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87-404: The Hershey–Chase experiments were a series of experiments conducted in 1952 by Alfred Hershey and Martha Chase that helped to confirm that DNA is genetic material . While DNA had been known to biologists since 1869, many scientists still assumed at the time that proteins carried the information for inheritance because DNA appeared to be an inert molecule, and, since it is located in

174-415: A "what-if" question, without a specific expectation about what the experiment reveals, or to confirm prior results. If an experiment is carefully conducted, the results usually either support or disprove the hypothesis. According to some philosophies of science , an experiment can never "prove" a hypothesis, it can only add support. On the other hand, an experiment that provides a counterexample can disprove

261-457: A 'true experiment' is a method of social research in which there are two kinds of variables . The independent variable is manipulated by the experimenter, and the dependent variable is measured. The signifying characteristic of a true experiment is that it randomly allocates the subjects to neutralize experimenter bias , and ensures, over a large number of iterations of the experiment, that it controls for all confounding factors. Depending on

348-556: A different elemental isotope . This allowed each to be observed and analyzed separately. Since phosphorus is contained in DNA but not amino acids, radioactive phosphorus-32 was used to label the DNA contained in the T2 phage . Radioactive sulfur-35 was used to label the protein sections of the T2 phage, because sulfur is contained in protein but not DNA. Hershey and Chase inserted the radioactive elements in

435-410: A disease), and informed consent . For example, in psychology or health care, it is unethical to provide a substandard treatment to patients. Therefore, ethical review boards are supposed to stop clinical trials and other experiments unless a new treatment is believed to offer benefits as good as current best practice. It is also generally unethical (and often illegal) to conduct randomized experiments on

522-410: A logical/ mental derivation. In this process of critical consideration, the man himself should not forget that he tends to subjective opinions—through "prejudices" and "leniency"—and thus has to be critical about his own way of building hypotheses. Francis Bacon (1561–1626), an English philosopher and scientist active in the 17th century, became an influential supporter of experimental science in

609-530: A net flow of water into the cell. Through observation of plasmolysis and deplasmolysis, it is possible to determine the tonicity of the cell's environment as well as the rate solute molecules cross the cellular membrane . The term plasmolysis is derived from the Latin word ‘plasma’ meaning ‘matrix’ and the Greek word ‘lysis’, meaning ‘loosening’. A plant cell in hypotonic solution will absorb water by endosmosis , so that

696-402: A protective protein coat was formed around the bacteriophage, but that the internal DNA is what conferred its ability to produce progeny inside a bacterium. They showed that, in growth, protein has no function, while DNA has some function. They determined this from the amount of radioactive material remaining outside of the cell. Only 20% of the P remained outside the cell, demonstrating that it

783-436: A range of chocolates to find a favorite), to highly controlled (e.g. tests requiring complex apparatus overseen by many scientists that hope to discover information about subatomic particles). Uses of experiments vary considerably between the natural and human sciences. Experiments typically include controls , which are designed to minimize the effects of variables other than the single independent variable . This increases

870-418: A strictly controlled test execution with a sensibility for the subjectivity and susceptibility of outcomes due to the nature of man is necessary. Furthermore, a critical view on the results and outcomes of earlier scholars is necessary: It is thus the duty of the man who studies the writings of scientists, if learning the truth is his goal, to make himself an enemy of all that he reads, and, applying his mind to

957-405: A test does not produce a measurable positive result. Most often the value of the negative control is treated as a "background" value to subtract from the test sample results. Sometimes the positive control takes the quadrant of a standard curve . An example that is often used in teaching laboratories is a controlled protein assay . Students might be given a fluid sample containing an unknown (to

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1044-466: A theory or hypothesis, but a theory can always be salvaged by appropriate ad hoc modifications at the expense of simplicity. An experiment must also control the possible confounding factors —any factors that would mar the accuracy or repeatability of the experiment or the ability to interpret the results. Confounding is commonly eliminated through scientific controls and/or, in randomized experiments , through random assignment . In engineering and

1131-432: Is a procedure similar to the actual experimental test but is known from previous experience to give a positive result. A negative control is known to give a negative result. The positive control confirms that the basic conditions of the experiment were able to produce a positive result, even if none of the actual experimental samples produce a positive result. The negative control demonstrates the base-line result obtained when

