One gene, one enzyme | Beadle and Tatum (article) | Khan Academy
Gene 1 encodes an mRNA, which is then translated to make a polypeptide .. a directional relationship known as the central dogma of molecular biology. The major breakthrough in demonstrating the relationship between genes and proteins Their research led them to propose the "one gene–one enzyme" hypothesis. Nirenberg added poly U to a test-tube mixture containing all 20 amino. Ultimately, one wishes to determine how genes—and the proteins they they represent a loss of function of a particular gene—a complementation test can be used to . Domain fusions reveal relationships between functionally linked genes. .. mutagenesis—one can determine exactly which parts of the polypeptide chain.
Typically the fluorescent DNA from the experimental samples labeled, for example, with a red fluorescent dye are mixed with a reference sample of cDNA fragments labeled with a differently colored fluorescent dye green, for example. Thus, if the amount of RNA expressed from a particular gene in the cells of interest is increased relative to that of the reference sample, the resulting spot is red.
Conversely, if the gene's expression is decreased relative to the reference sample, the spot is green. Using such an internal reference, gene expression profiles can be tabulated with great precision. Arrays that contain probes representing all yeast genes have been used to monitor the changes that occur in gene expression as yeast shift from fermenting glucose to growing on ethanol; as they respond to a sudden shift to heat or cold; and as they proceed through different stages of the cell cycle.
One gene–one enzyme hypothesis - Wikipedia
The first study showed that, as yeast use up the last glucose in their medium, their gene expression pattern changes markedly: About half of these genes have no known function, although this study suggests that they are somehow involved in the metabolic reprogramming that occurs when yeast cells shift from fermentation to respiration. Comprehensive studies of gene expression also provide an additional layer of information that is useful for predicting gene function.
Earlier we discussed how identifying a protein 's interaction partners can yield clues about that protein's function. A similar principle holds true for genes: Using a technique called cluster analysis, one can identify sets of genes that are coordinately regulated. Genes that are turned on or turned off together under a variety of different circumstances may work in concert in the cell: Figure Using cluster analysis to identify sets of genes that are coordinately regulated.
Genes that belong to the same cluster may be involved in common cellular pathways or processes. To perform a cluster analysis, microarray data are obtained from cell samples more Targeted Mutations Can Reveal Gene Function Although in rapidly reproducing organisms it is often not difficult to obtain mutants that are deficient in a particular process, such as DNA replication or eye developmentit can take a long time to trace the defect to a particular altered protein.
Recently, recombinant DNA technology and the explosion in genome sequencing have made possible a different type of genetic approach. Instead of beginning with a randomly generated mutant and using it to identify a gene and its protein, one can start with a particular gene and proceed to make mutations in it, creating mutant cells or organisms so as to analyze the gene's function.
Because the new approach reverses the traditional direction of genetic discovery—proceeding from genes and proteins to mutants, rather than vice versa—it is commonly referred to as reverse genetics. Reverse genetics begins with a cloned genea protein with interesting properties that has been isolated from a cell, or simply a genome sequence.
If the starting point is a protein, the gene encoding it is first identified and, if necessary, its nucleotide sequence is determined. The gene sequence can then be altered in vitro to create a mutant version. This engineered mutant gene, together with an appropriate regulatory region, is transferred into a cell.
One Gene-One Polypeptide Concept
Inside the cell, it can integrate into a chromosomebecoming a permanent part of the cell's genome. All of the descendants of the modified cell will now contain the mutant gene. If the original cell used for the gene transfer is a fertilized eggwhole multicellular organisms can be obtained that contain the mutant gene, provided that the mutation does not cause lethality.
In some of these animals, the altered gene will be incorporated into the germ cells—a germline mutation—allowing the mutant gene to be passed on to their progeny. Genetic transformations of this kind are now routinely performed with organisms as complex as fruit flies and mammals. Technically, even humans could now be transformed in this way, although such procedures are not undertaken, even for therapeutic purposes, for fear of the unpredictable aberrations that might occur in such individuals.
Earlier in this chapter we discussed other approaches to discover a gene 's function, including searching for homologous genes in other organisms and determining when and where a gene is expressed. This type of information is especially useful in suggesting what sort of phenotypes to look for in the mutant organisms.
A gene that is expressed only in adult liver, for example, may have a role in degrading toxins, but is not likely to affect the development of the eye. All of these approaches can be used either to study single genes or to attempt a large-scale analysis of the function of every gene in an organism—a burgeoning field known as functional genomics.
