Incidents (September 2006).DNA Analysis for “Minor” Crimes: A Major Benefit for Law Enforcement (January 2006).Identifying Victims Using DNA: A Guide for Families (April 2005).Report to the Attorney General on Delays in Forensic DNA Analysis (March 2003).Using DNA to Solve Cold Cases (October 2002).Improved Analysis of DNA Short Tandem Repeats With Time-of-Flight Mass Spectrometry (October 2001).Understanding DNA Evidence: A Guide for Victim Service Providers (May 2001).The Future of Forensic DNA Testing: Predictions of the Research and Development Working Group (November 2000).Postconviction DNA Testing: Recommendations for Handling Requests (September 1999).What Every Law Enforcement Officer Should Know About DNA Evidence (September 1999).http://health.discovery.com/minisites/dna/zs_forensics.html ForensicScience and Genetic Variation - Discovery Health DNA sciences. -->Explore Forensics WebsiteDNA and the Criminal Justice System (Book, Nov. 2004)DNA Fingerprinting in Human Health and SocietyHow DNA evidence works - From the "How Stuff Works" Web site.DNA in the Courtroom: A Trial Watcher's Guide - Online technical guide originally developed for reporters covering the O.J. Simpson trial.DNA & Serial Killer Search - NPR "Morning Edition" audio February 2003 - DNA gathering involved in a search for a serial killer may be infringing on the rights of some innocent people.Maryland Man's Exoneration Didn't End Nightmare - First death row inmate cleared by DNA. Article from the Washingtion Post. (February 24, 2003).DNA Evidence - NPR "Morning Edition" audio January 10, 2002.DNA Exonerations - NPR "All Things Considered" audio January 2002 - Prisoner released as a result of DNA evidence.DNA Technology Helps Reunite 418 Families People's Daily September 20, 2000DNA Evidence and Prior Convictions - NPR "Morning Edition" audio August 2000 - A report on lost evidence in a North Carolina murder case.Interview with DNA Forensics Authority Dr. Bruce Weir.
Kingdom Monera
Characteristics:no membrane-covered nuclei and organellesmostly unicellularreproduce asexually by binary fissionproduce food through photosynthesis but use a wider variety of substances as raw materials than eukaryotestiny organismsa cell wall, usually surrounded by a layer of slime, encloses the cellBacteria-simplest microorganisms, single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that produced by fissionClassification according to shape:1.coccus-spherical2.bacillus-rodlike3.spirillus-spiralFunctions of Monerans:Sewage disposalProduction of cheese and vinegarUsed in tanning leather and curing meatproduction of anibiotics like neomycinBiological control of harmful insectsCharacteristics of Monerans:1.cell wall-peptidoglycan2.flagellum-for movement3.pili-for attachment4.mode of reproductionAsexual:Binary FissionSexual:Conjugation, Transduction and TransformationCyanobacteria-predominantly photosynthetic prokaryotic organisms containing blue pigment in addition to chlorophyll-occur singly or in colonies in diverse habitats that can form filaments that they split up in 2 or break into fragments for reproduction-examples:Anacbaena, oscillatoria, nostoc-can carry out photosynthesis and absorb food from surroundingsTwo Prokaryotic Kingdoms:ArchaebacteriaKingdom of prokaryotes more like eukaryotic cells than eubacteria.Major Groups of Archaebacteria:Methanogens(methane maker)live at swamps, sewage, stockyards, animal guts and other oxygen free habitatstheir anaerobic pathway ends in methanethey release 2 billion tons of methane from termite guts, ruminant guts, wetlands, rice paddies and landfillsthis tremendous quantities of this by-product influence carbon dioxide levels in the atmosphere and the global carbon dioxide cycleExtreme halophiles(salt lovers)live in very salty water, as in brackish ponds and salt lakes, and near hydrotherml ventsthey spoil salted fish, animal hides, and commercial sea saltmost of them make ATP by aerobic pathwayssome also switch to a photosynthetic pathway when oxygen is lowExtreme thermophiles(heat lovers)live in highly acidic soils, hot springs, even coal mine wastessome start the food webs at hydrothermal vents, where water reaches 110 degrees Celsiusthey get electrons from hydrogen sulfidethey are cited as evidence that life originated deep in the oceansChemosynthesizersInstead of using the Sun's energy, chemosynthesizers absorb compounds that contain sulfur, iron and nitrogen, and get their energy through a process called oxidation. They use the energy to change carbon dioxide into organic food molecules, which support a