1218-408: Is always irreversible while concave plasmolysis is usually reversible. During concave plasmolysis, the plasma membrane and the enclosed protoplast partially shrinks from the cell wall due to half-spherical, inwarding curving pockets forming between the plasma membrane and the cell wall. During convex plasmolysis, the plasma membrane and the enclosed protoplast shrinks completely from the cell wall, with

1305-406: Is clearly impossible, when testing the hypothesis "Stars are collapsed clouds of hydrogen", to start out with a giant cloud of hydrogen, and then perform the experiment of waiting a few billion years for it to form a star. However, by observing various clouds of hydrogen in various states of collapse, and other implications of the hypothesis (for example, the presence of various spectral emissions from

1392-432: Is hypertonic, exosmosis takes place and the space between the cell wall and cytoplasm is filled with solutes, as most of the water drains away and hence the concentration inside the cell becomes more hypertonic. There are some mechanisms in plants to prevent excess water loss in the same way as excess water gain. Plasmolysis can be reversed if the cell is placed in a hypotonic solution. Stomata close to help keep water in

1479-502: Is induced in the laboratory by immersing cells in strong saline or sugar (sucrose) solutions to cause exosmosis , often using Elodea plants or onion epidermal cells , which have colored cell sap so that the process is clearly visible. Methylene blue can be used to stain plant cells. Plasmolysis is mainly known as shrinking of cell membrane in hypertonic solution and great pressure. Plasmolysis can be of two types, either concave plasmolysis or convex plasmolysis. Convex plasmolysis

1566-926: Is possible depends on the observed correlation between explanatory variables in the observed data. When these variables are not well correlated, natural experiments can approach the power of controlled experiments. Usually, however, there is some correlation between these variables, which reduces the reliability of natural experiments relative to what could be concluded if a controlled experiment were performed. Also, because natural experiments usually take place in uncontrolled environments, variables from undetected sources are neither measured nor held constant, and these may produce illusory correlations in variables under study. Much research in several science disciplines, including economics , human geography , archaeology , sociology , cultural anthropology , geology , paleontology , ecology , meteorology , and astronomy , relies on quasi-experiments. For example, in astronomy it

1653-406: Is sulfur-free enters the bacterial cell after phage adsorption. Further research was necessary to conclude that it was solely bacteriophages' DNA that entered the cell and not a combination of protein and DNA where the protein did not contain any sulfur. Hershey and Chase concluded that protein was not likely to be the hereditary genetic material. However, they did not make any conclusions regarding

1740-417: Is the step in the scientific method that helps people decide between two or more competing explanations—or hypotheses . These hypotheses suggest reasons to explain a phenomenon or predict the results of an action. An example might be the hypothesis that "if I release this ball, it will fall to the floor": this suggestion can then be tested by carrying out the experiment of letting go of the ball, and observing

1827-418: Is uncommon. In medicine and the social sciences , the prevalence of experimental research varies widely across disciplines. When used, however, experiments typically follow the form of the clinical trial , where experimental units (usually individual human beings) are randomly assigned to a treatment or control condition where one or more outcomes are assessed. In contrast to norms in the physical sciences,

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1914-588: The Avery–MacLeod–McCarty experiment , and successors served to unequivocally establish that hereditary information was carried by DNA. This finding has numerous applications in forensic science , crime investigation and genealogy . It provided the background knowledge for further applications in DNA forensics, where DNA fingerprinting uses data originating from DNA, not protein sources, to deduce genetic variation . Experiment A child may carry out basic experiments to understand how things fall to

2001-505: The English renaissance . He disagreed with the method of answering scientific questions by deduction —similar to Ibn al-Haytham —and described it as follows: "Having first determined the question according to his will, man then resorts to experience, and bending her to conformity with his placets, leads her about like a captive in a procession." Bacon wanted a method that relied on repeatable observations, or experiments. Notably, he first ordered

2088-481: The Manhattan Project implied the use of nuclear reactions to harm human beings even though the experiments did not directly involve any human subjects. Plasmolysis Plasmolysis is the process in which cells lose water in a hypertonic solution. The reverse process, deplasmolysis or cytolysis , can occur if the cell is in a hypotonic solution resulting in a lower external osmotic pressure and

2175-416: The branches of science . For example, agricultural research frequently uses randomized experiments (e.g., to test the comparative effectiveness of different fertilizers), while experimental economics often involves experimental tests of theorized human behaviors without relying on random assignment of individuals to treatment and control conditions. One of the first methodical approaches to experiments in

