Biology Reference
In-Depth Information
Fig. 17.1 Centrosome
structure. Centrosomes are
composed of two centrioles.
Each centriole is cylindrical
in structure and is made up by
nine microtubule triplets. The
older centriole, termed
mother centriole, carries
distal (purple) as well as
subdistal (cyan) appendages
where microtubule (MT)
anchorage and nucleation
takes place. Both centrioles
of a centrosome are
embedded in an amorphous
protein mass, termed
pericentriolar material (PCM;
blue) and are tethered
together in most cell cycle
phases (Adopted from
Anderhub et al.
2012
)
et al.
1994
; Stearns et al.
1991)
. One centriole of each centrosome, termed the
mother centriole, features distal as well as subdistal appendages which are
essential for MT anchorage and nucleation (Bornens
2002
). In addition, cilia
formation is initiated by the mother centriole and various diseases are directly
linked to defects in ciliogenesis (Nigg and Raff
2009
).
Cells in the G
1
-phase of the cell cycle harbor a single centrosome consisting of two
loosely connected centrioles. New centrioles are formed at the proximal part of each
of the two pre-existing centrioles upon transition from G
1
- to S-phase. These newly
formed centrioles elongate and mature during S- until late G
2
-phase, giving rise to
two pairs of centrioles per cell before these enter mitosis. In early mitosis, the two
centrosomes are separated in order to build up a bipolar mitotic spindle array, thereby
ensuring proper segregation of duplicated chromosomes. In late mitosis or early
G
1
-phase the tight connection between the two centrioles of each centrosome is
loosened, a process termed disengagement (Kuriyama and Borisy
1981
). Centro-
some duplication, like DNA replication, is regulated precisely to ensure that
centrosomes are duplicated only once per cell cycle. Recently, it has been shown that
the formation of new procentrioles is blocked intrinsically by the engagement of
centrioles. Interestingly, the activity of separase, a protein known to be important for
separation of duplicated chromatids during mitosis, is also required for centriole
disengagement (Tsou and Stearns
2006
). More precisely, a recent study revealed that
