Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 75
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Hybridization is principally in the regions adjacent to the centromeres(arrows). Note that some chromosomes are apparently free of this satellite DNA.They contain a different satellite DNA not examined in this experiment.This cell is unusual in that each chromosome has undergone two roundsof chromosome duplication without an intervening mitosis; this is whyeach chromosome in the normal diploid set is present as a pair.[Courtesy of David Prescott.]Page 241Figure 6.20(A) Metaphase chromosomes of the ground squirrel Ammospermophilus harrissi, stained to show theheterochromatic regions near the centromere of most chromosomes (red arrows) and the telomeresof some chromosomes (black arrows).
(B) An interpretive drawing.[Micrograph courtesy of T.C. Hsu.]the heterochromatin remains highly condensed throughout the cell cycle. The euchromatin, which makes up mostof the genome, is visible only in the mitotic cycle. The major heterochromatic regions are adjacent to thecentromere; smaller blocks are present at the ends of the chromosome arms (the telomeres) and interspersed withthe euchromatin. In many species, an entire chromosome, such as the Y chromosome in D.
melanogaster and inmammals, is almost completely heterochromatic. Different highly repetitive sequences have been purified from D.melanogaster, and in situ hybridization has shown that each sequence has its own distinctive distribution amongthe chromosomes.The genetic content of heterochromatin is summarized in the following generalization:The number of genes located in heterochromatin is small relative to the number in euchromatin.The relatively small number of genes means that many large blocks of heterochromatin are genetically almost inert,or devoid of function.
Indeed, heterochromatic blocks can often be rearranged in the genome, duplicated, or evendeleted without major phenotypic consequences.Middle-Repetitive SequencesMiddle-repetitive sequences constitute about 12 percent of the D. melanogaster genome and 40 percent or more ofthe human and other eukaryotic genomes. These sequences differ greatly in the number of copies and theirdistribution within a genome. They comprise many families of related sequences and include several groups ofgenes.
For example, the genes for the RNA components of the ribosomes (the particles on which proteins aresynthesized; Chapter 10) and the genes for tRNA molecules (which also participate in protein synthesis) arerepeated in the genomes of all organisms. The two major ribosomal RNA molecules come from a tandem pair ofgenes that is repeated several hundred times in most eukaryotic genomes. The genomes of all eukaryotes alsocontain multiple copies of the histone genes. Each histone gene is repeated about 10 times per genome in chickens,20 times in mammals, about 100 times in Drosophila, and as many as 600 times in certain sea urchin species.The dispersed middle-repetitive DNA of the D.
melanogaster genome consists ofPage 242about 50 families of related sequences, and from 20 to 60 copies of each family are widely scattered throughout thechromosomes. The positions of these sequences differ from one fly to the next except in completely homozygouslaboratory strains. There is variability in position, because many of these sequences are able to move from onelocation to another in a chromosome and between chromosomes; they are said to be transposable elements.Analogous types of sequences are also found in the genomes of yeast, maize, and bacteria (Chapter 8) and areprobably present in all organisms. An important dimension has been added to our understanding of the genome as astructural and functional unit by the discovery of these mobile genetic elements, because they can, in some cases,cause chromosome breakage chromosome rearrangements, modification of the expression of genes, and noveltypes of mutations.6.8—Transposable ElementsIn the 1940s, in a study of the genetics of kernel mottling in maize (Figure 6.21), Barbara McClintock discoveredan element that not only regulated the mottling but also caused breakage of the chromosome carrying the genes forcolor and consistency of the kernels.
The element was called Dissociation (Ds). Mapping data showed that thechromosome breakage always occurs at or very near the location of Ds. McClintock's critical observation wasFigure 6.21Sectors of purple and yellow tissue in the endosperm of maize kernels resulting from the presenceof the transposable elements Ds and Ac. The heavier sectoring in some ears results from dosageeffects of Ac.
The least speckled ear has one copy of Ac; that in the middle has two (Ac Ac); and themost speckled ear has three (Ac Ac Ac).[Courtesy of Jerry L. Kermicle.]Page 243Figure 6.22Sequence organization of a copia transposable element of Drosophila melanogaster.that Ds does not have a constant location but occasionally moves to a new position (transposition), causingchromosome breakage at the new site. Furthermore, Ds moves only if a second element, called Activator (Ac), isalso present. In addition, Ac itself moves within the genome and can cause, in the expression of genes at or near itsinsertion site, alterations similar to the modifications resulting from the presence of Ds.Other transposable elements with characteristics and genetic effects similar to those of Ac and Ds are known inmaize.
Much of the color variegation seen in the kernels of varieties used for decorative purposes are attributable tothe presence of one or more of these elements.Since McClintock's discovery, transposable nucleotide sequences have been observed to be widespread ineukaryotes and prokaryotes. In D. melanogaster, they constitute from 5 to 10 percent of the genome and compriseabout 50 distinct families of sequences. One well-studied family of closely related, but not identical, sequences iscalled copia. This element is present in about 30 copies per genome. The copia element (Figure 6.22) containsabout 5000 base pairs with two identical sequences of 267 base pairs located terminally and in the sameorientation. Repeated DNA sequences with the same orientation are called direct repeats (Figure 6.23A).
The endsof each of these copia direct repeats contain two segments of 17 base pairs, whose sequences are also nearlyidentical; these shorter segments have opposite orientations. Repeated DNA sequences with opposite orientationsare called inverted repeats (Figure 6.23B).Figure 6.23(A) In a direct repeat, a DNA sequence is repeated in the same left-to-rightorientation.(B) In an inverted repeat, the sequence is repeated in thereverse left-to-right orientation in the opposite strand. The opposite strandis stipulated in order to maintain the correct 5'-to-3' polarity.Other transposable elements have a similar organization with direct or inverted terminal repeats, as do many suchelements in other organisms, such as the transposable elements in E.
coli described in Chapter 8.The molecular processes responsible for the movement of transposable elementsPage 244are not well understood (some information will be presented in Chapter 8). A common feature is that transpositionof the element is usually accompanied by the duplication of a small number of base pairs orginally present at theinsertion site, with the result that a copy of this short chromosomal sequence is found immediately adjacent to bothends of the inserted element (Figure 6.24). The length of the duplicated segmentFigure 6.24The sequence arrangement of one type of transposable element (in this case, Ds of maize) and thechanges that take place when it inserts into the genome. Ds is inserted into the maize sh gene at theposition indicated.
In the insertion process, a sequence of eight base pairs next to the site ofinsertion is duplicated and flanks the Ds element.Page 245Connection Her Feeling for the OrganismBarbara McClintock 1950Cold Spring Harbor Laboratory,Cold Spring Harbor, New YorkThe Origin and Behavior of MutableLoci in MaizeMany geneticists regard McClintock's papers on transposable elements as difficult. Her discoveries werecompletely novel. Genes that could move from one place to another in the genome were unheard of.
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