Endoreduplication

Endoreplication (including polyploidization ) occurred in somatic tissues through changes in the program ( mitotic ) cell cycle to Endozyklus. Although the DNA replicates (several cycles ), but the nucleus and therefore the cell can not divide and grow. The process is called endoreduplication when the DNA content is doubled in each case exactly, from 2 C after 4 → C 8 C → 16 C → 32 C, etc. The DNA content is on interphase nuclei by flow or microscope photometry determined. The C value represents the size of the ( haploid ) genome of a particular biological Article A C- value in picograms (pg) or megabase pairs ( Mbp ) indicated (plant genomes: animal genomes :). Since the value of C 4 is similar to the DNA content, the produces an S- phase in the mitotic cycle, a endoreplizierter interphase nucleus can be seen only from 8 C.

Endoreplication occurs in normal development in eukaryotes: in unicellular organisms, in plants and in animals. Examples are the endosperm and the trichome development in plants in general, the fruit tissue of tomato. Reach the nuclei of the larval silk glands of the flour moth with 12 Endozyklen up to 8,192 C DNA of the silkworm with 17 Endozyklen about 300,000 C. The proportion of endoreplication in the global biomass growth is expected to be up to fifty percent. The advantage lies in increased protein synthesis and thus a more efficient metabolism. The downside is the risk that cells grow uncontrollably; this requires precise regulation of endoreplication.

Lets see a endoreplizierter nucleus chromosomes in the shape of two types of endoreplication can be distinguished: the obvious polyploidy and the Polytänie. Between the two, there may be transitional forms.

Polyploidy

On a endoreplication of the loose chromatin in an S- phase ( interphase ) as always follows a G- phase. The Endomitosis begins with a Endoprophase in which the chromosomes condense. Her daughter chromatids separate and can be counted in a Endometaphase. True polyploidy ( multiple ) sentences show at existing nuclear membranes 4 n, 8 n, 16 n, 32 n, etc. chromosomes. Such nuclei are called ( Greek) tetraploid, oktoploid, dekaexiploid, trianta - dyo - ploid, etc. The numbers should be according to the total DNA content of these nuclei 4 C, 8 C, 16 C, 32 C amount etc.. Endomitoses first described Lothar Geitler.

Polytänie

After Endoreplikationen in temporally separated S -phase, the maternal and paternal chromatids remain connected. Why show such nuclei multi-stranded ( polytene ) Giant chromosomes in diploid number ( 2n ). If the maternal connect beyond with the paternal chromatids show such nuclei somatically paired polytene chromosomes in haploid number (n-1 ). In the latter type of chromosome pairing sometimes gaps between the parental Chromatidensträngen are observed. Incomplete pairing follows necessarily, if the partners differ by mutated sections.

Polytänie be seen approximately from a DNA content of 32 C ( according to the fourth endoreplication ) in the light microscope. Since the connected chromatids are hard to count, Polytäniestufen be determined by Mikrofotometrie. Polytene chromosomes were first detected in the Garden Hair mosquito. Especially with the fruit fly Drosophila melanogaster, the cytogenetics of Polytänie was established. Basic knowledge about the gene activity in polytene chromosomes came from the Midge Chironomus tentans.

A special feature, the endoreplizierenden Nährzellkerne on the ovary of some Diptera. At first, they show Endometaphasen with oligotänen chromosomes. These primary giant chromosomes disintegrate after a few Endozyklen in their few chromatids serve subsequent Endozyklen produce morphologically polyploid chromosome sets.

Selective endoreplication

Each chromosomal replication is subject to strict genetic control. Therefore, it is possible that not all genomic DNA sequences equally, but are multiplied selected. Such selective endoreplication is part of normal development program and is described according to their relative importance in two aspects: as amplification or as a replication.

Amplification

This case, only a small portion endorepliziert short of a chromosome. Such a local gene augmentation creates the conditions to meet a developmental high demand for certain proteins.

Under replication

First time this phenomenon has been observed in Drosophila virilis, at which half of the genomic DNA did not participate in the Polytänisierung. The specific exclusion of heterochromatin, repetitive DNA of the endoreplication begins in the late embryonic development. Particularly striking is the effect of sub- replication, when they mainly relates to a single chromosome pair. The lower replication intercalary heterochromatin is difficult to quantify, reveals himself to the giant chromosomes but by weaknesses ( weak points).

Under replication it is not only in polytene structures, but also occurs in polyploid nuclei. Endometaphasen provide evidence. Hypothesis: heterochromatic repetitive DNA sequences require large nuclei and allowed to begin embryonic development with correspondingly large cells. Once endoreplication uses, dispenses with that placeholder DNA. The same effect as lower replication, the DNA elimination ( chromatin diminution ).

Human Biology

Endoreplizierte nuclei with mutated chromosomes often accompany tumorigenesis. In contrast to the above examples from the normal development they are pathologic.