whole community of other organisms. Chemosynthesizers can live in harsh environments where no other producer could survive, like the hot sulfur vents on the ocean floor.EubacteriaUnlike archaebacteria, they have fatty acids in their plasma membrane. In most cases their cell wall incorporates peptidoglycan.Modes of Nutrition:Photoautotrophscyanobacteria, or blue-green algae, are common oxygen releasing, photosynthetic typesyou may see them at ponds and lakes where mucus-sheathed chains of cells form slimy mats in nutrient enriched waterAnabaena and other types can convert nitrogen to ammonia for biosynthesisif nitrogen compounds dwindle, modified cells call heterocysts synthesize a nirogen fixing enzyme. they produce and share nitrogen compounds with other cells in the chains and get carbohydrates in return.anaerobic photosynthesizers get electrons from hydrogen sulfide or hydrogen gas, not water. They may resemble anaerobic bacteria in which the cyclicathway of photosynthesis is involved.Chemoautotrophsthey have mighty roles in the cycling of nitrogen and other nutrients Chemoheterotrophsmany have roles as decomposers and as human helpersLactobacillus is used to help make pickles, buttermilk and yoghurtActinomycetes to make antibioticsE. coli in our gut produces Vitamin K and compounds that help us digest fat. It also helps newborns digest milk, and it prevents many food-borne pathogens from colonizing the human gutsugarcane and corn rely on the nirogen-fixing spirochete Azospirillum. They take up some nitrogen fixed by this symbiont and give some sugars to it.Gram Positive Bacteria-the bacteria's cell wall is mad eup of a protein-sugar complex that takes on a purple color during the Gram Staining .Gram Negative Bacteria-the gram negative bacteriahas an extra layer of lipid on the outside of lipid on the outside of the cell wall and appear pink during the Gram Staining
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http://biology-g10p.blogspot.com/2007/01/virus.html
Virus
VIRUS
Virus (biology) (Latin, “poison”), any of a number of organic entities consisting simply of genetic material surrounded by a protective coat. The term “virus” was first used in the 1890s to describe agents that caused diseases but were smaller than bacteria. By itself a virus is a lifeless form, but within living cells it can replicate many times and harm its host in the process. There are at least 3,600 types of virus, hundreds of which are known to cause a wide range of diseases in humans, other animals, insects, bacteria, and plants.
The existence of viruses was established in 1892, when Russian scientist Dmitry I. Ivanovsky discovered microscopic particles later known as the tobacco mosaic virus. The name virus was applied to these infectious particles in 1898 by the Dutch botanist Martinus W. Beijerinck. A few years later, viruses were found growing in bacteria; these viruses were dubbed bacteriophages. Then, in 1935, the American biochemist Wendell Meredith Stanley crystallized tobacco mosaic virus and showed that it is composed only of the genetic material called ribonucleic acid (RNA) and a protein covering. In the 1940s development of the electron microscope made visualization of viruses possible for the first time. This was followed by development of high-speed centrifuges used to concentrate and purify viruses. The study of animal viruses reached a major turning point in the 1950s with the development of methods to culture cells that could support virus replication in test tubes. Numerous viruses were subsequently discovered, and in the 1960s and 1970s most were analysed to determine their physical and chemical characteristics.
Characteristics
Viruses are submicroscopic intracellular parasites that consist of either RNA or deoxyribonucleic acid (DNA)—never both—plus a protective coat of protein or of protein combined with lipid or carbohydrate components. The nucleic acid is usually a single molecule, either singly or doubly stranded. Some viruses, however, may have nucleic acid that is segmented into two or more pieces. The protein shell is termed the capsid, and the protein subunits of the capsid are called capsomeres. Together these form the nucleocapsid. Other viruses have an additional envelope that is usually acquired as the nucleocapsid buds from the host cell. The complete virus particle is called the virion. Viruses are obligate intracellular parasites; that is, their replication can take place only in actively metabolizing cells. Outside living cells, viruses exist as inert macromolecules (very large molecules).
