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Lecture 10, part 2 of 2
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D. In mitotic chromosomes, matrix attachment sites correspond to sites of attachment of chromatin to the chromosome scaffold.

Evidence indicates that  scaffold is derived from matrix during chromosome condensation in prophase, and presumably scaffold helps in the formation of matrix as the nucleus re-forms at telophase.

a. Lampbrush chromosomes in salamanders: evidence that matrix attachment regions are consistent within a species.

In salamander oocytes, meiosis is arrested in meiotic metaphase I and the chromatin becomes extended, allowing transcription to resume. Oocytes build up a supply of mRNA in this fashion. The extended appearance of the chromatin on these chromosomes has led to the name "lampbrush" chromosomes.

Light micrograph of the highly extended lampbrush chromosomes seen in an amphibian oocyte.

The symmetry of lampbrush chromosomes provides evidence that attachment of chromatin loops to the scaffold is not random.

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It has been estimated that the set of lampbrush chromosomes contains a total of about 10,000 different chromatin loops in many amphibians, with the remainder of the DNA being highly condensed in the chromosomes. Note that each loop corresponds to a particular DNA sequence, and that four copies of each loop are present in each cell, since the structure shown at the top consists of two paired homologous chromosomes and each chromosome is composed of two closely apposed sister chromatids. This four-stranded structure is characteristic of this stage of development of the oocyte (the diplotene stage of meiosis).

i. First, within a given oocyte, the "lampbrush" pattern of loops appears symmetrical, suggesting that the loops are the same in both chromatids. The loop pattern is also constant from one oocyte to the next and from one individual to the next.

ii. Secondly, when 3H-RNA probes of specific genes are hybridized in-situ to "lampbrush" chromosomes, a given probe always hybridizes to the same loop.

Figure 4-37. A model for the structure of a lampbrush
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 The use of nucleic acid hybridization to demonstrate the unusual transcription on amphibian lampbrush chromosomes. A single-stranded DNA radiolabled probe was prepared, corresponding to a repeated DNA sequence containing histone genes. The chromosomes in (A) were annealed with this probe, washed extensively, and then subjected to autoradiography (B). The extended loop that becomes radioactive here is synthesizing unusually long RNA transcripts that contain copies of several clustered histone genes. The fact that these long RNA transcripts hybridize with the DNA probe reveals that the repeated DNA sequence of the probe is copied into RNA, even though in other cells this sequence serves as a nontranscribed spacer between the histone genes.

Undisplayed Graphic

From M.O. Diaz, G. Barsacchi-Pilone, K.A. Mahon, and J. Gall, Cell 24:649-659, 1981.

Incidentally, this is an extraordinary case of transcription during meiosis.

Xenopus lampbrush chromosomes, showing chromatin domains attached to a central axis (scaffold). The scaffold is visualized with antibodies to Topoisomerase II.

(Keep in mind that there is increasing evidence that chromatin in the nucleus is almost always in the 10 nm fiber, not the 30 nm fiber, as this figure indicates.)

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TopoisomeraseII, nicking-closing enzyme . (MW=170Kd)

See Berg et al. BIOCHEMISTRY, Chapter 27.

In its active form, TopoII is present as a dimer (~309Kd). 


By balancing the activities of the topoisomerases, the cell can regulate the degree of supercoiling in a particular region of a linear chromosome.

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