Cells and Animals Containing Mutated Genes Can Be Made to Order We have seen that searching for homologous genes and analyzing gene expression patterns can provide clues about gene function, but they do not reveal what exactly a gene does inside a cell. Genetics provides a powerful solution to this problem, because mutants that lack a particular gene may quickly reveal the function of the protein that it encodes. Genetic engineering techniques allow one to specifically produce such gene knockouts, as we will see.
However, one can also generate mutants that express a gene at abnormally high levels overexpressionin the wrong tissue or at the wrong time misexpressionor in a slightly altered form that exerts a dominant phenotype.
One gene, one enzyme
To facilitate such studies of gene function, the coding sequence of a gene and its regulatory regions can be engineered to change the functional properties of the protein product, the amount of protein made, or the particular cell type in which the protein is produced.
Altered genes are introduced into cells in a variety of ways, some of which are described in detail in Chapter 9. DNA can be microinjected into mammalian cells with a glass micropipette or introduced by a virus that has been engineered to carry foreign genes.
In plant cells, genes are frequently introduced by a technique called particle bombardment: DNA samples are painted onto tiny gold beads and then literally shot through the cell wall with a specially modified gun. Electroporation is the method of choice for introducing DNA into bacteria and some other cells. In this technique, a brief electric shock renders the cell membrane temporarily permeable, allowing foreign DNA to enter the cytoplasm.
We will now examine how the study of such mutant cells and organisms allows the dissection of biological pathways. In these organisms an artificially introduced DNA molecule carrying a mutant gene can, with a relatively high frequency, replace the single copy of the normal gene by homologous recombination see p. In this way cells can be made to order that produce an altered form of any specific protein or RNA molecule instead of the normal form of the molecule. If the mutant gene is completely inactive and the gene product normally performs an essential function, the cell dies; but in this case a less severely mutated version of the gene can be used to replace the normal gene, so that the mutant cell survives but is abnormal in the process for which the gene is required.
Often the mutant of choice is one that produces a temperature-sensitive gene product, which functions normally at one temperature but is inactivated when cells are shifted to a higher or lower temperature. Figure Gene replacement, gene knockout, and gene addition. A normal gene can be altered in several ways in a genetically engineered organism.
A The normal gene green can be completely replaced by a mutant copy of the gene reda process called gene replacement. The ability to perform direct gene replacements in lower eucaryotes, combined with the power of standard genetic analyses in these haploid organisms, explains in large part why studies in these types of cells have been so important for working out the details of those processes that are shared by all eucaryotes.
As we shall see, gene replacements are possible, but more difficult to perform in higher eucaryotes, for reasons that are not entirely understood. Moreover, transfection with an altered gene generally leads to gene addition rather than gene replacement: Because gene addition is much more easily accomplished than gene replacement in higher eucaryotic cells, it is useful to create specific dominant negative mutations in which a mutant gene eliminates the activity of its normal counterparts in the cell.
One ingenious approach exploits the specificity of hybridization reactions between two complementary nucleic acid chains.
Normally, only one of the two DNA strands in a given portion of double helix is transcribed into RNAand it is always the same strand for a given gene see Figure If a cloned gene is engineered so that the opposite DNA strand is transcribed instead, it will produce antisense RNA molecules that have a sequence complementary to the normal RNA transcripts. A related method involves synthesizing short antisense nucleic acid molecules chemically or enzymatically and then injecting or otherwise delivering them into cells, again blocking although only temporarily production of the corresponding protein.
Figure The antisense RNA strategy for generating dominant negative mutations. Mutant genes that have been engineered to produce antisense RNA, which is complementary in sequence to the RNA made by the normal gene X, can cause double-stranded RNA to form inside more As investigators continued to explore the antisense RNA strategy, they made an interesting discovery.
An antisense RNA strand can block gene expressionbut a preparation of double-stranded RNA dsRNAcontaining both the sense and antisense strands of a target gene, inhibit the activity of target genes even more effectively see Figure This phenomenon, dubbed RNA interference RNAihas now been exploited for examining gene function in several organisms.
The RNAi technique has been widely used to study gene function in the nematode C. When working with worms, introducing the dsRNA is quite simple: RNA can be injected directly into the intestine of the animal, or the worm can be fed with E.
