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Centrosome and Centriole

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Structure of centrosome

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• The centrosome generally lies next to the nucleus, near the center of animal cell
• In interphase, it is typically located to one side of the nucleus, close to the outer surface of the
nuclear envelope
• Contains centriole pair and pericentriolar matrix

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• Centriole
• A pair of cylindrical structures arranged at right angles (orthogonally) to each other in an Lshaped configuration embedded in pericentriolar matrix (See later)

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• Pericentriolar material or Pericentriolar matrix or PCM or Centrosome matrix or
Centrosphere or Kinoplasm
• Surrounding each centriole pair, in both interphase and mitosis, is a region of the
cytoplasm made of a network of small fibers
• Highly ordered and granular mass of dense material
• Serves as an organizing center for microtubules (MTs)
• It nucleates MT polymerization
• It contains a number of centrosome-specific proteins, including a special minor
form of tubulin, called -tubulin
• -tubulin interacts with the normal α/β tubulin dimer to help nucleate MTs
• Contains more than 50 copies of -TuRC (-tubulin ring complex) that serves as
template that nucleate a MT with 13 protofilaments
• -TuRC are MT nucleation sites
• During interphase the centrosome matrix nucleates a cytoplasmic array of MTs,
which project outward toward the cell perimeter with their minus ends attached to
the centrosome

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Centrosome is the major MTOC of animal cell

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Centrosomes are the primary site of nucleation of MTs in animal cells
• Centrosome is the major microtubule-organizing center (MTOC) in animal
cells
• During interphase the centrosome matrix nucleates a cytoplasmic array of
MTs, which project outward toward the cell perimeter with their minus ends
attached to the centrosome
• Starlike network (aster) of MTs emanates from the centrosome and then
elongate toward the cell periphery until the original MT distribution is
reestablished
• At any one time, several hundred MTs are growing outward from a
centrosome
• Cytoplasmic array of MTs emanating from the centrosome can act as a
surveying device that is able to find the center of the cell
• Centrosomes protect the minus ends of MTs and continually nucleate the
formation of new MTs, which grow out in random directions
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Starlike network of
MTs emanating
from centrosome

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Molecular mechanism of MT nucleation is likely to be highly conserved
In plants
• Plant cells lack both centrosomes and centrioles, or any other type of obvious MTOC
• Instead, MTs in a plant cell are nucleated around the surface of nucleus and widely throughout the
cortex (just beneath plasma membrane)
In fungi and diatoms
• In fungi and diatoms the MTOC is a plaque called the spindle pole body, which is embedded in
the nuclear envelope
• Contains a matrix that nucleates MT polymerization
• Contain -tubulin and other centrosome-specific proteins

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Centrioles or Diplosomes

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Centrioles
• Paired organelles, hence also called diplosome
• Typically located near the nucleus in the centrosome
• Embedded in the centrosome is a pair of cylindrical structures arranged at right angles to each other in
an L-shaped configuration
• Two centrioles of a pair lie at right angles to each other in pericentriolar matrix of centrosome
• Orthogonally arranged
• Cylindrical hollow structures
• ~0
...
4 μ long
• Found only in animal cells
• Not all MTOCs contain centrioles
• Arise by the duplication of pre-existing centrioles
• Centriole doubling begins at around the time that DNA synthesis begins
• First the two members of a pair separate, and then a daughter centriole is formed perpendicular to
each original centriole

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• Consists of a structurally mature (older) mother centriole and an
immature (new) daughter centriole
• Both are orthogonally oriented
• Both are linked by interconnecting fibers (comprising of cohesin)
• Closer ends of two centrioles are proximal ends and distant ends are
distal ends
• Proximal end has minus end of MT
• Distal end has plus end of MT
• Mother centriole is attached to its daughter at the proximal end
• Mother centriole has radiating appendages at the distal end of its long
axis
• Distal appendages
• Subdistal appendages

• Mother centriole functions as a MTOC for the cell
• It can later transform into basal body giving rise to the cilium
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Structure of centriole
• Made up of 9 bundles of MTs (three MTs per bundle) arranged in a ring
• 9 sets of three MTs each
• Wall of each centriole is made of 9 interconnected triplet MTs, arranged
as a pinwheel or cartwheel
• Interior of each centriole appears empty, except for a cartwheel structure
which is formed as a result of 9 protein spokes that radiate from the central
core (hub) region towards each triplet
• There is no membrane outside centrioles

