Lampbrush chromosome

Heatwole V.M. and Haynes S. 1996. Association of RB97D, an RRM protein required for male fertility, with a Y chromosome lampbrush loop in Drosophila spermatocytes. Chromosoma 105 285-292. 

A somewhat similar structure appears to be present in the specialized eukaryotic lampbrush chromosomes (Fig. 27-6), which are observed during the meiotic prophase of oocytes. They have been studied intensively in amphibians such as Xenopus. A lampbrush chromosome is actually a homologous pair of chromosomes, each one in turn consisting of two closely associated chromatids. The chromosomes are highly expanded, and about 5% of the DNA is extended in the form of -4000 perfectly paired loops visible with an electron microscope. Each loop consists of - 50 pm or -150 kb of extended DNA. No evidence of any breaks in the DNA is seen, a fact that supports the... 

Like the puffs of polytene chromosomes (Chapter 28), which may have a similar structure, lampbrush chromosomes appear to be actively engaged in transcription. Approximately 3% of the DNA may be functional in producing mRNA that is accumulated within the oocyte and is used as a template for protein synthesis during early embryonic development.90... 

Lampbrush chromosome. Giant diplo-tene chromosome found in the oocyte nucleus. The loops that are observed are the sites of extensive gene expression. 

Portion of a "lampbrush chromosome" from an oocyte of the newt Nophthalmus viridescens] hnRNP protein associated with nascent RNA transcripts fluoresces red after staining with a monoclonal antibody. [Courtesy... 

Segment of an extended "lampbrush" chromosome from an oocyte of a newt. See Fig. 27-6. Courtesy of L. M. Mays. 

Figure 27-6 (A) Photomicrograph of a lampbrush chromosome from the nucleus of an oocyte of the newt Triturus. From...

In the same way that amphibian oocytes are an accessible scource of giant lampbrush chromosomes (as well as nuclear envelopes), the salivary glands of dipteran larvae are a source of giant polytene chromosomes, which are formed by repeated endomitotic reduplication of the DNA, while retaining lateral association of chromatids. In Chironomus tentans, polytene chromosomes in salivary... 

Fig. 2 Electron micrograph of a typical transcription unit on a lampbrush chromosome loop from a stage VI Xenopus oocyte nucleus. These arc seen less frequently than endogenous ribosomal genes and injected plasmid genes. It is not possible to identify these genes beyond the fact that they are Pol II genes, but t hey lend to be very long and to have a reasonably high density of nascent transcripts. Scale bar, 1 pm.

Gall, J. G. (1966). Techniques for the study of lampbrush chromosomes. Methods Cell Physiol. 

Scheer, U.. Hinssen, H., Frankc, W. W., and Jockush, B. M. (1984). Microinjection of actin-binding proteins and actin antibodies demonstrates involvement of nuclear actin in transcription of lampbrush chromosomes. Cell (Cambridge, Mass.) 39, 111-122. 

Miller, O. L., Jr. 1965. Fine structure of lampbrush chromosomes. Nat. Cancer Inst. Monogr. 18 79-99. 

The lampbrush chromosomes contract and apparently become inactive in RNA synthesis while oocytes still are quite small (Ficq, 1961). On the other hand, considerable sequence homology has been demon-... 

Animals which have not ovulated recently contain a population of oocytes, usually larger in diameter than 1.2 mm, which exhibit unpig-mented equatorial bands. These stage 6 oocytes (Dumont, 1972) are considered to represent the terminal stage of oocyte development and are always responsive to hormonal stimulation, both in vivo and in vitro. While these are not the only oocytes capable of undergoing maturation, the white band provides a convenient marker for isolation, and such oocytes have been utilized in our studies of RNA synthesis. For comparison, oocytes between 0.5 and 0.6 mm diameter (lampbrush chromosome stage 4 according to Davidson and Mirsky, 1965) were also studied. 

The oocytes of the newt Triturus veridens mature for months and grow to 2 mm in diameter with nuclei 1 mm in diameter. By micromanipulation, the chromosomes and nucleoli can be extracted. After appropriate treatment of the nucleoli, it can be shown under the electron microscope that their core is made of an axial fiber of deoxyribonucleoprotein from which ribonu-cleoprotein fibrils emerge in clusters of 80-100. Between the clusters are portions of nontranscribed DNA referred to as spacer DNA. A distinct granule 125 A in diameter links the DNP to the RNP fibers. The granule is believed to be RNA polymerase. Somewhat similar pictures were obtained when portions of the lampbrush chromosomes were examined. 

The best arguments for possible differential activity of autosomal genes are cases of heteromorphism of chromosomal regions for which the relation between cytological and functional expressions have been clearly demonstrated. These include puffs of polytene chromosomes, lateral loops of lampbrush chromosomes and nucleoli. 

The lateral loops of lampbrush chromosomes are active chromosomal regions, similar to puffs. These chromosomes have been studied less than polytene chromosomes from thp point of view of the differential activity of homologous chromosomes. However, there are examples of the functional heteromorphism in the behavior of homologous loops (Callan, 1963, 1965). 

In Xenopus, from early embryogenesis until the 500-2000 cell stage (early blastula), the store of mRNA that was synthesized on the lampbrush chromosomes did not change. However, 28% of this store disappeared during the 1.5 h-2.0 h that separate the early and late blastual stages (7 and 9 respectively) (Crippa et al., 1967). 

Franke et al. (1976) studied transcriptionally active chromatin from lampbrush chromosomes and nucleoli of oocytes of amphibians and Acetabularia nuclei and noted that the nucleosomes which are visible in the control preparations are not detectable in transcriptionally active chromatin. This was also seen in spacer regions of the transcriptionally active chromatin. DNA of nucleoli is packed into nucleosomes when the main part of the ribosomal genes is not active. 

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