The RNA is distributed throughout the body of the worm and is found to inhibit expression of the target gene in different tissue types. Further, as explained in Figurethe interference is frequently inherited by the progeny of the injected animal. Because the entire genome of C. In this way, they identified genes involved in cell division in C.
Of these, only 11 had been previously ascribed a function by direct experimentation. Figure Dominant negative mutations created by RNA interference. B Wild-type worm embryo. For unknown reasons, RNA interference does not efficiently inactivate all genes. And interference can sometimes suppress the activity of a target gene in one tissue and not another.
An alternative way to produce a dominant negative mutation takes advantage of the fact that most proteins function as part of a larger protein complex. Such complexes can often be inactivated by the inclusion of just one nonfunctional component. Therefore, by designing a gene that produces large quantities of a mutant protein that is inactive but still able to assemble into the complex, it is often possible to produce a cell in which all the complexes are inactivated despite the presence of the normal protein Figure Figure A dominant negative effect of a protein.
Here a gene is engineered to produce a mutant protein that prevents the normal copies of the same protein from performing their function. In this simple example, the normal protein must form a multisubunit complex more If a protein is required for the survival of the cell or the organisma dominant negative mutant dies, making it impossible to test the function of the protein.
To avoid this problem, one can couple the mutant gene to control sequences that have been engineered to produce the gene product only on command—for example, in response to an increase in temperature or to the presence of a specific signaling molecule.
Cells or organisms containing such a dominant mutant gene under the control of an inducible promoter can be deprived of a specific protein at a particular time, and the effect can then be followed. Inducible promoters also allow genes to be switched on or off in specific tissues, allowing one to examine the effect of the mutant gene in selected parts of the organism.
In the future, techniques for producing dominant negative mutations to inactivate specific genes are likely to be widely used to determine the functions of proteins in higher organisms. Gain-of-Function Mutations Provide Clues to the Role Genes Play in a Cell or Organism In the same way that cells can be engineered to express a dominant negative version of a proteinresulting in a loss-of-function phenotypethey can also be engineered to display a novel phenotype through a gain-of-function mutation.
Such mutations may confer a novel activity on a particular protein, or they may cause a protein with normal activity to be expressed at an inappropriate time or in the wrong tissue in an animal. Regardless of the mechanism, gain-of-function mutations can produce a new phenotype in a cell, tissue, or organism. Often, gain-of-function mutants are generated by expressing a gene at a much higher level than normal in cells.
Such overexpression can be achieved by coupling a gene to a powerful promoter sequence and placing it on a multicopy plasmid —or integrating it in multiple copies in the genome. In either case, the gene is present in many copies and each copy directs the transcription of unusually large numbers of mRNA molecules. Although the effect that such over- expression has on the phenotype of an organism must be interpreted with caution, this approach has provided invaluable insights into the activity of many genes.
In an alternate type of gain-of-function mutationthe mutant protein is made in normal amounts, but is much more active than its normal counterpart.
Such proteins are frequently found in tumors, and they have been exploited to study signal transduction pathways in cells discussed in Chapter Genes can also be expressed at the wrong time or in the wrong place in an organism—often with striking results Figure Such misexpression is most often accomplished by re-engineering the genes themselves, thereby supplying them with the regulatory sequences needed to alter their expression.
Figure Ectopic misexpression of Wnt, a signaling protein that affects development of the body axis in the early Xenopus embryo. In this experiment, mRNA coding for Wnt was injected into the ventral vegetal blastomere, inducing a second body axis discussed in more Genes Can Be Redesigned to Produce Proteins of Any Desired Sequence In studying the action of a gene and the protein it encodes, one does not always wish to make drastic changes—flooding cells with huge quantities of hyperactive protein or eliminating a gene product entirely.
Intro to gene expression (central dogma) (article) | Khan Academy
It is sometimes useful to make slight changes in a protein's structure so that one can begin to dissect which portions of a protein are important for its function. The activity of an enzymefor example, can be studied by changing a single amino acid in its active site. Special techniques are required to alter genes, and their protein products, in such subtle ways.
The first step is often the chemical synthesis of a short DNA molecule containing the desired altered portion of the gene's nucleotide sequence. The synthetic oligonucleotide will now serve as a primer for DNA synthesis by DNA polymerasethereby generating a DNA double helix that incorporates the altered sequence into one of its two strands. After transfectionplasmids that carry the fully modified gene sequence are obtained. The appropriate DNA is then inserted into an expression vector so that the redesigned protein can be produced in the appropriate type of cells for detailed studies of its function.
