Somatic genome processing
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The genome of most cells of eukaryotes remains mainly constant during life. However, there are cases of genome being altered in specific cells or in different life cycle stages during development. For example, not every human cell has the same genetic content as red blood cells which are devoid of nucleus. One of the best known groups in respect of changes in somatic genome are ciliates. The process resulting in a variation of somatic genome that differs from germline genome is called somatic genome processing.
Genome loss
The result of this process is the removal of a whole genome from a cell. The most known example is the enucleation process of erythrocytes. Processed stem cell goes through changes causing it to lose a nucleus. In the begining phase, pro-erythroblast goes through another mitotic divisions, in which erythroblast with smaller nucleus is created and moved to the side of the cell. The nucleus becomes isolated from cytoplasm and then erythroblast is divided into reticulocyte with cytoplasm and pyrenocyte with condensed nucleus. Pyrenocyte with all genetic material from cell is degraded then by macrophage. Loss of genome is in this case advantageous since pyrenocyte can accumulate more hemoglobin. Mature red blood cell without nucleus, can properly deliver oxygen.[1]
Chromatin diminution
Chromatin diminution is a process of partial elimination of a chromatin genetic material from genome of prospective somatic cells. This process was found to occur during the early developmental stage in 3 groups: nematodes, copepods, and hagfish. One of the first studies regarding somatic genome processing was observed by Boveri large-scale chromatin elimination in Parascaris univalens.[2]
Targetting rearrangements
Targetting rearrangements do not affect whole genome, only specific loci. Process of rearrangement helps maintaining variety of immunoglobulins in vertebrates. During life organism have contact with large number of antigenes, it means that immune system needs to synthesize a wide range of antibodies. Each immunoglobulin is tetramer consisting of four polypeptides connected by disulfide bridges. They form two long, heavy chains and two short, light chains. But vertebrate genome does not code entire genes of heavy and light immunoglobulins, only gene segments. Segments of heavy chain are located on chromosome 14, they include 11 constant gene segments (CH), that are preceded by 123-129 variable segments (VH), 27 diversity gene segments (DH) and 9 joining segments (JH), coding different versions of components V, D, J. Loci of light chains on chromosome 2 (locus κ) and chromosome 22 (locus λ) have similar structure, but they do not contain D segments. At the early stage of lymphocyte B development , loci of immunoglobulins are rearranged. During rearrangement, segment VH on heavy chain locus is connected with one DH segment, then V-D group is combined with JH segment. Eventually, exon with open reading frame coding segments: VH, DH, JH of immunoglobulin. Through RNA splicing during transcription, this exon becomes connected to exon for CH segment. Complementary mRNA of heavy chain can be translated into immunoglobulin specific only for one lymphocyte.[3]
References
- ^ Migliaccio, Anna Rita (2017-04-27). "Erythroblast enucleation". Haematologica. 95 (12): 1985–1988. doi:10.3324/haematol.2010.033225. ISSN 0390-6078. PMC 2995553. PMID 21123437.
{{cite journal}}: CS1 maint: PMC format (link) - ^ Boveri, Theodor (1887). "Über Differenzierung der Zellkerne während der Furchung des Eies von Ascaris megalocephala". Anatomischer Anzeiger.
- ^ Brown, T.A. Genomes 3. pp. 439–441. ISBN 9780815341383.