Following DNA replication in S-phase, chromosomes are linked together
(cohesion), compacted (condensation), untangled and then moved to
opposite poles of the cell (segregation). All these events are coordinated
and executed with precision to prevent aneuploidy, a condition of
inappropriate chromosome number often associated with birth defects
The poleward movement of chromosomes to the two daughter cells relies
on the spindle, a complex microtubule-based machine, which captures
sister chromatids during metaphase and pulls them to opposite poles
at anaphase. Chromosomes are not passive substrates during segregation,
but actively contribute to the process by virtue of three specific
structural features: (i) Each sister chromatid has a region that mediates
the attachment and movement of chromosomes on the microtubules of
the spindle, named centromere, (ii) Sister chromatids are linked by
protein bridges (sister chromatid cohesion). The resolution of cohesion
not only serves as a key regulatory step for the onset of chromosome
segregation but it is also important during bipolar attachment of
sister chromatids to the mitotic spindle, and (iii) Sister chromatids
are compacted during segregation to minimise the entanglement of chromosomes
while they move to the poles in anaphase. This is necessary to ensure
that mitotic chromosomes are less than half as long as the spindle,
thus preventing the lagging ends of segregating chromosomes from being
severed by cytokinesis.
Late anaphase cell [+}
(Click image to enlarge)
type yeast cell in late anaphase. The cell expresses
the nucleolar marker Net1 fused to the cyan fluorescent protein
(CFP) and contain a tetO chromosome tag inserted in the telomere
of chromosome XII (visialised through tetR-Green Fluorescent
Protein, GFP). The nuclei is shown in red, the ribosomal DNA
in blue and the telomere tag in green.
In eukaryotes cohesion and condensation are largely dependent upon
two multiprotein complexes named cohesin and condensin respectively.
The core subunits of cohesin and condensin belong to the SMC protein
family of chromosomal ATPases.
Smc6-9 mutant cell [+]
(Click image to enlarge)
mutant cell in anaphase. The cell expresses the nucleolar
marker Net1 fused to the green fluorescent protein (GFP).
The nuclei are shown in red and the ribosomal DNA in green.
Note that during anaphase rDNA separates into two in wild
type cells but as it can be seen in the image rDNA separates
into three or more signals in the smc6-9 mutant. The reason
for this is that chromosome XII breaks at the rDNA region
during mitosis in this mutant.
SMC protein family
SMC proteins form complexes that have essential roles in chromosome
behaviour. The members of the family are long proteins with two globular
domains separated by two long coiled-coils and a central globular
hinge region. The signature domain is a C-terminal 'DA' box. Both
N- and C-terminal domains contain candidate Walker B motifs (ATP hydrolysis).
In the genome of S. cerevisiae there are three structurally
distinct SMC containing protein complexes: (i) cohesin, (ii) condensin,
and (iii) the “SMC5/6 complex”.
Eukaryotic sister chromatids are held together by proteins from the
time of their replication during S phase, until they are segregated
at anaphase. Cohesion is mediated by cohesin, which contains a core
of two SMC proteins, Smc1p and Smc3p and two further subunits, Scc3p
and Scc1p (Mcd1p/Rad21p). In budding yeast cohesin holds chromatids
together. Cohesion is resolved during the metaphase to anaphase transition
through the concerted cleavage of the Scc1p subunit by a cysteine
protease, named separase. This resolution allows the spindle-mediated
poleward movement of chromatids to the poles. The resolution of cohesion
is also linked to cell cycle control through the spindle checkpoint.
Eukaryotic chromosome condensation is a form of chromatin organisation
required to densely compact chromosomes during mitosis. Condensation
is mediated by the action of the condensin complex, which in S.
cerevisiae consists of Smc2p, Smc4p, Brn1p, Ycs4p and Ycs5p.
Condensin was originally identified in Xenopus laevis egg extracts,
where it is required for mitotic chromosome condensation. The present
view is that condensin accomplishes condensation in vivo by
reconfiguring chromatin, a feature consistent with the presence of
secondary structures resembling motor proteins in members of the SMC
Studies in yeast support a role for the condensin complex in chromosome
compaction in this organism. However, it has been also shown that
S. cerevisiae condensin plays additional roles in the
transmission of the rDNA locus. Recent experiments in a variety of
organisms suggests that condensin is likely to perform several unexpected
roles in widely diverse processes, including regulation of gene expression,
cell-cycle checkpoints and centromere organisation.
(iii) The 'SMC5/6
Eukaryotic genomes have a third conserved SMC-containing protein complex:
the 'SMC5/6'. The S. pombe, rad18 gene and its S. cerevisiae
homologue, SMC6 (RHC18), are essential for proliferation and sensitive
to ionising irradiation and UV. These studies suggest putative roles
in higher-order chromosome organisation and DNA damage. The requirement
of the SMC5/6 complex for viability is predicted to originate from
a possible role linking replication, repair and mitotic control. The
complex is likely to consist of multiple subunits including Smc5p,
Smc6p, Nse1p and Nse2p, Nse3p, Nse4p Nse5p and Nse6p.