By changing selected amino acids in a protein in this way—a technique called site-directed mutagenesis —one can determine exactly which parts of the polypeptide chain are important for such processes as protein folding, interactions with other proteins, and enzymatic catalysis.
The use of a synthetic oligonucleotide to modify the protein-coding region of a gene by site-directed mutagenesis. A A recombinant plasmid containing a gene insert is separated into its two DNA strands. A synthetic oligonucleotide primer corresponding more Engineered Genes Can Be Easily Inserted into the Germ Line of Many Animals When engineering an organism that is to express an altered geneideally one would like to be able to replace the normal gene with the altered one so that the function of the mutant protein can be analyzed in the absence of the normal protein.
As discussed above, this can be readily accomplished in some haploidsingle-celled organisms. We shall see in the following section that much more complicated procedures have been developed that allow gene replacements of this type in mice.
Foreign DNA can, however, be rather easily integrated into random positions of many animal genomes. In mammals, for example, linear DNA fragments introduced into cells are rapidly ligated end-to-end by intracellular enzymes to form long tandem arrays, which usually become integrated into a chromosome at an apparently random site.
Fertilized mammalian eggs behave like other mammalian cells in this respect. A mouse egg injected with copies of a linear DNA molecule often develops into a mouse containing, in many of its cells, a tandem array of copies of the injected gene integrated at a single random site in one of its chromosomes.
If the modified chromosome is present in the germ line cells eggs or spermthe mouse will pass these foreign genes on to its progeny. Animals that have been permanently reengineered by either gene insertion, gene deletionor gene replacement are called transgenic organismsand any foreign or modified genes that are added are called transgenes. When the normal gene remains present, only dominant effects of the alteration will show up in phenotypic analyses.
Nevertheless, transgenic animals with inserted genes have provided important insights into how mammalian genes are regulated and how certain altered genes called oncogenes cause cancer. It is also possible to produce transgenic fruit flies, in which single copies of a gene are inserted at random into the Drosophila genome. In this case the DNA fragment is first inserted between the two terminal sequences of a Drosophila transposon called the P element. The terminal sequences enable the P element to integrate into Drosophila chromosomes when the P element transposase enzyme is also present see p.
To make transgenic fruit flies, therefore, the appropriately modified DNA fragment is injected into a very young fruit fly embryo along with a separate plasmid containing the gene encoding the transposase.
When this is done, the injected gene often enters the germ line in a single copy as the result of a transposition event. Gene Targeting Makes It Possible to Produce Transgenic Mice That Are Missing Specific Genes If a DNA molecule carrying a mutated mouse gene is transferred into a mouse cell, it usually inserts into the chromosomes at random, but about once in a thousand times, it replaces one of the two copies of the normal gene by homologous recombination.
The technique works as follows: After a period of cell proliferation, the rare colonies of cells in which a homologous recombination event is likely to have caused a gene replacement to occur are isolated. The correct colonies among these are identified by PCR or by Southern blotting: In the second step, individual cells from the identified colony are taken up into a fine micropipette and injected into an early mouse embryo. The transfected embryo-derived stem cells collaborate with the cells of the host embryo to produce a normal-looking mouse; large parts of this chimeric animal, including—in favorable cases—cells of the germ lineoften derive from the artificially altered stem cells Figure Figure Summary of the procedures used for making gene replacements in mice.
In the first step Aan altered version of the gene is introduced into cultured ES embryonic stem cells. Only a few rare ES cells will have their corresponding normal genes replaced more The mice with the transgene in their germ line are bred to produce both a male and a female animal, each heterozygous for the gene replacement that is, they have one normal and one mutant copy of the gene.
When these two mice are in turn mated, one-fourth of their progeny will be homozygous for the altered gene. Studies of these homozygotes allow the function of the altered gene—or the effects of eliminating a gene activity—to be examined in the absence of the corresponding normal gene.
The ability to prepare transgenic mice lacking a known normal gene has been a major advance, and the technique is now being used to dissect the functions of a large number of mammalian genes Figure Related techniques can be used to produce conditional mutants, in which a selected gene becomes disrupted in a specific tissue at a certain time in development.