Viruses vary considerably in size and shape. Three basic structural groups exist: isometric; rod shaped or elongated; and tadpole-like, with head and tail (as in some bacteriophages). The smallest viruses are icosahedrons (20-sided polygons) that measure about 18 to 20 nanometres wide (one-millionth of a millimetre = 1 nanometre). The largest viruses are rod shaped. Some rod-shaped viruses may measure several microns in length, but they are still usually less than 100 nanometres in width. Thus, the widths of even the largest viruses are below the limits of resolution of the light microscope, which is used to study bacteria and other large micro-organisms.
Many of the viruses with helical internal structure have outer coverings (also known as envelopes) composed of lipoprotein or glycoprotein, or both. These viruses appear roughly spherical or in various other shapes, and they range from about 60 to more than 300 nanometres in diameter. Complex viruses, such as some bacteriophages, have heads and a tubular tail, which attaches to host bacteria. The pox viruses are brick shaped and have a complex protein composition. Complex and pox viruses are exceptions, however; most viruses have a simple shape.
Replication
Viruses do not contain the enzymes and metabolic precursors necessary for self-replication. They have to get these from the host cells that they infect. Viral replication, therefore, is a process of separate synthesis of viral components and assembly of these into new virus particles. Replication begins when a virus enters the cell. The virus coat is removed by cellular enzymes, and the virus RNA or DNA comes into contact with ribosomes (cell organs that synthesize proteins) inside the cell. There the virus RNA or DNA directs the synthesis of proteins specified by the viral nucleic acid. The nucleic acid replicates itself, and the protein subunits constituting the viral coat are synthesized. Thereafter, the two components are assembled into a new virus. One infecting virus can give rise to thousands of progeny viruses. Some viruses are released by destruction of the infected cell. Others are released by budding through cell membranes and do not kill the cell. In some instances, infections are “silent”—that is, viruses may replicate within the cell but cause no obvious cell damage.
Lytic and Lysogenic Cycles of a Bacteriophage. All bacteriophages (viruses that parasitize bacteria) have a lytic or infectious cycle, in which the virus, incapable of replicating itself, injects its genetic material into a bacterium. By pirating its host’s enzymes and protein-building capacities, the virus can reproduce and repackage, making about 100 new copies before it bursts from and destroys the bacterium. Some bacteriophages, however, behave differently when they infect a bacterium. The injected genetic material instead integrates itself into its host DNA, passively replicating with it to be inherited by bacterial daughter cells. In about 1 in 100,000 of these lysogenic cells, the viral DNA spontaneously activates and starts a new lytic cycle.
The RNA-containing viruses are unique among replicative systems in that the RNA can replicate itself independently of DNA. In some cases, the RNA can function as messenger RNA (see Genetics), indirectly replicating itself using the cell's ribosomal and metabolic precursor systems. In other cases, RNA viruses carry within the coat an RNA-dependent enzyme that directs the synthesis of virus RNA. Some RNA viruses, which have come to be known as retroviruses, may produce an enzyme that can synthesize DNA from the RNA molecule. The DNA thus formed then acts as the viral genetic material.Viral Replication Outside of a host cell, a virus is an inert particle. Once inside a cell, a virus can replicate many times, creating thousands of viruses that leave the cell to find host cells of their own. Viruses that cause disease do so by destroying or damaging cells as they leave them.
Bacterial viruses and animal viruses differ somewhat in their interaction with the cell surface during infection. The “T even” bacteriophage that infects the bacterium Escherichia coli, for instance, first attaches to the surface and injects its DNA directly into the bacterium. No absorption and uncoating take place. The basic events of virus replication, however, are the same after the nucleic acid enters the cell.
Viruses in Medicine
Viruses represent a major challenge to medical science in combating infectious diseases. Many cause diseases that are of major importance to humans and that are extraordinary in their diversity.
