Amphibian lampbrush chromosomes

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... 

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... 

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. 

Crippa, M., Davidson, E. H., and Mirsky, A. E. (1967). Persistence in early amphibian embryos of informational RNA s from the lampbrush chromosome stage of oogenesis. Proc. Natl. Acad. Sci. U.S. 57, 885-892. 

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