The strategy takes advantage of a site-specific recombination system to excise—and thus disable—the target gene in a particular place or at a particular time.
In this case the target gene in ES cells is replaced by a fully functional version of the gene that is flanked by a pair of the short DNA sequences, called lox sites, that are recognized by the Cre recombinase protein.
The transgenic mice that result are phenotypically normal. They are then mated with transgenic mice that express the Cre recombinase gene under the control of an inducible promoter. In the specific cells or tissues in which Cre is switched on, it catalyzes recombination between the lox sequences—excising a target gene and eliminating its activity.One gene one enzyme hypothesis
Similar recombination systems are used to generate conditional mutants in Drosophila see Figure Figure Mouse with an engineered defect in fibroblast growth factor 5 FGF5. FGF5 is a negative regulator of hair formation. In a mouse lacking FGF5 rightthe hair is long compared with its heterozygous littermate left.
Transgenic mice with phenotypes that more In some circumstances the dedifferentiated cells can even form an apical meristemwhich can then give rise to an entire new plant, including gametes.
This remarkable plasticity of plant cells can be exploited to generate transgenic plants from cells growing in culture. When a piece of plant tissue is cultured in a sterile medium containing nutrients and appropriate growth regulators, many of the cells are stimulated to proliferate indefinitely in a disorganized manner, producing a mass of relatively undifferentiated cells called a callus.
If the nutrients and growth regulators are carefully manipulated, one can induce the formation of a shoot and then root apical meristems within the callus, and, in many species, a whole new plant can be regenerated. Callus cultures can also be mechanically dissociated into single cells, which will grow and divide as a suspension culture. In several plants—including tobacco, petunia, carrot, potato, and Arabidopsis—a single cell from such a suspension culture can be grown into a small clump a clone.
At the time of the experimentsnon-geneticists still generally believed that genes governed only trivial biological traits, such as eye color, and bristle arrangement in fruit flies, while basic biochemistry was determined in the cytoplasm by unknown processes. Also, many respected geneticists thought that gene action was far too complicated to be resolved by any simple experiment. Thus Beadle and Tatum brought about a fundamental revolution in our understanding of genetics.
The nutritional mutants of Neurospora also proved to have practical applications; in one of the early, if indirect, examples of military funding of science in the biological sciences, Beadle garnered additional research funding from the Rockefeller Foundation and an association of manufacturers of military rations to develop strains that could be used to assay the nutrient content of foodstuffs, to ensure adequate nutrition for troops in World War II.
In a review, Beadle suggested that "the gene can be visualized as directing the final configuration of a protein molecule and thus determining its specificity. However, the proposed connection between a single gene and a single protein enzyme outlived the protein theory of gene structure. In a paper, Norman Horowitz named the concept the "one gene—one enzyme hypothesis".
For many who did accept the results, it strengthened the link between genes and enzymes, so that some biochemists thought that genes were enzymes; this was consistent with other work, such as studies of the reproduction of tobacco mosaic virus which was known to have heritable variations and which followed the same pattern of autocatalysis as many enzymatic reactions and the crystallization of that virus as an apparently pure protein.
At the start of the s, the Neurospora findings were widely admired, but the prevailing view in was that the conclusion Beadle had drawn from them was a vast oversimplification. This insight provided the foundation for the concept of a genetic code. However, it was not until the experiments were performed showing that DNA was the genetic material, that proteins consist of a defined linear sequence of amino acids, and that DNA structure contained a linear sequence of base pairs, was there a clear basis for solving the genetic code.
By the early s, advances in biochemical genetics—spurred in part by the original hypothesis—made the one gene—one enzyme hypothesis seem very unlikely at least in its original form.
Beginning inVernon Ingram and others showed through electrophoresis and 2D chromatography that genetic variations in proteins such as sickle cell hemoglobin could be limited to differences in just a single polypeptide chain in a multimeric proteinleading to a "one gene—one polypeptide" hypothesis instead.
Davis"By — indeed, even by — one gene, one enzyme was no longer a hypothesis to be resolutely defended; it was simply the name of a research program. This splicing was discovered in by Phillip Sharp and Richard J.
Roberts  Possible anticipation of Beadle and Tatum's results[ edit ] Historian Jan Sapp has studied the controversy in regard to German geneticist Franz Moewus who, as some leading geneticists of the s and 50s argued, generated similar results before Beadle and Tatum's celebrated work.