Included among viral diseases is the common cold, which affects millions of people every year. Recent research has even indicated that the AD-36 virus, which causes cold-like symptoms, affects food-energy absorption and more than doubles the normal layer of body fat in animals. About 30 per cent of obese people had contracted AD-36 compared with 5 per cent of lean people, and so this virus may contribute to obesity in a percentage of people. Other viral diseases are important because they are frequently fatal. These diseases include rabies, haemorrhagic fevers, encephalitis, poliomyelitis, and yellow fever. Most viruses, however, cause diseases that usually only create acute discomfort unless the patient develops serious complications from the virus or from a bacterial infection. Some of these diseases are influenza, measles, mumps, cold sores (also known as herpes simplex), chickenpox, shingles (also known as herpes zoster), respiratory diseases, acute diarrhoea, warts, and hepatitis. Still others, such as rubella (also known as German measles) virus and cytomegalovirus, may cause serious abnormalities or death in unborn infants. Acquired immune deficiency syndrome (AIDS) is caused by a retrovirus. Only two retroviruses are unequivocally linked with human cancers (see Leukaemia and HTLV), but some papilloma virus forms are suspected. Increasing evidence also indicates that other viruses may be involved in some types of cancer and in chronic diseases such as multiple sclerosis and other degenerative diseases. Some of the viruses take a long time to cause disease; kuru and Creutzfeldt-Jakob disease, both of which gradually destroy the brain, are slow virus diseases.
Viruses that cause important human disease are still being discovered. Most can be isolated and identified by laboratory methods, but these usually take several days to complete. One of the most recently discovered viruses is rotavirus, the causal agent of infant gastroenteritis.
Spread
To cause new cases of disease, viruses must be spread from person to person. Many viruses, such as those causing influenza and measles, are transmitted by the respiratory route when virus-containing droplets are put into the air by people coughing and sneezing. Other viruses, such as those that cause diarrhoea, are spread by the faecal-oral route. Still others, such as yellow fever and viruses called arboviruses, are spread by biting insects. Viral diseases are either endemic (present most of the time), causing disease in susceptible people, or epidemic—that is, they come in large waves and attack thousands of people. An example of an epidemic viral disease is the worldwide occurrence of influenza almost every year.
Treatment
Smallpox Vaccination This drawing shows a doctor administering the smallpox vaccine, first discovered in 1796 by British physician Edward Jenner. Jenner found that infecting a patient with cowpox, a minor disease, produced immunity to smallpox, which can cause disfigurement or death. His discoveries won him worldwide renown.
Currently, no completely satisfactory treatments exist for viral infections, because most drugs that destroy viruses also damage the cell. The drug amantadine is used extensively in some countries for treatment of respiratory infections caused by influenza-A viruses, and the drug AZT is used in the treatment of HIV.
One promising antiviral agent, interferon, is produced by the cell itself. This non-toxic protein, which is produced by some animal cells infected with viruses, can protect other cells against such infection. The use of interferon for treating cancer is under intensive study. Until recently, study of the use of interferon has been restricted by its limited availability in pure form. However, new techniques of molecular cloning of genetic material (see Genetic Engineering) now make it possible for scientists to obtain the protein in larger quantities. Its relative value as an antiviral agent has already been established.The only effective way to prevent viral infection is by the use of vaccines. For example, vaccination for smallpox on a worldwide scale in the 1970s eradicated this disease. Many antiviral vaccines have been developed for humans and other animals. Those for humans include vaccines for rubeola (also known as measles), rubella, poliomyelitis, and influenza. Immunization with a virus vaccine stimulates the body's immune mechanism to produce a protein—called an antibody—that will protect against infection with the immunizing virus. The viruses are always altered before they are used for immunization so that they cannot themselves produce disease.
Plant Diseases
Viruses cause a wide variety of diseases in plants and frequently cause serious damage to crops. Common plant-disease viruses are turnip yellow mosaic virus, potato leaf roll virus, and tobacco mosaic virus. Plants have rigid cell walls that plant viruses cannot penetrate, so the most important means of plant-virus spread is provided by animals that feed on plants. Often, healthy plants are infected by insects that carry on their mouthparts viruses acquired while feeding on other infected plants. Nematodes (also known as roundworms) may also transmit viruses while feeding on the roots of healthy plants.
Plant viruses can accumulate in enormous quantities within infected cells. For instance, tobacco mosaic virus may represent as much as 10 per cent of the dry weight of infected plants. Studies on the interaction of plant viruses with plant cells are limited, because plants often cannot be infected directly, but only by means such as an insect vector. Cell cultures in test tubes, which can be infected with plant viruses, are not generally available.
Role in Research
The study of viruses and their interaction with host cells has been a major motivation for the host of fundamental biological studies at a molecular level. For example, the existence of messenger RNA, which carries the genetic code from DNA to define what proteins are made by a cell, was discovered during studies of bacteriophages replicating in bacteria. Studies of bacteriophages have also been instrumental in delineating the biochemical factors that start and stop the utilization of genetic information. Knowledge of how virus replication is controlled is fundamental to understanding biochemical events in higher organisms.
