Lampbrush chromosomes and associated bodies: new insights into principles of nuclear structure and function.

The lampbrush chromosomes and assorted nuclear bodies of amphibian and avian oocytes provide uniquely advantageous and amenable experimental material for cell biologists to study the structure and function of the eukaryotic nucleus, and in particular to address the processes of nuclear gene expression. Recent findings discussed here include the molecular analysis of the actively elongating RNA polymerase complexes associated with lampbrush chromosome loops and of the association between loop nascent transcripts and RNA processing components. In addition, several types of chromosome structure that do not outwardly resemble simple extended loops and that may house novel nuclear functions have recently been studied in detail. Among these a type of chromosomal body that can also exist free in the oocyte nucleus, the Cajal body, has been shown to possess a range of characteristics that suggest it is involved in the assembly of macromolecular complexes required for gene expression. Homologous structures have also been described in somatic nuclei. Fundamental aspects of the looped organization exhibited by lampbrush as well as other chromosomes have also been addressed, most notably by the application of a technique for de-novo chromosome assembly.