Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 80
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H. Blackburn. 1996. Telomeres, telomerase and cancer. Scientific American, February.Hartl, D. L., E. R. Lozovskaya, D. I. Nurminsky, and A. R. Lohe. 1997. What restricts the activity of mariner-liketransposable elements? Trends in Genetics 13: 197.Haseltine, W. A., and F. Wong-Stahl. 1988. The molecular biology of the AIDS virus. Scientific American, October.Hsu, T. H. 1979. Human and Mammalian Cytogenetics. New York: Springer-Verlag.Jacks, T. 1996. Tumor suppressor gene mutations in mice. Annual Review of Genetics 30: 603.Manning, C.
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Annual Review ofGenetics 30: 141.Page 258A model of the telomerase enzyme showing protein components in green and the RNA component in blue.The "palm" of the protein is the active site of telomere elongation, whereas the"fingers" and "thumb" are protruding. The DNA strand, whose telomere is being elongated, is shownin red:[Courtesy of Joachim Lingner and Thomas Cech, from J. Lingner, T. R. Hughes, A.
Shevchenko,M. Mann, V. Lundblad and T. R. Cech. 1997. Science 276: 561.]Page 259Chapter 7—Variation in Chromosome Number and StructureCHAPTER OUTLINE7-1 Centromeres and the Genetic Stability of Chromosomes7-2 Polyploidy7-3 Monoploid Organisms7-4 Extra or Missing Chromosomes7-5 Human ChromosomesTrisomy in Human BeingsDosage CompensationThe Calico Cat as Evidence for X-Chromosome InactivationSex-Chromosome AbnormalitiesThe Fragile-X SyndromeChromosome Abnormalities in Spontaneous Abortion7-6 Abnormalities in Chromosome StructureDeletionsDuplicationsUnequal Crossing-Over in Human Red-Green Color BlindnessInversionsReciprocal TranslocationsRobertsonian Translocations7-7 Position Effects on Gene Expression7-8 Chromosome Abnormalities and CancerRetinoblastoma and Tumor-Suppressor GenesChapter SummaryKey TermsReview the BasicsGuide to Problem SolvingAnalysis and ApplicationsChallenge ProblemsFurther ReadingGeNETics on the webPRINCIPLES• Duplication of the entire chromosome complement present in a species—or in a hybrid between species—is amajor process in the evolution of higher plants.• The genetic unbalance caused by a single chromosome that is extra or missing may have a more seriousphenotypic effect than an entire extra set of chromosomes.• Chromosome abnormalities are an important cause of human genetic disease and are a major factor inspontaneous abortions.• Aneuploid (unbalanced) chromosome rearrangements usually have greater phenotypic effects than euploid(balanced) chromosome rearrangements.• By a process of mispairing and unequal crossing-over, genes that are duplicated in tandem along the chromosomecan give rise to chromosomes with even more copies.• Gene-for-gene pairing between a wildtype chromosome and one that contains an inversion of a segment of genesresults in the formation of a loop in one of the chromosomes; crossing-over within the "inversion loop" leads tochromosomal abnormalities.• Reciprocal translocations result in abnormal gametes because they upset segregation.• Some types of cancer are associated with particular chromosome rearrangements.CONNECTIONSCONNECTION: The First Human Chromosomal DisorderJerome Lejeune, Marthe Gautier, And Raymond Turpin 1959Study of the somatic chromosomes of nine Down syndrome childrenCONNECTION: Lyonization of an X ChromosomeMary F.
Lyon 1961Gene Action In The X Chromosome Of The Mouse (Mus Musculus L.)Page 260In all species, an occasional organism is found that has extra chromosomes or that lacks a particular chromosome.Such a deviation from the norm is an abnormality in chromosome number. Other organisms, usually rare, are foundto have alterations in the arrangement of genes in the genome, such as by having a chromosome with a particularsegment missing, reversed in orientation, or attached to a different chromosome.
These variations are abnormalitiesin chromosome structure. This chapter will deal with the genetic effects of both numerical and structuralchromosome abnormalities. We will see that animals are much less tolerant of such changes than are plants.Furthermore, inFigure 7.1(A) Diagram of a chromosome that is dicentric (two centromeres) andone that is acentric (no centromere). Dicentric and acentric chromosomesare frequently lost in cell division, the former because the twocentromeres may bridge between the daughter cells, and the latterbecause the chromosome cannot attach to the spindle fibers. (B) Threepossible shapes of monocentric chromosomes in anaphase asdetermined by the position of the centromere. The centromeresare shown in dark blue.animals, numerical alterations often produce greater effects on phenotype than do structural alterations.7.1—Centromeres and the Genetic Stability of ChromosomesChromosomes that have a single centromere are usually the only ones that are transmitted reliably from parentalcells to daughter cells and from parental organisms to their progeny.