The reason that viruses are so useful as model systems for studying events that control genetic information is that viruses are, in essence, small pieces of genetic information that is different from the genetic information of the cell. This allows scientists to study a smaller and simpler replicating system, but one that works on the same principle as that of the host cell. Much of the research on viruses is aimed at understanding their replicative mechanism in order to find ways to control their growth, so that viral diseases can be eliminated. Studies on viral diseases have also contributed greatly to understanding the body's immune response to infectious agents. Antibodies in blood serum, as well as secretions of the mucous membranes, all of which help the body eliminate foreign elements such as viruses, have been more thoroughly characterized by studying their responses to viral infection. Intense scientific interest is now concentrated on studies designed to isolate certain viral genes. These genes can be used in molecular-cloning systems to produce large amounts of particular virus proteins, which can in turn be used as vaccines.
Virus (biology) (Latin, “poison”), any of a number of organic entities consisting simply of genetic material surrounded by a protective coat. The term “virus” was first used in the 1890s to describe agents that caused diseases but were smaller than bacteria. By itself a virus is a lifeless form, but within living cells it can replicate many times and harm its host in the process. There are at least 3,600 types of virus, hundreds of which are known to cause a wide range of diseases in humans, other animals, insects, bacteria, and plants.
The existence of viruses was established in 1892, when Russian scientist Dmitry I. Ivanovsky discovered microscopic particles later known as the tobacco mosaic virus. The name virus was applied to these infectious particles in 1898 by the Dutch botanist Martinus W. Beijerinck. A few years later, viruses were found growing in bacteria; these viruses were dubbed bacteriophages. Then, in 1935, the American biochemist Wendell Meredith Stanley crystallized tobacco mosaic virus and showed that it is composed only of the genetic material called ribonucleic acid (RNA) and a protein covering. In the 1940s development of the electron microscope made visualization of viruses possible for the first time. This was followed by development of high-speed centrifuges used to concentrate and purify viruses. The study of animal viruses reached a major turning point in the 1950s with the development of methods to culture cells that could support virus replication in test tubes. Numerous viruses were subsequently discovered, and in the 1960s and 1970s most were analysed to determine their physical and chemical characteristics.
Characteristics
Viruses are submicroscopic intracellular parasites that consist of either RNA or deoxyribonucleic acid (DNA)—never both—plus a protective coat of protein or of protein combined with lipid or carbohydrate components. The nucleic acid is usually a single molecule, either singly or doubly stranded. Some viruses, however, may have nucleic acid that is segmented into two or more pieces. The protein shell is termed the capsid, and the protein subunits of the capsid are called capsomeres. Together these form the nucleocapsid. Other viruses have an additional envelope that is usually acquired as the nucleocapsid buds from the host cell. The complete virus particle is called the virion. Viruses are obligate intracellular parasites; that is, their replication can take place only in actively metabolizing cells. Outside living cells, viruses exist as inert macromolecules (very large molecules).
Viruses vary considerably in size and shape. Three basic structural groups exist: isometric; rod shaped or elongated; and tadpole-like, with head and tail (as in some bacteriophages). The smallest viruses are icosahedrons (20-sided polygons) that measure about 18 to 20 nanometres wide (one-millionth of a millimetre = 1 nanometre). The largest viruses are rod shaped. Some rod-shaped viruses may measure several microns in length, but they are still usually less than 100 nanometres in width. Thus, the widths of even the largest viruses are below the limits of resolution of the light microscope, which is used to study bacteria and other large micro-organisms.
Many of the viruses with helical internal structure have outer coverings (also known as envelopes) composed of lipoprotein or glycoprotein, or both. These viruses appear roughly spherical or in various other shapes, and they range from about 60 to more than 300 nanometres in diameter. Complex viruses, such as some bacteriophages, have heads and a tubular tail, which attaches to host bacteria. The pox viruses are brick shaped and have a complex protein composition. Complex and pox viruses are exceptions, however; most viruses have a simple shape.
