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Stress-induced condensation occurs by a non-random, zipper-like convergence of sister chromosomes. This convergence appears to depend on the ability of identical double-stranded DNA molecules to specifically identify each other, a process that culminates in the proximity of homologous sites along the paired chromosomes.
The outcome can be activation of transcription or repression of a gene. For example, the combination of acetylation and phosphorylation have synergistic effects on the chromosomes overall structural condensation level and, hence, induces transcription activation of immediate early gene. [22]
During the leptotene stage, the duplicated chromosomes - each consisting of two sister chromatids - condense from diffuse chromatin into long, thin strands that are more visible within the nucleoplasm (nucleus contents). The chromosomes become visible as thin threadlike structures known as leptonema under a light microscope. [1]: 27 [2]: 353
The mitotic kinase aurora B phosphorylates histone H3 at serine 10, triggering a cascade of changes that mediate mitotic chromosome condensation. [ 92 ] [ 93 ] Condensed chromosomes therefore stain very strongly for this mark, but H3S10 phosphorylation is also present at certain chromosome sites outside mitosis, for example in pericentric ...
[55] [56] Condensin II is present within the cell nucleus during interphase and participates in an early stage of chromosome condensation within the prophase nucleus. On the other hand, condensin I is present in the cytoplasm during interphase, and gains access to chromosomes only after the nuclear envelope breaks
The appearance of a prematurely condensed chromosome depends on the stage that the interphase cell was in. [1] Chromosomes that are condensed during the G 1 phase are usually long and have a single strand, while chromosomes condensed during the S phase appear crushed. Condensation during the G 2 phase yields long chromosomes with two chromatids.
Chromosome scaffolds play an important role to hold the chromatin into compact chromosomes. Loops of 30 nm structure further condense with scaffold, into higher order structures. [ 21 ] Chromosome scaffolds are made of proteins including condensin , type IIA topoisomerase and kinesin family member 4 (KIF4). [ 22 ]
The chromosomes reach their highest level of condensation during pachytene. Each chromosome consists of two closely associated sister chromatids along their entire length. The chromosomes appear as distinct, well-defined threadlike structures under the microscope.