Polytene chromosome

Polytänchromosom as a chromosome is referred to, which contains many parallel single strands ( chromatids ) each with the same gene sequences. Polytene chromosomes are usually much larger than "normal" mitotic chromosomes and to detect the light microscope even at moderate power. They are also called giant chromosomes, a term that was sometimes used for the different structure lampbrush chromosomes. Prerequisite for Polytänisierung is endoreplication. In this operation, the chromosomal DNA is multiplied several rounds within a nucleus; so it comes to no cell division.

Morphology

Chromatids a Polytänchromosoms are closely and precisely aligned with the corresponding DNA - sequences together. Densely packed areas of euchromatin appear as " bands " separated from " interbandings ". Even denser regions form heterochromatin. Due to the band pattern that characterizes each individual Polytänchromosom, you have already created early maps of chromosomes. In those organisms that form giant chromosomes that are homologous ( corresponding to each other, obtained from mother and father) chromosomes often already associated with one another in pairs in mitotic cells. This fact should help to narrow pairing of maternal with paternal chromatids of giant chromosomes. Polytene sets with fully or partially paired homologs show the haploid chromosome number (n-1 ).

The Polytänisierung may be ten or more rounds of replication ( Endozyklen ), whereby " cable " with 2048 or more chromatids arise. As one chromatid contains a single DNA double helix, one set has giant chromosomes after 10 full Endozyklen a DNA content of 2048 C. The value of "C " stands for the species-specific genome size.

Some heterochromatic regions do not participate ( or only partially ) on the Polytänisierung, first described in Drosophila virilis. This so-called sub- replication can be determined by comparison with mitotic metaphase. See " Selective endoreplication ".

Gene activity

During transcription activity leads to a loosening ( decondensation ), whereby a band in the light microscope appear brighter. Such a region is called Puff. Particularly large puffs after its discoverer Edouard -Gérard Balbiani (1823-1899) called Balbiani rings. The transcription of puffs and Balbianiringen in Chironomus tentans has been impressively shown with a high resolution scanning electron microscope. Those DNA sequences from which the ribosomal RNA is read constitutional form, the nucleolus, the ever largest puff of a cell nucleus.

Occurrence

Particularly large polytene chromosomes found in the salivary glands of the larvae of some insect species such as the midge ( Chironomus sp. ), Where she first described in 1881 Balbiani. At the Hamburg Botanical Institute Emil Heitz and Hans Bauer succeeded in 1933 with the Garden Hair mosquito proof: The huge " core loops " in the oversized nuclei are actually 1 n = 5 chromosomes. Best known are the 1 n = 4 giant chromosomes of the fruit fly Drosophila melanogaster. In Drosophila species and other Diptera polytene chromosomes come in cell nuclei and other tissues before (eg, bristle -forming cells, Malpighian vessel cells, nurse cells ). In silk glands of insect larvae shortly before and during pupation one also finds polytene chromosomes. The genes encoding the silk proteins are there to identify the very high rate of transcription for the Balbiani ring. Giant chromosomes also occur in springtails ( Collembola ) in ciliates ( Ciliophora ) and in some plants in the suspensor. When people allow trophoblast nuclei with polytene chromosomes implantation in the uterus at the beginning of pregnancy.