Replication
Viruses do not contain the enzymes and metabolic precursors necessary for self-replication. They have to get these from the host cells that they infect. Viral replication, therefore, is a process of separate synthesis of viral components and assembly of these into new virus particles. Replication begins when a virus enters the cell. The virus coat is removed by cellular enzymes, and the virus RNA or DNA comes into contact with ribosomes (cell organs that synthesize proteins) inside the cell. There the virus RNA or DNA directs the synthesis of proteins specified by the viral nucleic acid. The nucleic acid replicates itself, and the protein subunits constituting the viral coat are synthesized. Thereafter, the two components are assembled into a new virus. One infecting virus can give rise to thousands of progeny viruses. Some viruses are released by destruction of the infected cell. Others are released by budding through cell membranes and do not kill the cell. In some instances, infections are “silent”—that is, viruses may replicate within the cell but cause no obvious cell damage.
Lytic and Lysogenic Cycles of a Bacteriophage. All bacteriophages (viruses that parasitize bacteria) have a lytic or infectious cycle, in which the virus, incapable of replicating itself, injects its genetic material into a bacterium. By pirating its host’s enzymes and protein-building capacities, the virus can reproduce and repackage, making about 100 new copies before it bursts from and destroys the bacterium. Some bacteriophages, however, behave differently when they infect a bacterium. The injected genetic material instead integrates itself into its host DNA, passively replicating with it to be inherited by bacterial daughter cells. In about 1 in 100,000 of these lysogenic cells, the viral DNA spontaneously activates and starts a new lytic cycle.
The RNA-containing viruses are unique among replicative systems in that the RNA can replicate itself independently of DNA. In some cases, the RNA can function as messenger RNA (see Genetics), indirectly replicating itself using the cell's ribosomal and metabolic precursor systems. In other cases, RNA viruses carry within the coat an RNA-dependent enzyme that directs the synthesis of virus RNA. Some RNA viruses, which have come to be known as retroviruses, may produce an enzyme that can synthesize DNA from the RNA molecule. The DNA thus formed then acts as the viral genetic material.Viral Replication Outside of a host cell, a virus is an inert particle. Once inside a cell, a virus can replicate many times, creating thousands of viruses that leave the cell to find host cells of their own. Viruses that cause disease do so by destroying or damaging cells as they leave them.
Bacterial viruses and animal viruses differ somewhat in their interaction with the cell surface during infection. The “T even” bacteriophage that infects the bacterium Escherichia coli, for instance, first attaches to the surface and injects its DNA directly into the bacterium. No absorption and uncoating take place. The basic events of virus replication, however, are the same after the nucleic acid enters the cell.
Viruses in Medicine
Viruses represent a major challenge to medical science in combating infectious diseases. Many cause diseases that are of major importance to humans and that are extraordinary in their diversity.
Included among viral diseases is the common cold, which affects millions of people every year. Recent research has even indicated that the AD-36 virus, which causes cold-like symptoms, affects food-energy absorption and more than doubles the normal layer of body fat in animals. About 30 per cent of obese people had contracted AD-36 compared with 5 per cent of lean people, and so this virus may contribute to obesity in a percentage of people. Other viral diseases are important because they are frequently fatal. These diseases include rabies, haemorrhagic fevers, encephalitis, poliomyelitis, and yellow fever. Most viruses, however, cause diseases that usually only create acute discomfort unless the patient develops serious complications from the virus or from a bacterial infection. Some of these diseases are influenza, measles, mumps, cold sores (also known as herpes simplex), chickenpox, shingles (also known as herpes zoster), respiratory diseases, acute diarrhoea, warts, and hepatitis. Still others, such as rubella (also known as German measles) virus and cytomegalovirus, may cause serious abnormalities or death in unborn infants. Acquired immune deficiency syndrome (AIDS) is caused by a retrovirus. Only two retroviruses are unequivocally linked with human cancers (see Leukaemia and HTLV), but some papilloma virus forms are suspected. Increasing evidence also indicates that other viruses may be involved in some types of cancer and in chronic diseases such as multiple sclerosis and other degenerative diseases. Some of the viruses take a long time to cause disease; kuru and Creutzfeldt-Jakob disease, both of which gradually destroy the brain, are slow virus diseases.
Viruses that cause important human disease are still being discovered. Most can be isolated and identified by laboratory methods, but these usually take several days to complete. One of the most recently discovered viruses is rotavirus, the causal agent of infant gastroenteritis.
