Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 76
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She had noterminology with which to discuss such things, so she had to adapt the conventional terminology to describe aunique situation. What we now call a transposable element, and believe to be universal among organisms,McClintock calls a ''chromatin element." McClintock was a superb geneticist and cytogeneticist, perhaps the bestof her generation. In tracking down transposable elements by genetic means, McClintock had to use considerableingenuity in designing crosses that were sometimes quite complex.
Her writing style is also uniquely McClintock.The excerpt that follows is a good example. It is a relentless marshaling of observation and hypothesis, experimentand result, interpretation and deduction. McClintock was awarded a Nobel Prize in 1983. The following passagedeals with the discovery of Ds (Dissociation) and Ac (Activator). In modern terminology, we call Ac anautonomous transposable element because it codes for all the proteins it needs for its own transposition; Ds is anonautonomous transposable element because it requires Ac to provide the proteins necessary for it to move.In the course of an experiment designed to reveal the genic composition of the short arm of chromosome 9, aphenomenon of rare occurrence (or recognition) in maize began to appear with remarkably high frequencies in thecultures.
The terms mutable genes, unstable genes, variegation, mosaicism, or mutable loci have been applied tothis phenomenon. Its occurrence in a wide variety of organisms has been recognized . . .A fortunate discovery wasmade early in the study of the mutable loci which proved to be of singular importance in showing the kinds ofevents that are associated with their origin and behavior. A locus was found in the short arm of chromosome 9 atwhich breaks were occurring in somatic cells.
The time and frequency of the breakage events occurring at this Ds(Dissociation) locus appeared to be the same as the time and frequency of the mutation-producing mutable loci. Anextensive study of the Ds locusTransposition of Ac takes place from one position in the chromosomal complement toanother—very often from one chromosome to another.has indicated the reason for this relationship and has produced the information required to interpret the eventsoccurring at mutable loci. It has been concluded that the changed phenotypic expression of such loci is related tochanges in a chromatin element other than that composing the genes themselves, and that mutable loci arise whensuch chromatin is inserted adjacent to the genes that are affected.
. . . The Ds locus is composed of this kind ofmaterial. Various types of alterations are observed as a consequence of events occurring at the Ds locus. . . . Theyinvolve chromosome breakage and fusion. The breaks are related, however, to events occurring at this one specificlocus in the chromosome—the Ds locus. .
. . [Among the known events is] transposition of Ds activity from oneposition to another in the chromosome complement with or without an associated gross chromosomalrearrangement. . . . It is from the transpositions of Ds that some of the new mutable loci may arise. . . . In one casethe transposed Ds locus appeared in a single gamete of a plant carrying chromosome 9 with a dominant C (coloredaleurone) allele.
. . . The new position of Ds corresponded to the known location of C. . . . Significantly, theappearance of Ds activity at this new position was correlated with the disappearance of the normal action of the Clocus. The resulting phenotype was the same as that produced by the known recessive allele c.
. . . That the cphenotype in this case was associated with the appearance of Ds at the C locus was made evident becausemutations from c to C occurred [in which] Ds action concomitantly disappeared. . . . The origin and behavior ofthis mutation at the c locus have been interpreted as follows: Insertion of the chromatin composed of Ds adjacent tothe C locus is responsible for complete inhibition of the action of C.
Removal of this foreign chromatin can occur.In many cases, the mechanism associated with the removal results in restoration of former genic organization andaction . . . The mutation-producing mechanisms involve only Ds. No gene mutations occur at the C locus; therestoration of its action is due to the removal of the inhibiting Ds chromatin . . . [The movement] of Ds requires anactivator. This activator has been designated Ac. .
. . Ac shows a very important characteristic not exhibited instudies of the usual genetic factors. This characteristic is the same as that shown by Ds. Transposition of Ac takesplace from one position in the chromosomal complement to another—very often from one chromosome to another.Again, as in Ds, changes in state may occur at the Ac locus .
