Diminution (chromatin)
As a diminution or Chromatindiminution a process is referred to, in which parts of the chromosomes of a cell are specifically degraded so that daughter cells have less chromatin, as the progenitor cells. This is an exception to the rule according to which daughter cells basically receive the same chromosomal structure as the precursor cells.
The process is distinguished from chromosome elimination, in which entire chromosomes are broken down.
The process was first described in 1887 by Theodor Boveri on the eggs of a roundworm , more precisely the horse powderworm ( Parascaris equorum , still called Ascaris megalocephala in Boveri's time ).
Processes in roundworms
In the horse roundworm examined by Boveri, the fertilized egg divides and the “primordial chromosomes” (Boveri) are preserved in one of the daughter cells (P1). In the other there is diminution. After three further divisions of P1, one of the offspring becomes the progenitor of the germ cells or sex cells . The other daughter cell in turn goes under the diminution.
The processes described by Boveri can be seen in the adjacent drawing: Fig. 26: The fertilized egg cell has already divided once and is about to divide a second time. The chromosomes are already condensed (in metaphase). The upper daughter cell (S1) will only have somatic cells as descendants; the defragmentation of the two chromosomes as the first step in diminution has already started. The lower cell (P1) is the precursor of the germ cells and other somatic cells. Here the unchanged “original chromosomes” are still present. Fig. 27: Both cells are now in the nucleus division, more precisely in the anaphase and the duplicated chromosomes are divided for the daughter cells. In the upper cell, the separated chromosome fragments remain in the equatorial plane. Fig. 28 .: After the division is complete, the chromosome fragments to be broken down are still visible at the border of the upper daughter cells. All cells are now in interphase. Fig. 29: In the next anaphase it becomes clear that the two lower cells also have different fates: the lower left cell (S2 (EMSt)) also undergoes diminution. The cell at the bottom right (P2) retains the complete chromosomes. With two further divisions, the daughter cell P3 is created first and from this P4, from which the sex cells will emerge. The other daughter cells undergo diminution.
In the somatic cells, the diminution begins with a fragmentation of the “original chromosomes” at specific chromosomal regions. New telomeres are created so that all fragments that are still needed can be passed on. The mechanisms of breakpoint selection and fragmentation are not known. The DNA between a break point and the telomere (with all the genes it contains) is broken down and a new telomere is generated at the break point by adding the consensus sequence TTAGGC. Presumably this is done by telomerase and not by recombination with the old telomere. During the next nuclear division, the microtubules of the spindle apparatus only attach to the new, shortened chromosomes, but not to the cut fragments that remain in the anaphase in the equatorial plate and are then broken down enzymatically. The rejected DNA contains mainly repetitive sequences .
More detailed investigations on Ascaris suum showed that of the 334 million base pairs of DNA present in the germline, 13% is broken down in the somatic cells. 52 breakpoints were identified at which a subsequent stabilization by telomere formation took place. A connection of different fragments to new chromosomes could not be proven. The removed DNA contained 70% mainly a 121 base pair long repetitive satellite DNA . However, the remaining 12.7 million base pairs were non-repetitive and contained nearly 700 genes. This corresponds to about 4% of all genes. For these genes, transcription could actually only be detected in the germline and in the early embryo. These data suggest that a role of diminution may be to remove germline-specific genes from somatic cells.
distribution
Diminution was not only found in nematodes (in Ascaris , Parascaris and Strongyloides , but not in Caenorhabditis elegans ). She was also described by copepods (Copepoda), ciliates (ciliates), the sea lamprey and at a Lauchart.
In the copepods, diminution has meanwhile been described in over 20 freshwater species, including several species of hoppers ( Cyclops ). The somatic cells lose between 45 and 94% of the original DNA. Large amounts of heterochromatin are removed.
In the sea lamprey ( Petromyzon marinus ), about 20% of the germline DNA is removed from the somatic cells during early embryonic development. In addition to large amounts of repetitive DNA, hundreds to thousands of genes are also removed, including those that are involved in basic cell functions such as transcription. As in Ascaris suum , no conserved sequences at the breakpoints could be found here either. However, palindromic sequences were found at some breakpoints.
Guranda Gwaladze and colleagues described diminution in a plant species, Allium atroviolaceum , which belongs to the genus Leek .
function
It is discussed that diminution is an alternative form of gene regulation . The more recent studies on Ascaris suum and Petromyzon marinus described above suggest that diminution is an effective method to save somatic cells the cost of replicating and maintaining DNA that is not needed there and to very effectively prevent the expression of germline-specific genes .
Individual evidence
- ^ A b Scott F. Gilbert: Developmental Biology . Tenth edition. Sinauer Associates, Inc., 2013, ISBN 978-1-60535-192-6 , Chapter 17.1, Chromatin Diminution. ( online version ). online version ( Memento of the original from February 28, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b c d A. Streit: Silencing by throwing away: a role for chromatin diminution. In: Developmental cell. Volume 23, Number 5, November 2012, pp. 918-919, doi : 10.1016 / j.devcel.2012.10.022 , PMID 23153488 .
- ↑ a b Theodor Boveri: The potencies of the Ascaris blastomeres with modified furrowing . In: Festschrift for Richard Hertwig's sixtieth birthday . tape III . Verlag von Gustav Fischer, Jena 1910, p. 133–214 (with 6 plates and 24 text figures).
- ^ S. Pimpinelli, C. Goday: Unusual kinetochores and chromatin diminution in Parascaris. In: Trends in genetics: TIG. Volume 5, Number 9, September 1989, pp. 310-315, PMID 2686123 doi : 10.1016 / 0168-9525 (89) 90114-5 , (Review).
- ^ A. Streit: How to become a parasite without sex chromosomes: a hypothesis for the evolution of Strongyloides spp. and related nematodes. In: Parasitology. Volume 141, Number 10, September 2014, pp. 1244-1254, doi : 10.1017 / S003118201400064X , PMID 24829037 (review).
- ^ A b F. Müller, V. Bernard, H. Tobler: Chromatin diminution in nematodes. In: BioEssays: news and reviews in molecular, cellular and developmental biology. Volume 18, Number 2, February 1996, pp. 133-138, doi : 10.1002 / bies.950180209 , PMID 8851046 (review).
- ^ Heinz Tobler, Fritz Müller: Chromatin Diminution . In: eLS . 2001, doi : 10.1038 / npg.els.0001181 .
- ↑ A. Grishanin: chromatin diminution in Copepoda (Crustacea): pattern, biological role and evolutionary aspects. In: Comparative cytogenetics. Volume 8, number 1, 2014, pp. 1-10, doi : 10.3897 / CompCytogen.v8i1.5913 , PMID 24744830 , PMC 3978239 (free full text).
- ↑ G. Gvaladze, N. Nadirashvili, M. Akhalkatsi: Chromatin diminution during endosperm development in Allium atroviolaceum Boiss. (Alliaceae) . In: Bull. Georg. Acad. Sci. tape 166 , no. 3 , 2002, p. 537-540 ( first page online [PDF]). first page online ( memento of the original from February 28, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