Spread
To cause new cases of disease, viruses must be spread from person to person. Many viruses, such as those causing influenza and measles, are transmitted by the respiratory route when virus-containing droplets are put into the air by people coughing and sneezing. Other viruses, such as those that cause diarrhoea, are spread by the faecal-oral route. Still others, such as yellow fever and viruses called arboviruses, are spread by biting insects. Viral diseases are either endemic (present most of the time), causing disease in susceptible people, or epidemic—that is, they come in large waves and attack thousands of people. An example of an epidemic viral disease is the worldwide occurrence of influenza almost every year.
Treatment
Smallpox Vaccination This drawing shows a doctor administering the smallpox vaccine, first discovered in 1796 by British physician Edward Jenner. Jenner found that infecting a patient with cowpox, a minor disease, produced immunity to smallpox, which can cause disfigurement or death. His discoveries won him worldwide renown.
Currently, no completely satisfactory treatments exist for viral infections, because most drugs that destroy viruses also damage the cell. The drug amantadine is used extensively in some countries for treatment of respiratory infections caused by influenza-A viruses, and the drug AZT is used in the treatment of HIV.
One promising antiviral agent, interferon, is produced by the cell itself. This non-toxic protein, which is produced by some animal cells infected with viruses, can protect other cells against such infection. The use of interferon for treating cancer is under intensive study. Until recently, study of the use of interferon has been restricted by its limited availability in pure form. However, new techniques of molecular cloning of genetic material (see Genetic Engineering) now make it possible for scientists to obtain the protein in larger quantities. Its relative value as an antiviral agent has already been established.The only effective way to prevent viral infection is by the use of vaccines. For example, vaccination for smallpox on a worldwide scale in the 1970s eradicated this disease. Many antiviral vaccines have been developed for humans and other animals. Those for humans include vaccines for rubeola (also known as measles), rubella, poliomyelitis, and influenza. Immunization with a virus vaccine stimulates the body's immune mechanism to produce a protein—called an antibody—that will protect against infection with the immunizing virus. The viruses are always altered before they are used for immunization so that they cannot themselves produce disease.
Plant Diseases
Viruses cause a wide variety of diseases in plants and frequently cause serious damage to crops. Common plant-disease viruses are turnip yellow mosaic virus, potato leaf roll virus, and tobacco mosaic virus. Plants have rigid cell walls that plant viruses cannot penetrate, so the most important means of plant-virus spread is provided by animals that feed on plants. Often, healthy plants are infected by insects that carry on their mouthparts viruses acquired while feeding on other infected plants. Nematodes (also known as roundworms) may also transmit viruses while feeding on the roots of healthy plants.
Plant viruses can accumulate in enormous quantities within infected cells. For instance, tobacco mosaic virus may represent as much as 10 per cent of the dry weight of infected plants. Studies on the interaction of plant viruses with plant cells are limited, because plants often cannot be infected directly, but only by means such as an insect vector. Cell cultures in test tubes, which can be infected with plant viruses, are not generally available.
Role in Research
The study of viruses and their interaction with host cells has been a major motivation for the host of fundamental biological studies at a molecular level. For example, the existence of messenger RNA, which carries the genetic code from DNA to define what proteins are made by a cell, was discovered during studies of bacteriophages replicating in bacteria. Studies of bacteriophages have also been instrumental in delineating the biochemical factors that start and stop the utilization of genetic information. Knowledge of how virus replication is controlled is fundamental to understanding biochemical events in higher organisms.
The reason that viruses are so useful as model systems for studying events that control genetic information is that viruses are, in essence, small pieces of genetic information that is different from the genetic information of the cell. This allows scientists to study a smaller and simpler replicating system, but one that works on the same principle as that of the host cell. Much of the research on viruses is aimed at understanding their replicative mechanism in order to find ways to control their growth, so that viral diseases can be eliminated. Studies on viral diseases have also contributed greatly to understanding the body's immune response to infectious agents. Antibodies in blood serum, as well as secretions of the mucous membranes, all of which help the body eliminate foreign elements such as viruses, have been more thoroughly characterized by studying their responses to viral infection. Intense scientific interest is now concentrated on studies designed to isolate certain viral genes. These genes can be used in molecular-cloning systems to produce large amounts of particular virus proteins, which can in turn be used as vaccines.
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