2262-449: The physical sciences , experiments are a primary component of the scientific method. They are used to test theories and hypotheses about how physical processes work under particular conditions (e.g., whether a particular engineering process can produce a desired chemical compound). Typically, experiments in these fields focus on replication of identical procedures in hopes of producing identical results in each replication. Random assignment

2349-498: The Hershey–Chase experiment helped to confirm that DNA, not protein, is the genetic material. Hershey and Chase showed that the introduction of deoxyribonuclease (referred to as DNase ), an enzyme that breaks down DNA, into a solution containing the labeled bacteriophages did not introduce any P into the solution. This demonstrated that the phage is resistant to the enzyme while intact. Additionally, they were able to plasmolyze

2436-425: The accuracy of the hypotheses. Experiments can be also designed to estimate spillover effects onto nearby untreated units. The term "experiment" usually implies a controlled experiment, but sometimes controlled experiments are prohibitively difficult, impossible, unethical or illegal. In this case researchers resort to natural experiments or quasi-experiments . Natural experiments rely solely on observations of

2523-410: The advantage that outcomes are observed in a natural setting rather than in a contrived laboratory environment. For this reason, field experiments are sometimes seen as having higher external validity than laboratory experiments. However, like natural experiments, field experiments suffer from the possibility of contamination: experimental conditions can be controlled with more precision and certainty in

2610-571: The amount of some cell or substance in the blood, physical strength or endurance, etc.) and not based on a subject's or a professional observer's opinion. In this way, the design of an observational study can render the results more objective and therefore, more convincing. By placing the distribution of the independent variable(s) under the control of the researcher, an experiment—particularly when it involves human subjects —introduces potential ethical considerations, such as balancing benefit and harm, fairly distributing interventions (e.g., treatments for

2697-466: The bacteriophages by adding the isotopes to separate media within which bacteria were allowed to grow for 4 hours before bacteriophage introduction. When the bacteriophages infected the bacteria, the progeny contained the radioactive isotopes in their structures. This procedure was performed once for the sulfur-labeled phages and once for phosphorus-labeled phages. The labeled progeny were then allowed to infect unlabeled bacteria. The phage coats remained on

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2784-436: The bacteriophages so that they went into osmotic shock, which effectively created a solution containing most of the P and a heavier solution containing structures called "ghosts" that contained the S and the protein coat of the virus. It was found that these "ghosts" could adsorb to bacteria that were susceptible to T2, although they contained no DNA and were simply the remains of the original viral capsule. They concluded that

2871-405: The cell wall and the membrane and making the plant cell shrink and crumple. A continued decrease in pressure eventually leads to cytorrhysis – the complete collapse of the cell wall. Plants with cells in this condition wilt. After plasmolysis the gap between the cell wall and the cell membrane in a plant cell is filled with hypertonic solution. This is because as the solution surrounding the cell

2958-421: The cell. Vacuoles play a role in turgor pressure when water leaves the cell due to hyperosmotic solutions containing solutes such as mannitol , sorbitol , and sucrose . If a plant cell is placed in a hypertonic solution, the plant cell loses water and hence turgor pressure by plasmolysis: pressure decreases to the point where the protoplasm of the cell peels away from the cell wall, leaving gaps between

3045-441: The centuries that followed, people who applied the scientific method in different areas made important advances and discoveries. For example, Galileo Galilei (1564–1642) accurately measured time and experimented to make accurate measurements and conclusions about the speed of a falling body. Antoine Lavoisier (1743–1794), a French chemist, used experiment to describe new areas, such as combustion and biochemistry and to develop

3132-551: The core and margins of its content, attack it from every side. He should also suspect himself as he performs his critical examination of it, so that he may avoid falling into either prejudice or leniency. Thus, a comparison of earlier results with the experimental results is necessary for an objective experiment—the visible results being more important. In the end, this may mean that an experimental researcher must find enough courage to discard traditional opinions or results, especially if these results are not experimental but results from

3219-437: The covariates that can be identified. Researchers attempt to reduce the biases of observational studies with matching methods such as propensity score matching , which require large populations of subjects and extensive information on covariates. However, propensity score matching is no longer recommended as a technique because it can increase, rather than decrease, bias. Outcomes are also quantified when possible (bone density,