When a cell divides, spindle fibers attach tothe centromere of each chromosome and pull the sister chromatids to opposite poles. Occasionally, a chromosomearises that has an abnormal number of centromeres, as diagrammed in Figure 7.1A. The chromosome on the lefthas two centromeres and is said to be dicentric. A dicentric chromosome is genetically unstable, which means it isnot transmitted in a predictable fashion.
The dicentric chromosome is frequently lost from a cell when the twocentromeres proceed to opposite poles in the course of cell division; in this case, the chromosome is stretched andforms a bridge between the daughter cells. This bridge may not be included in either daughter nucleus, or it maybreak, with the result that each daughter nucleus receives a broken chromosome. The chromosome on the right inFigure 7.1A is an acentric chromosome, which lacks a centromere. Acentric chromosomes also are geneticallyunstable because they cannot be maneuvered properly during cell division and tend to be lost.In eukaryotic organisms, virtually all chromosomes have a single centromere and are rod-shaped.
(Rarely, a ringchromosome is found, which results from breakage and loss of the telomere at each end of a rod chromosome andsubsequent fusion of the broken ends.) Monocentric rod chromosomes are often classified according to the relativeposition of their centromeres. A chromosome with its centromere about in the middle is a metacentricchromosome; the arms are of approximately equal length and form a V shape at anaphase (Figure 7.1B). When thecentromere is somewhat off center, thePage 261chromosome is a submetacentric chromosome, and the arms form a J shape at anaphase.
A chromosome with thecentromere very close to one end appears I-shaped at anaphase because the arms are grossly unequal in length;such a chromosome is acrocentric.The distinction among metacentric, submetacentric, and acrocentric chromosomes is useful because it drawsattention to the chromosome arms.
In the evolution of chromosomes, often the number of chromosome arms isconserved without conservation of the individual chromosomes. For example, Drosophila melanogaster has twolarge metacentric autosomes, but many other Drosophila species have four acrocentric autosomes instead of thetwo metacentrics. Detailed comparison of the genetic maps of these species reveals that the acrocentricchromosomes in the other species correspond, arm for arm, with the large metacentrics in Drosophila melanogaster(Figure 7.2). Among higher primates, chimpanzees and human beings have 22 pairs of chromosomes that aremorphologically similar, but chimpanzees have two pairs of acrocentrics not found in human beings, and humanbeings have one pair of metacentrics not found in chimpanzees.
In this case, the human metacentric chromosomewas formed by fusion of the telomeres between the short arms of the chromosomes that, in chimpanzees, remainacrocentrics. The metaphase chromosome resulting from the fusion is human chromosome 2.7.2—PolyploidyThe genus Chrysanthemum illustrates polyploidy, an important phenomenon found frequently in higher plants inwhich a species has a genome composed of multiple complete sets of chromosomes. One Chrysanthemum species,a diploid species, has 18 chromosomes. A closely related species has 36 chromosomes.
However, comparison ofchromosome morphology indicates that the 36-chromosome species has two complete sets of the chromosomesfound in the 18-chromosome species (Figure 7.3). The basic chromosome set in the group, from which all the othergenomes are formed, is called the monoploid chromosome set. In Chrysanthemum, the monoploid chromosomenumber is 9. The diploid species has two complete copies of the monoploid set, or 18 chromosomes altogether. The36-chromosome species has four copies of the monoploid set (4 × 9 = 36) and is a tetraploid.
Other species ofChrysanthemum have 54 chromosomes (6 × 9, constituting the hexaploid), 72 chromosomes (8 × 9, constitutingthe octoploid), and 90 chromosomes (10 × 9, constituting the decaploid).In meiosis, the chromosomes of all Chrysanthemum species synapse normally in pairs to form bivalents (Section3.3). The 18-chromosome species forms 9 bivalents, the 36-chromosome species forms 18 bivalents, the 54chromosome species forms 27Figure 7.2The haploid chromosome complement of twospecies of Drosophila. Color indicates homology ofchromosome arms.
The large metacentricchromosomes of Drosophila melanogaster(chromosomes 2 and 3) correspond arm forarm with the four large acrocentric autosomes ofDrosophila virilis.Page 262Figure 7.3Chromosome numbers indiploid and polyploid species ofChrysanthemum. Each set ofhomologous chromosomesis depicted in a differentcolor.bivalents, so forth. Gametes receive one chromosome from each bivalent, so the number of chromosomes in thegametes of any species is exactly half the number of chromosomes in its somatic cells.
The chromosomes presentin the gametes of a species constitute the haploid set of chromosomes. In the species of Chrysanthemum with 90chromosomes, for example, the haploid chromosome number is 45; in meiosis, 45 bivalents are formed, so eachgamete contains 45 chromosomes. When two such gametes come together in fertilization, the complete set of 90chromosomes in the species is restored.
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