Endoreplication

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Endoreplication (also polyploidization ) occurs in somatic tissues through program changes from the (mitotic) cell cycle to the endocycle. While replicating the DNA (in several cycles), but the nucleus and therefore the cell can not divide and become larger. The process is called endoreduplication when the DNA content is exactly doubled in each case, from 2 C to 4 C → 8 C → 16 C → 32 C etc. The DNA content is measured on interphase nuclei using flow-throughor microscope photometry. The C value stands for the size of the (haploid) genome of a certain biological species . A C value is given in picograms (pg) or megabase pairs (Mbp) (plant genomes, animal genomes). Since the value 4 C equals the DNA content produced by an S phase in the mitotic cycle, an endoreplicated interphase nucleus can only be recognized from 8 C.

Endoreplication occurs in the normal development of eukaryotes : in protozoa, in plants and in animals. Examples are the endosperm and trichome development in plants in general, the fruit tissue of tomatoes. The cores of the larval silk glands of the flour moth reached 12 Endozyklen up to 8,192 C DNA silkworm 17 Endozyklen about 300,000 C . The share of endoreplication in global biomass growth is likely to be up to fifty percent. The advantage lies in increased protein synthesis and thus a more efficient metabolism. The downside is the risk of cells growing out of control; this requires precise regulation of endoreplication.

If an endo-replicated cell nucleus reveals chromosomes , two types of endoreplication can be distinguished by their shape: obvious polyploidy and polytene . There can be transitional forms between the two.

Polyploidy

Endoreplication of the loose chromatin in an S phase (interphase) is always followed by a G phase. The endomitosis begins with a Endoprophase in which the chromosomes condense. Their daughter chromatids separate and can be counted in an endometric phase. True polyploid (multiple) sets show 4 n, 8 n, 16 n, 32 n, etc. chromosomes if nuclear membranes are present. Such nuclei are called (Greek) tetraploid, octoploid, decaexiploid, trianta-dyo-ploid etc. According to the numbers, the total DNA content of such cell nuclei should be 4 C, 8 C, 16 C, 32 C etc. Lothar Geitler was the first to describe endomitoses .

Polythene

After endoreplications in temporally separated S phases, the maternal and paternal chromatids remain connected. Therefore, such cell nuclei show multi-stranded (polytine) giant chromosomes in diploid numbers (2 n). If, in addition, the maternal and paternal chromatids combine, such cell nuclei show somatically paired polytene chromosomes in haploid numbers (1 n). In the latter type of chromosome, mating gaps can sometimes be observed between the parental chromatid strands. Incomplete pairing arises when the partners differ through mutated sections.

Polytenia can be seen in the light microscope from a DNA content of about 32 C (after the fourth endoreplication). Since the connected chromatids can hardly be counted, polytene levels are determined by microphotometry. Polytene chromosomes were first detected in the garden hair mosquito . Especially with the fruit fly Drosophila melanogaster , the cytogenetics of polytene was established. Basic knowledge about the gene activity in polytene chromosomes came from the mosquito Chironomus tentans .

The endoreplicating nutrient cell nuclei in the ovary of some diptera have a special feature . At first they show endometaphases with oligotene chromosomes. After a few endocycles, these primary giant chromosomes disintegrate into their few chromatids, with which subsequent endocycles produce morphologically polyploid sets of chromosomes.

Selective endoreplication

Every chromosomal replication is subject to strict genetic control. It is therefore possible that not all genomic DNA sequences are multiplied in the same way, but rather selectively. Such selective endoreplication is part of the normal development program and is described according to its relative extent under two aspects: as amplification or as under-replication .

Amplification

Here, only a small section of a chromosome is endoreplicated for a short time. Such local gene reproduction creates the prerequisite for meeting a development-related high demand for certain proteins.

Sub-replication

This phenomenon was observed for the first time in Drosophila virilis , in which half of the genomic DNA does not take part in polytenization. The specific exclusion of heterochromatic , repetitive DNA from endoreplication begins in the late embryonic development. Under-replication is particularly noticeable when it mainly affects a single pair of chromosomes. The under-replication of intercalar heterochromatin is difficult to quantify, but can be recognized by weak points in the giant chromosomes.

Under-replication does not only exist in polytene structures, but also occurs in polyploid cell nuclei. Endometaphases provide information about this. Hypothesis: Heterochromatically repetitive DNA sequences require large cell nuclei and allow embryonic development to begin with correspondingly large cells. As soon as endoreplication starts, this placeholder DNA is dispensed with. DNA elimination (chromatin diminution) has the same effect as under-replication.

Human biology

Also in humans (as in other vertebrates, mammals) endoreplicated cell nuclei are to be found in tissues that obviously perform at high physiological levels. These include heart muscle cells, megakaryocytes (giant cells of the bone marrow), as well as certain neurons in the nervous system. Endoreplicative cell nuclei from extrevillous trophoblasts of the human placenta with polytene chromosomes enable implantation in the uterus at the beginning of pregnancy.

Endoreplicated cell nuclei with mutated chromosomes often accompany tumor development . In contrast to the above examples from normal development, they are pathological.

literature

  • Brodsky VY and Uryvaeva IV: Genome multiplication in growth and development. Biology of polyploid and polytene cells. Cambridge University Press, Cambridge, London, New York 1985. ISBN 0-521-25323-3
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  • Nagl Walter: Endopolyploidy and polyteny in differentiation and evolution. North Holland, Amsterdam 1978.
  • Sumner Adrian T .: Chromosomes. Organization and function. Blackwell, Oxford (UK) 2003. ISBN 0-632-05407-7
  • Therman Eeva and Susman Millard: Human chromosomes. Structure, behavior, and effects. Third edition. Springer, New York, Berlin, Heidelberg 1993. ISBN 0-387-97871-2
  • Traut Walther: Chromosomes. Classical and Molecular Cytogenetics. Springer, Berlin, Heidelberg 1991. ISBN 3-540-53319-2
  • Ullah Zau, Lee CY, Lilly MA, DePamphilis Melvin L .: Developmentally programmed endoreduplication in animals. In: Cell Cycle. 8/2009, pp. 1501-1509.

Individual evidence

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