|previous page||PLNT3140 Introductory Cytogenetics
Lecture 10, part 2 of 2
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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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.
Displayed by direct hypertext link to Alberts et al., Molecular Biology of the Cell
| 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
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.
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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nicking-closing enzyme . (MW=170Kd)
al. BIOCHEMISTRY, Chapter 27.
DEMO: TYGON TUBING MODEL
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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