Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 78
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The hydrogen bonding in the G-quartet is calledHoogstein base pairing to distinguish it from ordinary Watson-Crick base pairing. A protein has been isolatedfrom the ciliated protozoan Oxytricha that binds specifically to the telomeric DNA of linear chromosomes. The βsubunit of this telomere-binding protein promotesPage 251Table 6.2 Sequences of telomeric DNAs.OrganismSequenceProtozoaTetrahymenaC4A2/T2G4ParameciumOxytrichaC4A4/T4G4PlasmodiumTrypanosomaC3TA2/T2AG3GiardiaC3TA/TAG3Slime moldsPhysarumC3TA2/T2AG3DidymiumC3TA2/T2AG3DictyosteliumC1–8T/AG1–8FungiSaccharomycesC2–3ACA1–6/T1–6GTG2–3KluyveromycesACAC2ACATAC2TA2TCA3TC2GA/TCG2AT3GAT2AG2TATGTG2TGTCandidaACAC2A2GA2GT2AGACATC2GT/ACG2ATGTCTA2CT2CT2G2TGTSchizosaccharomycesC1–6G0–1T0–1GTA1–2/T1–2ACA0–1C0–1G1–6NeurosporaC3TA2/TA2AG3PodosporaC3TA2/T2AG3CryptococcusA2C3–5T/AG3–5T2CladosporiumC3TA2/T2AG3InvertebratesCaenorhabditisGC2TA2/T2AG2CAscarisGC2TA2/T2AG2CParascarisTGCA2/T2GCABombyx, other insectsC2TA2/T2AG2VertebratesC3TA2/T2AG3PlantsChlamydomonasC3TA4/T4AG3ChlorellaC3TA3/T3AG3ArabidopsisC3TA3/T3AG3TomatoSource: From V.A.
Zakian. 1995, Science 270:1602G-quartet formation by the telomeric DNA of Oxytricha:. . .5'-TTTTGGGGTTTTGGGGT-3' . . .or Tetrahymena:. . .5'-TTGGGGTTGGGGT-3' . . .It is hypothesized that within telomeres, telomeric DNA may be organized in special three-dimensionalconformations that include such G-quartets. Models for Oxytricha and Tetrahymena telomeric DNA, showing thepossible positions of G-quartets, are presented in Figure 6.29.Page 252Figure 6.28G-quartet structure formed by hydrogen bonding between four guanine bases present in asingle DNA strand folded back upon itself. The rectangles on the right show how theguanines are oriented, with the deoxyribose sugars attached at the outside corners.
On the left,the sugars are indicated by the small blue boxes. There is also a monovalent cation in thecenter of the quartet (not shown).[From J. R. Williamson, M. K. Raghuraman and T. R. Cech. 1982 Cell 59: 871.]Figure 6.29Models of telomere structure in Oxytricha (A) and Tetrahymena (B) thatincorporate the G-quartet structure. The arrowhead indicates the 3' end ofthe DNA strand. The indicated G bases participate in the Hoogstein basepairing, and the G-quartets are drawn as green squares, as in Figure 6.28.There are two G-quartets postulated in the Oxytricha telomere, three inTetrahymena.[From J. R.
Williamson, M. K. Raghuraman, and T. R. Cech. 1982. Cell 59:871.]Page 253Chapter SummaryThe DNA content of organisms varies widely. Small viruses exist whose DNA contains only a few thousandnucleotides, and among the higher animals and plants, the DNA content can be as large as 1.5 × 1011 nucleotides.Generally speaking, DNA content increases with the complexity of the organism, but within particular groups oforganisms, the DNA content can vary as much as tenfold.DNA molecules come in a variety of forms. Except for a few of the smallest viruses, whose DNA is singlestranded, and some viruses in which RNA is the genetic material, all organisms contain double-stranded DNA. Thechromosomal DNA of higher organisms is almost always linear. Bacterial DNA is circular, as is the DNA of manyanimal viruses and of some bacteriophages.
Circular DNA molecules are invariably supercoiled. The bacterialchromosome consists of independently supercoiled domains.The DNA of both prokaryotic and eukaryotic cells and of viruses is never in a fully extended state but rather isfolded in an intricate way, which reduces its effective volume. In viruses, the DNA is tightly folded but withoutbound protein molecules. In bacteria, the DNA is folded to form a multiply looped structure called a nucleoid,which includes several proteins that are essential for folding.
In eukaryotes, the DNA is compacted intochromosomes, which contain several proteins and which are thick enough to be visible by light microscopy duringthe mitotic phase of the cell cycle. The DNA-protein complex of eukaryotic chromosomes is called chromatin. Theprotein component of chromatin consists primarily of five distinct proteins: histones H1, H2A, H2B, H3, and H4.The last four histones aggregate to form an octameric protein that contains two molecules of each. DNA is wrappedaround the histone octamer, forming a particle called a nucleosome.