. . It should be emphasized that when no Ac is presentin a nucleus, no mutation-producing events occur at Ds.Source:Proceedings of the National Academy of Sciences of the USA 36: 344–355Page 246ranges from 2 to 12 base pairs, depending on the particular transposable element. Insertion of a transposableelement is not a sequence-specific process in that the element is flanked by a different duplicated sequence at eachlocation in the genome; however, the number of duplicated base pairs is usually the same at each location in thegenome and is characteristic of a particular transposable element. Experimental deletion or mutation of part of theterminal base sequences of several different elements has shown that the short terminal inverted repeats areessential for transposition, probably because they are necessary for binding an enzyme called a transposase that isrequired for transposition.
Many transposable elements code for their own transposase by means of a gene locatedin the central region between the terminal repeats, so these elements are able to promote their own transposition.Elements in which the transposase gene has been lost or inactivated by mutation are transposable only if a relatedelement is present in the genome to provide this activity. The inability of the maize Ds element to transposewithout Ac results from the absence of a functional transposase gene in Ds.
Transposable elements are sometimesreferred to as selfish DNA, because each type of element maintains itself in the genome as a result of its ability toreplicate and transpose.Transposable elements are responsible for many visible mutations. A transposable element is even responsible forthe wrinkled-seed mutation in peas studied by Gregor Mendel. The wildtype allele of the gene codes for starchbranching enzyme I (SBEI), which is used in the synthesis of amylopectin (starch with branched chains).
In thewrinkled mutation, a transposable element inserted into the gene renders the enzyme nonfunctional. The peatransposable element has terminal inverted repeats that are very similar to those in the maize Ac element, and theinsertion site in the wrinkled allele is flanked by a duplication of eight base pairs of the SBEI coding sequence.This particular insertion appears to be genetically quite stable: The transposable element does not seem to havebeen excised in the long history of wrinkled peas.6.9—Centromere and Telomere StructureEukaryotic chromosomes contain regions specialized for maneuvering the chromosomes in cell division and forcapping the ends.
These regions are discussed next.Molecular Structure of the CentromereThe centromere is a specific region of the eukaryotic chromosome that becomes visible as a distinct morphologicalentity along the chromosome during condensation. It serves as a central component of the kinetochore, the complexof DNA and proteins to which the spindle fibers attach as they move the chromosomes in both mitosis and meiosis.The kinetochore is also the site at which the spindle fibers shorten, causing the chromosomes to move toward thepoles.
Electron microscopic analysis has shown that in some organisms—for example, the yeast Saccharomycescerevisiae—a single spindle-protein fiber is attached to centromeric chromatin. Most other organisms havemultiple spindle fibers attached to each centromeric region.Centromeres exhibit considerable structural variation among species. At one extreme are holocentricchromosomes, which appear to have centromeric sequences spread throughout their length (constituting what iscalled a diffuse centromere).
The nematode Caenorhabiditis elegans, which is widely used in genetic research,has holocentric chromosomes. In a holocentric chromosome, microtubules attach along the entire length of eachchromatid. If a holocentric chromosome is broken into fragments by x rays, each fragment behaves as a separate,smaller holocentric chromosome that moves normally in cell division. Diffuse centromeres are very poorlyunderstood and will not be discussed further.The conventional type of centromere is the localized centromere, in which microtubules attach to a single regionof the chromosome (the kinetochore). Localized centromeres fall into two types, point centromeres and regionalcentromeres.Page 247Point centromeres are found in a number of different yeasts, including Saccharomyces cerevisiae, and they arerelatively small in terms of their DNA content. Other eukaryotes, including higher eukaryotes, have regionalcentromeres that may contain hundreds of kilobases of DNA.The chromatin segment of the centromeres of Saccharomyces cerevisiae has a unique structure that is exceedinglyresistant to the action of various DNases; it has been isolated as a protein-DNA complex containing from 220 to250 base pairs.
The nucleosomal constitution and DNA base sequences of all of the yeast centromeres have beendetermined. Several common features of the base sequences are shown in Figure 6.25A. There are four regions,labeled CDE1, CDE2, CDE3, and CDE4. All yeast centromeres have sequences highly similar to those indicatedfor regions 1, 2, and 3, but the sequence of region CDE4 varies from one centromere to another. Region 2 isnoteworthy in that approximately 90 percent of the base pairs are A—T pairs.
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