Sister-chromatid cohesion is essential for proper chromosome segregation and faithful transmission

of the genome during the cell cycle (Morales and Losada， 2018; Uhlmann， 2016). Failure to estab-

lish or resolve cohesion in a timely manner leads to genomic instability and aneuploidy. Sister-chro-

matid cohesion is mediated by cohesin， a ring-shaped ATPase machine that consists of SMC1A，

SMC3， RAD21， and either STAG1 or STAG2 in human somatic cells (Haarhuis et al.， 2014;

Losada and Hirano， 2005; Nasmyth and Haering， 2009; Onn et al.， 2008; Peters et al.， 2008;

Zheng and Yu， 2015). Cohesin rings topologically entrap DNA to generate physical linkages

beteen sister chromatids and enable cohesion. Cohesin regulates other chromosome-based pro-

cesses， such as DNA repair， transcription， and chromosome folding (Merkenschlager and Odom，

2013; Wu and Yu， 2012). These other functions of cohesin likely also involve the topological entrap-

ment of chromosomes or possibly the extrusion of DNA loops (Barrington et al.， 2017;

Davidson et al.， 2016; Haarhuis et al.， 2017).

Cohesin is loaded onto chromosomes in telophase and G1 by the SCC2/4 plex (NIPBL/MAU2

in humans)(Ciosk et al.， 2000; Gillespie and Hirano， 2004; Takahashi et al.， 2004; Tonkin et al.，

2004; Watrin et al.， 2006). Before DNA replication， the chromosome-bound cohesin is dynamic and

is actively removed from chromosomes by the cohesin-releasing factor WAPL ith the help of thescaffolding protein PDS5A or PDS5B (Chan et al.， 2012; Kueng et al.， 2006; Lopez-Serra et al.，

2013; Ouyang and Yu， 2017; Ouyang et al.， 2013; Ouyang et al.， 2016). During DNA replication

in S phase， a pool of cohesin is converted to the cohesive form， hich stably associates ith chromo-

somes and mediates sister-chromatid cohesion (Gerlich et al.， 2006; Kueng et al.， 2006). In human

cells， cohesion establishment requires the acetylation of SMC3 by the acetyltransferases ESCO1 and

ESCO2 and subsequent recruitment of sororin， hich antagonizes WAPL to stabilize cohesin on

chromosomes (Alomer et al.， 2017; Hou and Zou， 2005; Nishiyama et al.， 2010; Ouyang et al.，

2016; Rankin et al.， 2005; Rolef Ben-Shahar et al.， 2008; Roland et al.， 2009; Unal et al.， 2008;

Zhang et al.， 2008a).

The checkpoint kinase proteins Mec1 and Rad53 are required in

the budding yeast， Saccharomyces cerevisiae， to maintain cell

viability in the presence of drugs causing damage to DNA or

arrest of DNA replication forks1±3. It is thought that they act by

inhibiting cell cycle progression， alloing time for DNA repair to

take place. Mec1 and Rad53 also slo S phase progression in

response to DNA alkylation4， although the mechanism for this

and its relative importance in protecting cells from DNA damage

have not been determined .Here e sho that the DNA-alkylating

agent methyl methanesulphonate (MMS) profoundly reduces the

rate of DNA replication fork progression; hoever， this moderation

does not require Rad53 or Mec1. The accelerated S phase in

checkpoint mutants4， therefore， is primarily a consequence of

inappropriate initiation events5±-type cells ultimately plete

DNA replication in the presence of MMS. In contrast，