This wrapping is the first level of compactionof the DNA in chromosomes. Each nucleosome unit contains about 200 nucleotide pairs, of which about 145 are incontact with the protein. The remaining 55 nucleotide pairs link adjacent nucleosomes. Histone H1 binds to thelinker segment and draws the nucleosomes nearer to one another. The DNA in its nucleosome form is furthercompacted into a helical fiber, the 30-nm fiber. In forming a visible chromosome, this unit undergoes severaladditional levels of folding, producing a highly compact visible chromosome.
The result is that a eukaryotic DNAmolecule, whose length and width are about 50,000 and 0.002 µm, respectively, is folded to form a chromosomewith a length of about 5 µm and a width of about 0.5 µm.Polytene chromosomes are found in certain organs in insects. These gigantic chromosomes consist of about 1000molecules of partly folded chromatin aligned side by side.Seen by microscopy, they have about 5000 transverse bands. Polytene chromosomes do not replicate further, andcells that contain them do not divide. They are useful to geneticists primarily as morphological markers forparticular genes and chromosome segments.The number of copies of individual base sequences in a DNA molecule can vary tremendously.
In prokaryoticDNA, most sequences are unique. However, in eukaryotic DNA, only a fraction of the DNA consists of uniquesequences present once per haploid genome. The sequence composition of complex genomes can be studied byDNA renaturation kinetics, or Cot analysis. In eukaryotes, many sequences are present in hundreds to millions ofcopies. Some highly repetitive sequences are primarily located in the centromeric regions of the chromosomes,whereas others are dispersed. A significant fraction of the DNA, the middle-repetitive DNA, consists of sequencesof which from 10 to 1000 copies are present per cell.
Much of middle-repetitive DNA in higher eukaryotes consistsof transposable elements, which are sequences able to move from one part of the genome to another. A typicaltransposable element is a sequence ranging in length from one to several thousand nucleotide pairs terminating inshort sequences that are repeated, either in the same orientation (direct repeats) or in reverse orientation (invertedrepeats). The terminal repeats and a transposase enzyme are necessary for the movement of these elements, aprocess known as transposition. Many transposable elements contain a gene coding for their own transposase.Insertion of most transposable elements causes duplication of a short chromosomal nucleotide sequence flankingthe point of insertion. Insertions of transposable elements are the cause of many visible mutations.Centromeres and telomeres are regions of eukaryotic chromosomes specialized for spindle fiber attachment andstabilization of the tips, respectively.
The centromeres of most higher eukaryotes are associated with localized,highly repeated, satellite DNA sequences. Telomeres are formed by a telomerase enzyme that contains a guideRNA that serves as a template for the addition of nucleotides to the 3' end of a telomerase addition site. Thecomplementary strand is synthesized in a manner analogous to the replication of the lagging strand at an ordinaryreplication fork. In mammals and other vertebrates, the 3' strand of the telomere terminates in tandem repeats of thesimple sequence 5'-TTAGGG-3'.
Relatively few copies of this sequence are needed to prime the telomerase. Thespecial properties of the telomere may be related to the ability of guanine nucleotides to form complex hydrogenbonded structures—in particular, the G-quartet.Key Termsalpha satellitesCotdiffuse centromerechromatinCot curvedirect repeatchromocentercovalent circleDNaseconserved sequencecytological mapeuchromatincore particledeoxyribonucleasefolded chromosomePage 254genomein situ hybridizationsatellite DNAguide RNAkilobase pairs (kb)scaffoldH1 histonelocalized centromereselfish DNAH2A histonemegabase pairs (Mb)single-copy sequenceH2B histonemicrococcal nucleasesupercoiled DNAH3 histonemiddle-repetitive sequencestelomeraseH4 histonenegative supercoiling30-nm fiberheterochromatinnucleoidtopoisomerase Ihighly repetitive sequencesnucleosometopoisomerase IIhistonepolytene chromosometransposable elementholocentric chromosomepulsed-field gel electrophoresistransposaseHoogstein base pairingrelaxed DNAtranspositioninverted repeatrepetitive sequenceunique sequenceReview the Basics• What are the principal differences between prokaryotes and eukaryotes in the organization of the geneticmaterial?• In a eukaryotic chromosome, where are the telomeres located? Is it possible to have a chromosome with thecentromere exactly at the tip? What shape of chromosome has no telomeres?• What is a transposable element?• Are all repetitive DNA sequences transposable elements? If a transposable element were present somewhere in along stretch of DNA, what features in the nucleotide sequence would help to identify it?• Histone proteins are positively charged (basic) molecules, and they interact with negatively charged (acidic)molecules.
What is the principal type of acid with which histones interact?• How many polypeptide chains are present in each nucleosome? How many different polypeptide chains arepresent in each nucleosome? Which type of histone is not part of the nucleosome core particle?• Are the ratios of the different histone types identical in the nucleosomes in all cells of a eukaryotic organism? Inall eukaryotic organisms?• Is it correct to say that no genes are present in heterochromatin? Explain your answer.Guide to Problem SolvingProblem 1: Dp(1;f)1187 is a minichromosome induced by x rays in Drosophila.
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