3306-483: The data in light of them (though this may be rare when social phenomena are under examination). For an observational science to be valid, the experimenter must know and account for confounding factors. In these situations, observational studies have value because they often suggest hypotheses that can be tested with randomized experiments or by collecting fresh data. Fundamentally, however, observational studies are not experiments. By definition, observational studies lack

3393-516: The design and analysis of experiments occurred in the early 20th century, with contributions from statisticians such as Ronald Fisher (1890–1962), Jerzy Neyman (1894–1981), Oscar Kempthorne (1919–2000), Gertrude Mary Cox (1900–1978), and William Gemmell Cochran (1909–1980), among others. Experiments might be categorized according to a number of dimensions, depending upon professional norms and standards in different fields of study. In some disciplines (e.g., psychology or political science ),

3480-407: The discipline, experiments can be conducted to accomplish different but not mutually exclusive goals: test theories, search for and document phenomena, develop theories, or advise policymakers. These goals also relate differently to validity concerns . A controlled experiment often compares the results obtained from experimental samples against control samples, which are practically identical to

3567-443: The effect of the tested variables. In the scientific method , an experiment is an empirical procedure that arbitrates competing models or hypotheses . Researchers also use experimentation to test existing theories or new hypotheses to support or disprove them. An experiment usually tests a hypothesis , which is an expectation about how a particular process or phenomenon works. However, an experiment may also aim to answer

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3654-495: The effect of the treatment (exposure) from the effects of the other covariates, most of which have not been measured. The mathematical models used to analyze such data must consider each differing covariate (if measured), and results are not meaningful if a covariate is neither randomized nor included in the model. To avoid conditions that render an experiment far less useful, physicians conducting medical trials—say for U.S. Food and Drug Administration approval—quantify and randomize

3741-413: The effects of substandard or harmful treatments, such as the effects of ingesting arsenic on human health. To understand the effects of such exposures, scientists sometimes use observational studies to understand the effects of those factors. Even when experimental research does not directly involve human subjects, it may still present ethical concerns. For example, the nuclear bomb experiments conducted by

3828-470: The experiment is to measure the response to the stimulus by a test method . In the design of experiments , two or more "treatments" are applied to estimate the difference between the mean responses for the treatments. For example, an experiment on baking bread could estimate the difference in the responses associated with quantitative variables, such as the ratio of water to flour, and with qualitative variables, such as strains of yeast. Experimentation

3915-414: The experimental sample except for the one aspect whose effect is being tested (the independent variable ). A good example would be a drug trial. The sample or group receiving the drug would be the experimental group ( treatment group ); and the one receiving the placebo or regular treatment would be the control one. In many laboratory experiments it is good practice to have several replicate samples for

4002-429: The first scholars to use an inductive-experimental method for achieving results. In his Book of Optics he describes the fundamentally new approach to knowledge and research in an experimental sense: We should, that is, recommence the inquiry into its principles and premisses, beginning our investigation with an inspection of the things that exist and a survey of the conditions of visible objects. We should distinguish

4089-413: The focus is typically on the average treatment effect (the difference in outcomes between the treatment and control groups) or another test statistic produced by the experiment. A single study typically does not involve replications of the experiment, but separate studies may be aggregated through systematic review and meta-analysis . There are various differences in experimental practice in each of

4176-456: The gene expression process. These steps include transcription , RNA splicing , translation , and post-translational modification which are used to control the chemical and structural nature of proteins. Moreover, genetic engineering gives engineers the ability to directly manipulate the genetic materials of organisms using recombinant DNA techniques. The first recombinant DNA molecule was created by Paul Berg in 1972 when he combined DNA from

4263-413: The genetic material, is responsible for the synthesis of the thousands of proteins found in cells. They had made this proposal based on the structural similarity that exists between the two macromolecules: both protein and DNA are linear sequences of monomers (amino acids and nucleotides, respectively). Once the Hershey–Chase experiment was published, the scientific community generally acknowledged that DNA

4350-466: The genetic structure of viruses". In the early twentieth century, biologists thought that proteins carried genetic information. This was based on the belief that proteins were more complex than DNA. Phoebus Levene's influential "tetranucleotide hypothesis", which incorrectly proposed that DNA was a repeating set of identical nucleotides , supported this conclusion. The results of the Avery–MacLeod–McCarty experiment , published in 1944, suggested that DNA

