Fri

anaphase It

four gameies (or spores)

Interphase

10-nm fiber

FIGURE 7-17 Forms of chromatin structure seen in the EM. (a) Electron micrographs of M phase and interphase OWA show the changes in the structure of chromatin, (b) Electron micrographs of different forms of chromatin in interphase cells show the SOnm and 10-nm chromatin fibers {beads on a string). (Source: (a) Courtesy of Victoria foe, <D 2Q02 from Alberts B. el jI 2002. Molecular biology of the coll, 4th edition. Reproduced by permission of Routledgc Inc., part of The Taytor & Francis Croup, (b) Courtesy of Barbara Hamkalo; <3 2002 from Alberts B. et al. 2002. Molecular biology of the cell, 4th edition. Reproduced by permission of Routlectge Inc, pari of The Taylor & Francis Group.)

30-nm fiber

Interphase

10-nm fiber

FIGURE 7-17 Forms of chromatin structure seen in the EM. (a) Electron micrographs of M phase and interphase OWA show the changes in the structure of chromatin, (b) Electron micrographs of different forms of chromatin in interphase cells show the SOnm and 10-nm chromatin fibers {beads on a string). (Source: (a) Courtesy of Victoria foe, <D 2Q02 from Alberts B. el jI 2002. Molecular biology of the coll, 4th edition. Reproduced by permission of Routledgc Inc., part of The Taytor & Francis Croup, (b) Courtesy of Barbara Hamkalo; <3 2002 from Alberts B. et al. 2002. Molecular biology of the cell, 4th edition. Reproduced by permission of Routlectge Inc, pari of The Taylor & Francis Group.)

Different Levels of Chromosome Structure Can Re Observed by Microscopy

Microscopy has long been used to observe chromosome structure and function. Indeed, long before it was clear that chromosomes were the source of the genetic information in the cell, their movements and changes during cell division were well understood. The compact nature of condensed mitotic chromosomes also makes them relatively easy to visualize even by simple light microscopy (Figure 7-17a). Indeed, it was in this form that chromosomes were first identified. Condensed chromosomes are also used to determine the chromosomal make-up of human cells to detect such abnormalities as ehromosoninl deletions or individuals with extra copies of a single chromosome.

Chromosomal DNA not in mitosis (that is, in interphase) is less compact {Figure 7-17a). In the electron microscope two states of chromatin are readily observed: fibers with a diameter of either 30-nm or 10 nm (Figure 7-17b). The 30-nm fiber is a more compact version of chromatin that is frequently folded into targe loops reaching oul from a protein core or scaffold. In contrast, the 10-nm fiber is a less compact form of chromatin that resembles a regular series of "beads on u string." These beads are nucleosontes. We will first focus on the nature of the nucleosome, including how they are formed, and theii describe how nucleosome-dependent structures control global effects on the accessibility of nuclear DNA.

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