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The major structurs in DNA compaction: DNA, the nucleosome, the 10 nm "beads-on-a-string" feebre, the 30 nm chromatin feebre an the metaphase chromosome.

Chromatin is a complex o macromolecules foond in cells, consistin o DNA, protein, an RNA.[1] Its primar function is packagin verra lang DNA molecules intae a mair compact, denser shape, that prevents the strands frae acomin tangled an plays important roles in reinforcin the DNA in cell diveesion, preventin DNA damage, an regulatin gene expression an DNA replication. In mitosis an meiosis, chromatin facilitates proper segregation o the chromosomes in anaphase; the chairactereestic shapes o chromosomes veesible in this stage are the result o DNA bein coiled intae heichly condensed netwarks o chromatin.

The primar protein components o chromatin are histones, that bind tae DNA an function as "anchors" aroond that the strands are wound. In general, thsre are three levels o chromatin organisation:

  1. DNA wraps around histone proteins, formin nucleosomes an the sae-cried "beads on a string" structur (euchromatin).
  2. Multiple histones wrap intzd a 30-nanometre feebre consistin o nucleosome arrays in thair maist compact form (heterochromatin).[lower-alpha 1]
  3. Heicher-level DNA supercoilin o the 30-nm feebre produces the metaphase chromosome (in mitosis an meiosis).

Mony organisms, houiver, dae nae follae this organisation scheme. For ensaumple, spermatozoa an avian reid bluid cells hae mair tichtly packed chromatin nor maist eukaryotic cells, an trypanosomatid protozoa dae nae condense thair chromatin intae veesible chromosomes at aw. Prokaryotic cells hae entirely different structures for organisin thair DNA (the prokaryotic chromosome equivalent is cried a genophore an is localised within the nucleoid region).

The oweraw structur o the chromatin netwark forder depends on the stage o the cell cycle. In interphase, the chromatin is structurally lowse tae allou access tae RNA an DNA polymerases that transcribe an replicate the DNA. The local structur o chromatin in interphase depends on the speceefic genes present in the DNA. Regions o DNA conteenin genes that are actively transcrived ("turned on") are less tichtly compactit an closely associatit wi RNA polymerases in a structur kent as euchromatin, while regions conteenin inactive genes ("turned aff") are generally mair condensed an associatit wi structural proteins in heterochromatin.[3][4] Epigenetic modification o the structural proteins in chromatin via methylation an acetylation an aw alters local chromatin structur an tharefore gene expression. The structur o chromatin netwarks is currently puirly unnerstuid an remeens an active aurie o resairch in molecular biology.

NotesEedit

  1. Tho it haes been definitively established tae exist in vitro, the 30-nanometre feebre wis nae seen in recent X-ray studies o human mitotic chromosomes.[2]

ReferencesEedit

  1. Monday, Tanmoy (July 2010). "Characterization of the RNA content of chromatin". Genome Res. 20 (7): 899-907. 
  2. Hansen, Jeffrey (March 2012). "Human mitotic chromosome structure: what happened to the 30-nm fibre?". The EMBO Journal. 31 (7): 1621–1623. doi:10.1038/emboj.2012.66. PMC 3321215 . PMID 22415369. 
  3. "Chromatin Network Home Page". Retrieved 2008-11-18. 
  4. Dame, R.T. (May 2005). "The role of nucleoid-associated proteins in the organization and compaction of bacterial chromatin". Molecular Microbiology. 56 (4): 858–870. doi:10.1111/j.1365-2958.2005.04598.x. PMID 15853876.