<P> Chromatin relaxation is one of the earliest cellular responses to DNA damage . The relaxation appears to be initiated by PARP1, whose accumulation at DNA damage is half complete by 1.6 seconds after DNA damage occurs . This is quickly followed by accumulation of chromatin remodeler Alc1, which has an ADP - ribose--binding domain, allowing it to be quickly attracted to the product of PARP1 . The maximum recruitment of Alc1 occurs within 10 seconds of DNA damage . About half of the maximum chromatin relaxation, presumably due to action of Alc1, occurs by 10 seconds . PARP1 action at the site of a double - strand break allows recruitment of the two DNA repair enzymes MRE11 and NBS1 . Half maximum recruitment of these two DNA repair enzymes takes 13 seconds for MRE11 and 28 seconds for NBS1 . </P> <P> Another process of chromatin relaxation, after formation of a DNA double - strand break, employs γH2AX, the phosphorylated form of the H2AX protein . The histone variant H2AX constitutes about 10% of the H2A histones in human chromatin . γH2AX (phosphorylated on serine 139 of H2AX) was detected at 20 seconds after irradiation of cells (with DNA double - strand break formation), and half maximum accumulation of γH2AX occurred in one minute . The extent of chromatin with phosphorylated γH2AX is about two million base pairs at the site of a DNA double - strand break . </P> <P> γH2AX does not, by itself, cause chromatin decondensation, but within seconds of irradiation the protein "Mediator of the DNA damage checkpoint 1" (MDC1) specifically attaches to γH2AX . This is accompanied by simultaneous accumulation of RNF8 protein and the DNA repair protein NBS1 which bind to MDC1 as MDC1 attaches to γH2AX . RNF8 mediates extensive chromatin decondensation, through its subsequent interaction with CHD4 protein, a component of the nucleosome remodeling and deacetylase complex NuRD . CHD4 accumulation at the site of the double - strand break is rapid, with half - maximum accumulation occurring by 40 seconds after irradiation . </P> <P> The fast initial chromatin relaxation upon DNA damage (with rapid initiation of DNA repair) is followed by a slow recondensation, with chromatin recovering a compaction state close to its predamage level in ∼ 20 min . </P>

Nucleosome location may be changed by a process called atp-dependent chromatin remodeling