4437-482: The ground, while teams of scientists may take years of systematic investigation to advance their understanding of a phenomenon. Experiments and other types of hands-on activities are very important to student learning in the science classroom. Experiments can raise test scores and help a student become more engaged and interested in the material they are learning, especially when used over time. Experiments can vary from personal and informal natural comparisons (e.g. tasting

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4524-402: The groups and that the groups should respond in the same manner if given the same treatment. This equivalency is determined by statistical methods that take into account the amount of variation between individuals and the number of individuals in each group. In fields such as microbiology and chemistry , where there is very little variation between individuals and the group size is easily in

4611-430: The increased volume of water in the cell will increase pressure, making the protoplasm push against the cell wall , a condition known as turgor . Turgor makes plant cells push against each other in the same way and is the main line method of support in non-woody plant tissue. Plant cell walls resist further water entry after a certain point, known as full turgor, which stops plant cells from bursting as animal cells do in

4698-452: The lab. Yet some phenomena (e.g., voter turnout in an election) cannot be easily studied in a laboratory. An observational study is used when it is impractical, unethical, cost-prohibitive (or otherwise inefficient) to fit a physical or social system into a laboratory setting, to completely control confounding factors, or to apply random assignment. It can also be used when confounding factors are either limited or known well enough to analyze

4785-410: The light of stars), we can collect data we require to support the hypothesis. An early example of this type of experiment was the first verification in the 17th century that light does not travel from place to place instantaneously, but instead has a measurable speed. Observation of the appearance of the moons of Jupiter were slightly delayed when Jupiter was farther from Earth, as opposed to when Jupiter

4872-403: The manipulation required for Baconian experiments . In addition, observational studies (e.g., in biological or social systems) often involve variables that are difficult to quantify or control. Observational studies are limited because they lack the statistical properties of randomized experiments. In a randomized experiment, the method of randomization specified in the experimental protocol guides

4959-454: The mean for each group is expected to be the same. For any randomized trial, some variation from the mean is expected, of course, but the randomization ensures that the experimental groups have mean values that are close, due to the central limit theorem and Markov's inequality . With inadequate randomization or low sample size, the systematic variation in covariates between the treatment groups (or exposure groups) makes it difficult to separate

5046-490: The millions, these statistical methods are often bypassed and simply splitting a solution into equal parts is assumed to produce identical sample groups. Once equivalent groups have been formed, the experimenter tries to treat them identically except for the one variable that he or she wishes to isolate. Human experimentation requires special safeguards against outside variables such as the placebo effect . Such experiments are generally double blind , meaning that neither

5133-456: The modern sense is visible in the works of the Arab mathematician and scholar Ibn al-Haytham . He conducted his experiments in the field of optics—going back to optical and mathematical problems in the works of Ptolemy —by controlling his experiments due to factors such as self-criticality, reliance on visible results of the experiments as well as a criticality in terms of earlier results. He was one of

5220-507: The monkey virus SV40 with that of the lambda phage . Experiments on hereditary material during the time of the Hershey–Chase experiment often used bacteriophages as a model organism . Bacteriophages lend themselves to experiments on hereditary material because they incorporate their genetic material into their host cell 's genetic material (making them useful tools), they multiply quickly, and they are easily collected by researchers. The Hershey–Chase experiment, its predecessors, such as

5307-490: The nature of genetic coding and, in turn, understand the process of protein synthesis. George Gamow proposed that the genetic code was composed of sequences of three DNA base pairs known as triplets or codons which represent one of the twenty amino acids. Genetic coding helped researchers to understand the mechanism of gene expression , the process by which information from a gene is used in protein synthesis . Since then, much research has been conducted to modulate steps in

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5394-567: The nucleus, its role was considered to be phosphorus storage. In their experiments, Hershey and Chase showed that when bacteriophages , which are composed of DNA and protein, infect bacteria, their DNA enters the host bacterial cell, but most of their protein does not. Hershey and Chase and subsequent discoveries all served to prove that DNA is the hereditary material. Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria for their "discoveries concerning

5481-452: The outside of the bacteria, while genetic material entered. Disruption of phage from the bacteria by agitation in a blender followed by centrifugation allowed for the separation of the phage coats from the bacteria. These bacteria were lysed to release phage progeny. The progeny of the phages that were labeled with radioactive phosphorus remained labeled, whereas the progeny of the phages labeled with radioactive sulfur were unlabeled. Thus,

