Lampbrush chromosome

Lampbrush chromosomes are temporary shape and function of the chromosomes through the formation of numerous loops ( loops) look like round brushes in the light microscope. Due to their size, they are sometimes called giant chromosomes. This term is also used for the differently configured polytene chromosomes.

Zoology

Particularly striking are lampbrush chromosomes in cell nuclei of oocytes I in newts and salamanders, because these have very large genomes. The loops occur in the ovaries of these amphibians during egg formation ( oogenesis ), in the diplotene (part of the meiotic prophase I). Here, the lampbrush chromosomes make known as bivalents as two homologous partners are connected by chiasmata. The diplotene nuclei have a DNA content of 4 C. lampbrush chromosomes first described Walther Flemming in the oocyte nucleus of his time. Than siredon pisciformis designated Axolotl The name forgave J. Rückert 1892. Lampbrush chromosomes were also in the oocytes of birds, reptiles, sharks and bony fish, found some mollusks and some insects.

Exceptions

Species of the fruit fly Drosophila develop Lampbrush, but only at the two chromatids of the Y chromosome in spermatocytes I, ie in males during sperm formation ( spermatogenesis). Among the plants is only in the seaweed Acetabularia a lampbrush stage known that occurs in their single nucleus prior to gamete formation.

Function

The loops unfold discontinuously from occupied with proteins DNA - axes of the four chromatids of each Bivalents. By the DNA of the loops large amounts RNA transcribed by RNA polymerase molecules from a start point, in close succession, reading along a loop. The growing RNA molecules are packed with proteins, of course, remain with their RNA polymerase and thus connected to the loop. Particularly in the electron microscope is clear that loops consist of one or even a plurality of transcription units. The transcribed DNA loops, however, makes only around 2% of the total DNA of the cell nucleus. Nevertheless enormous RNA synthesis in oocytes supplies the material, which is required after the impregnation in order to allow a fast growth of the embryo.

Those insects that show Lampbrush, produce the necessary material in large nurse cells surrounding the oocyte. And mammals have a long gestation that eliminates a previous fitting of the oocyte.

Chromosome structure

The lampbrush chromosomes provided fundamental insight into the organization of chromosomes. Each chromatid contains only one, namely a continuous DNA double helix. Evidence provided digestion with DNase I, an enzyme that single-strand breaks caused. Such studded with proteins DNA axis can decondense at various points and thereby develop temporarily transcribed loops. On completion of the gene activity of the loops according to the program back into the axle. Similar, temporary loops form the chromatids in the ( active transcription ) puffs of polytene chromosomes; on large Balbiani rings that's good to see.

Measurements of the viscoelasticity of DNA solutions confirmed that a double helix through a whole chromosome, without being interrupted by the centromere. In this regard, matching findings Drosophila americana, D. hydei, D. melanogaster and D. virilis revealed.