Chromosome structure

FIGURE 12.31 A model for chromosome structure, human chromosome 4. The 2-um DNA helix is wound twice around histone octamers to form 10-um uucleosomes, each of which contains 160 bp (80 per turn). These uucleosomes are then wound in solenoid fashion with six uucleosomes per turn to form a 30-nm filament. In this model, the 30-nm filament forms long DNA loops, each containing about 60,000 bp, which are attached at their base to the nuclear matrix. Eighteen of these loops are then wound radially around the circumference of a single turn to form a miniband unit of a chromosome. Approximately 10 of these minibands occur in each chromatid of human chromosome 4 at mitosis. 

Genotoxicity studies are required to identify compounds that can induce genetic damage ranging from single point gene mutations to gross alterations of chromosomal structure. Such effects are taken as indicative of the potential to cause cancer or heritable defects in humans. A standard battery of three types of test is recommended ... 

It is possible to observe effects of organotin(lV) compounds exposure such as inhibition of cleavage of fertilized eggs, interference with the formation of the mitotic spindle, damages affecting chromosome structure, and electron-dense precipitate formation in organelles. 

Lewis, C.D. and Laemmli, U.K. (1982). Higher order metaphase chromosome structure evidence for metal-loprotein interactions. Cell 29, 171-181. 

Sigrist S, Jacobs H, Stratmann R, Lehner CF 1995 Exit from mitosis is regulated by Drosophila fizzy and the sequential destruction of cyclins A, B and B3. EMBO J 14 4827-4838 Skibbens RV, Corson LB, Koshland D, Hieter P 1999 Ctf7p is essential for sister chromatid cohesion and links mitotic chromosome structure to the DNA replication machinery. Genes... 

The chromosome structure is visible only during the mitotic portion of the cell cycle. The constituent parts of the chromosomes are nucleoprotein fibers called chromatin. When condensed, chromatin forms a microscop-ically visible chromosome-like structure. The chromosomes are composed of DNA, RNA, and proteins. The relative amounts of the three vary, but chromatin is primarily protein and DNA. 

Sobti RC, Obe G. Eukaryotic Chromosomes Structural and Functional Aspects, Springer-Verlag, New York, 1991. 

Fio. 4. Chromosome structure and terminology. Iq21.1 stands for chromosome 1, armq, region 2, band 1, subband 1. 

Chromosomal disorders can also be caused by changes in chromosome structure. These changes are caused by the breakage and reunion of chromosome segments when an egg or sperm cell is formed or in early fetal development. Pieces of DNA can be rearranged within one chromosome, or transferred between two or more chromosomes. The effects of structural changes depend on their size and location. Many different structural changes are possible some cause medical problems, while others may have no effect on a person s health. 

Changes in chromosome structure can also cause chromosomal disorders. Some changes in chromosome structure can be inherited, while others occur as random accidents during the formation of reproductive cells or in early fetal development. Because the inheritance of these changes can be complex, people concerned about this type of chromosomal abnormality may want to talk with a genetics professional. 

Figure 1. Hierarchical model of chromosome structure, (a) In interphase cells, DNA is packed in a nucleus as forming nucleosome and chromatin, (b) DNA forms nucleosome structure together with core histone octamer, which is then folded up into 30nm fiber with a help of linker histone HI. This 30 nm fiber is further folded into 80 nm fiber and 300 nm loop structures in a nucleus. In mitosis, chromosome is highly condensed. Proteins which are involved in each folding step are indicated above and non-protein factors are indicated below, (c) The amino acid sequences of histone tails (H2A, H2B, H3 and H4) are shown to indicate acetylation, methylation and phosphorylation sites. (See Colour Plate 1.)...

During the cell cycle, chromosome structures shuttle between de-condensed interphase and condensed mitosis states. Dynamic changes also occur at the lower levels of architectures, i.e., at the chromatin and nucleosome levels. Upon gene activation and inactivation, folding and unfolding of the nucleosome structure and the chromatin fibers occur at limited loci of the genome. Namely, the structures of the chromosome are dynamic and mobile. Nevertheless, there are basic structural units that remain stable and constitute the fundamental chromosome architecture. 

Mahy NL, Perry PE, Gilchrist S, Baldock RA, Bickmore WA (2002) Spatial organization of active and inactive genes and noncoding DNA within chromosome territories. J Cell Biol 157 579-589 Mangenot S, Leforestier A, Vachette P, Durand D, Livolant F (2002) Salt-induced conformation and interaction changes of nucleosome core particles. Biophys J 82 345-356 Marsden MP, Laeimnh UK (1979) Metaphase chromosome structure evidence for a radial loop model. Cell 17 849-858... 

Simpson RT, Thoma F, Brubaker JM (1985) Chromatin reconstituted from tandemly repeated cloned DNA fragments and core histones a model system for study of higher order structure. Cell 42 799-808 Sugiyama S, Yoshino T, Kanahara H, Kobori T, Ohtani T (2003) Atomic force microscopic imaging of 30 nm chromatin fiber from partially relaxed plant chromosomes. Scanning 25 132-136 Sugiyama S, Yoshino T, Kanahara H, Shichiri M, Fukushi D, Ohtani T (2004) Effects of acetic acid treatment on plant chromosome structures analyzed by atomic force microscopy. Anal Biochem 324 39 4... 

Yoshimura SH, Kim J, Takeyasu K (2003) On-substrate lysis treatment combined with scanning probe microscopy revealed chromosome structures in eukaryotes and prokaryotes. J Electron Microsc (Tokyo) 52 415-423... 

Allshire RC (1995) Elements of chromosome structure and function in fission yeast. Semin Cell Biol 6 55-64... 

During mitosis, aU the DNA is highly condensed to allow separation of the sister chromatids. This is the only time in the ceE cycle when the chromosome structure is visible. Chromosome abnormalities may be assessed on mitotic chromosomes by karyotype analysis (metaphase chromosomes) and by banding techniques (prophase or prometaphase), which identify aneu-ploidy, translocations, deletions, inversions, and duplications. 

Endonuclease activation and chromatin fr mentation are characteristic features of eukaryotic cell death by apoptosis. Which of the following chromosome structures would most likely be degraded first in an apoptotic cell ... 

To visualize chromosomes more accurately, various stains are plied so that the bands in chromosomes can be seen (Fig II-3-1). The bands tend to reflect differences in chromosome structure or composition (e.g., regions rich in CG bases versus those rich in AT bases). 

M. G. Poirier, A. Nemani, P. Gupta, S. Eroglu, and J. E. Marko, Probing chromosome structure with dynamic force relaxation. Phys. Rev. Lett. 86, 360-363 (2001). 

D. A. Jackson, and A. Pombo, Replicon clusters are stable units of chromosome structure evidence that nuclear organization contributes to the efficient activation and propagation of S phase in human cells. J. Cell Biol. 140, 1285-1295 (1998). 

Clemson, C.M., McNeil, J.A., Willard, H.F., and Lawrence, J.B. (1996) XIST RNA paints the inactive X chromosome at interphase evidence for a novel RNA involved in nuclear/chromosome structure. J. Cell Biol. 132, 259-275. 

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