5568-505: The plant so it does not dry out. Wax also keeps water in the plant. The equivalent process in animal cells is called crenation . The liquid content of the cell leaks out due to exosmosis. The cell collapses, and the cell membrane pulls away from the cell wall (in plants). Most animal cells consist of only a phospholipid bilayer ( plasma membrane ) and not a cell wall, therefore shrinking up under such conditions. Plasmolysis only occurs in extreme conditions and rarely occurs in nature. It

5655-539: The properties of particulars, and gather by induction what pertains to the eye when vision takes place and what is found in the manner of sensation to be uniform, unchanging, manifest and not subject to doubt. After which we should ascend in our inquiry and reasonings, gradually and orderly, criticizing premisses and exercising caution in regard to conclusions—our aim in all that we make subject to inspection and review being to employ justice, not to follow prejudice, and to take care in all that we judge and criticize that we seek

5742-500: The protein protected the DNA from DNase, but that once the two were separated and the phage was inactivated, the DNase could hydrolyze the phage DNA. Hershey and Chase were also able to prove that the DNA from the phage is inserted into the bacteria shortly after the virus attaches to its host. Using a high-speed blender they were able to force the bacteriophages from the bacterial cells after adsorption . The lack of P-labeled DNA remaining in

5829-409: The reagents for the protein assay but no protein. In this example, all samples are performed in duplicate. The assay is a colorimetric assay in which a spectrophotometer can measure the amount of protein in samples by detecting a colored complex formed by the interaction of protein molecules and molecules of an added dye. In the illustration, the results for the diluted test samples can be compared to

5916-455: The reliability of the results, often through a comparison between control measurements and the other measurements. Scientific controls are a part of the scientific method . Ideally, all variables in an experiment are controlled (accounted for by the control measurements) and none are uncontrolled. In such an experiment, if all controls work as expected, it is possible to conclude that the experiment works as intended, and that results are due to

6003-747: The results of the observational studies are inconsistent and also differ from the results of experiments. For example, epidemiological studies of colon cancer consistently show beneficial correlations with broccoli consumption, while experiments find no benefit. A particular problem with observational studies involving human subjects is the great difficulty attaining fair comparisons between treatments (or exposures), because such studies are prone to selection bias , and groups receiving different treatments (exposures) may differ greatly according to their covariates (age, height, weight, medications, exercise, nutritional status, ethnicity, family medical history, etc.). In contrast, randomization implies that for each covariate,

6090-422: The results of the standard curve (the blue line in the illustration) to estimate the amount of protein in the unknown sample. Controlled experiments can be performed when it is difficult to exactly control all the conditions in an experiment. In this case, the experiment begins by creating two or more sample groups that are probabilistically equivalent, which means that measurements of traits should be similar among

6177-407: The results. Formally, a hypothesis is compared against its opposite or null hypothesis ("if I release this ball, it will not fall to the floor"). The null hypothesis is that there is no explanation or predictive power of the phenomenon through the reasoning that is being investigated. Once hypotheses are defined, an experiment can be carried out and the results analysed to confirm, refute, or define

6264-430: The same conditions. This is also the reason that plants stand upright. Without the stiffness of the plant cells the plant would fall under its own weight. Turgor pressure allows plants to stay firm and erect, and plants without turgor pressure (known as flaccid) wilt. A cell will begin to decline in turgor pressure only when there is no air spaces surrounding it and eventually leads to a greater osmotic pressure than that of

6351-504: The scientific method as we understand it today. There remains simple experience; which, if taken as it comes, is called accident, if sought for, experiment. The true method of experience first lights the candle [hypothesis], and then by means of the candle shows the way [arranges and delimits the experiment]; commencing as it does with experience duly ordered and digested, not bungling or erratic, and from it deducing axioms [theories], and from established axioms again new experiments. In

6438-404: The solution after the bacteriophages had been allowed to adsorb to the bacteria showed that the phage DNA was transferred into the bacterial cell. The presence of almost all the radioactive S in the solution showed that the protein coat that protects the DNA before adsorption stayed outside the cell. Hershey and Chase concluded that DNA, not protein, was the genetic material. They determined that

6525-507: The specific function of DNA as hereditary material, and only said that it must have some undefined role. Confirmation and clarity came a year later in 1953, when James D. Watson and Francis Crick correctly hypothesized, in their journal article " Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid ", the double helix structure of DNA, and suggested the copying mechanism by which DNA functions as hereditary material. Furthermore, Watson and Crick suggested that DNA,

6612-417: The statistical analysis, which is usually specified also by the experimental protocol. Without a statistical model that reflects an objective randomization, the statistical analysis relies on a subjective model. Inferences from subjective models are unreliable in theory and practice. In fact, there are several cases where carefully conducted observational studies consistently give wrong results, that is, where

6699-463: The student) amount of protein. It is their job to correctly perform a controlled experiment in which they determine the concentration of protein in the fluid sample (usually called the "unknown sample"). The teaching lab would be equipped with a protein standard solution with a known protein concentration. Students could make several positive control samples containing various dilutions of the protein standard. Negative control samples would contain all of

6786-461: The test being performed and have both a positive control and a negative control . The results from replicate samples can often be averaged, or if one of the replicates is obviously inconsistent with the results from the other samples, it can be discarded as being the result of an experimental error (some step of the test procedure may have been mistakenly omitted for that sample). Most often, tests are done in duplicate or triplicate. A positive control

6873-403: The theory of conservation of mass (matter). Louis Pasteur (1822–1895) used the scientific method to disprove the prevailing theory of spontaneous generation and to develop the germ theory of disease . Because of the importance of controlling potentially confounding variables, the use of well-designed laboratory experiments is preferred when possible. A considerable amount of progress on

6960-458: The transformation of one strain of Streptococcus pneumoniae to another. The results of these experiments provided evidence that DNA was the biomolecule that carried genetic information. Hershey and Chase needed to be able to examine different parts of the phages they were studying separately, so they needed to distinguish the phage subsections. Viruses were known to be composed of a protein shell and DNA, so they chose to uniquely label each with

7047-523: The truth and not to be swayed by opinion. We may in this way eventually come to the truth that gratifies the heart and gradually and carefully reach the end at which certainty appears; while through criticism and caution we may seize the truth that dispels disagreement and resolves doubtful matters. For all that, we are not free from that human turbidity which is in the nature of man; but we must do our best with what we possess of human power. From God we derive support in all things. According to his explanation,

7134-443: The variables of the system under study, rather than manipulation of just one or a few variables as occurs in controlled experiments. To the degree possible, they attempt to collect data for the system in such a way that contribution from all variables can be determined, and where the effects of variation in certain variables remain approximately constant so that the effects of other variables can be discerned. The degree to which this

7221-422: The volunteer nor the researcher knows which individuals are in the control group or the experimental group until after all of the data have been collected. This ensures that any effects on the volunteer are due to the treatment itself and are not a response to the knowledge that he is being treated. In human experiments, researchers may give a subject (person) a stimulus that the subject responds to. The goal of

7308-503: Was closer to Earth; and this phenomenon was used to demonstrate that the difference in the time of appearance of the moons was consistent with a measurable speed. Field experiments are so named to distinguish them from laboratory experiments, which enforce scientific control by testing a hypothesis in the artificial and highly controlled setting of a laboratory. Often used in the social sciences, and especially in economic analyses of education and health interventions, field experiments have

7395-399: Was incorporated with DNA in the cell's genetic material. All of the S in the protein coats remained outside the cell, showing it was not incorporated into the cell, and that protein is not the genetic material. Hershey and Chase's experiment concluded that little sulfur-containing material entered the bacterial cell. However no specific conclusions can be made regarding whether material that

7482-415: Was the genetic code material. This discovery led to a more detailed investigation of DNA to determine its composition as well as its 3D structure. Using X-ray crystallography , the structure of DNA was discovered by James Watson and Francis Crick with the help of previously documented experimental evidence by Maurice Wilkins and Rosalind Franklin . Knowledge of the structure of DNA led scientists to examine

7569-460: Was the genetic material, but there was still some hesitation within the general scientific community to accept this, which set the stage for the Hershey–Chase experiment. Hershey and Chase, along with others who had done related experiments, confirmed that DNA was the biomolecule that carried genetic information. Before that, Oswald Avery , Colin MacLeod , and Maclyn McCarty had shown that DNA led to

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