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•
•
•
•

Nine sets of three MTs, fused into triplets, form the wall of the centriole, alike basal body
Triplets named A, B, C (A present towards inner side, B in middle, C towards outer side)
Only MT ‘A’ is round and complete with 13 protofilaments
MTs B and C share protofilaments
• MT B shares 3 subunits with A
• MT C shares 3 subunits with B
• Each triplet is tilted inward like the blades of a turbine
• Triplets are tilted in such a way that each forms an angle of ~40 to the radius of the cylinder
• Adjacent triplets are linked at intervals along their length (A-C linker)
• MT A is linked to MT C of adjacent triplet by protein linkers
• Protein spokes radiate out to each triplet from a central core (hub), forming a pattern like a cartwheel
or pinwheel

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Centrosome cycle

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Centrosome cycle
• The process of centrosome duplication and separation is known as the centrosome cycle
• Centrosome duplicates and splits into two equal parts during interphase
• Each half contains a duplicated centriole pair
• Centrosome duplication occurs once per cell cycle

• Centrosome duplication begins during the S and G2 phases of the cell cycle
• Centrioles and other components of the centrosome are duplicated but remain together as a single
complex on one side of the nucleus until the beginning of M phase
• Duplicated centrosomes separate and move to opposite sides of the nucleus at the onset of M phase
to form the two poles of the mitotic spindle

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Duplication of centrioles
• Centrioles replicate autonomously like mitochondria and peroxisomes
• Before DNA replication, cells contain two centrioles, an older mother centriole, and a younger
daughter centriole
• Centrioles start duplicating when DNA replicates
• During cell division, new centrioles grow at the proximal ends of both mother and daughter
centrioles

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During interphase
• G1 phase - Two centrioles separate by few micrometres
• S phase - Daughter centrioles begin to grow near the base (proximal end) of each old centriole and at
right angle to it (formation of procentriole) and elongate
• G2 phase - Elongation or maturation of daughter centrioles is completed; Centriole pairs separate
During mitosis
• As mitosis begins, this complex splits in two
• These two daughter centrosomes move to opposite sides of the nucleus when mitosis begins
• Each centriole pair becomes part of a separate MTOC that nucleates a radial array of MTs called an
aster
• They form the two poles of the mitotic spindle
• As mitosis ends and the nuclear envelope re-forms around the separated chromosomes, each
daughter cell receives a centrosome in association with its chromosomes

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• Centriole duplication begin from centers which contain proteins, like tubulin, needed for their
formation
• This leads to the formation of procentrioles
• Procentriole is a centriole or basal body in a stage of early development
• Each procentriole grows out a single MT from which the triplet can form
• Once a centriole is made, daughter centrioles can grow out from the tubules at right angles
• After duplication, the two centriole pairs (the freshly assembled centriole is now a daughter centriole
in each pair) will remain attached to each other orthogonally until mitosis
• The two centriole pairs in the centrosome are tied to one another
• At that point centriole pairs separate dependently on an enzyme called separase
• Separase helps in cleavage of cohesin (a protein in interconnecting fibers)
• Each daughter cell formed after cell division will inherit one of these pairs
• These then add to the daughter cell (in a dividing cell) or they move to the periphery and form
the basal body for the cilium

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Centrosome cycle

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Functions of centrosome and centrioles

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• Centrosome functions as MTOC in which minus ends of MTs are anchored
• Stabilizes and protects minus ends of MTs
• Plays a key role in determining intracellular organization of MTs
• System of MTs radiating from the centrosome acts as a device to survey the outlying regions of cell
and position the centrosome at its center (in animal cells)
• This ability of MTs to find the center of cell establishes a general coordinate system, which is
then used to position many organelles within the cell
• Role in formation of spindle during cell division
• Help organize spindle fibers and astral rays during mitosis and meiosis in animal cells
• Cytoplasmic MTs emanate from centrosome in a star-like ‘astral’ conformation
• Centrosomes are associated with the nuclear membrane during prophase
• As the nuclear membrane breaks down and the centrosome nucleated MTs then interact with the
chromosomes to build mitotic spindle
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Functions of centrioles
• Mother centriole functions as MTOC
• Cell division
• MTs with the help of centrioles become organized into a spindle shaped formation that span the
cell
• These spindle fibers act as guides for the alignment of chromosomes as they separate later
during the process of cell division
• Formation of basal body, cilia and flagella
• In cells that feature cilia or flagella, the basal bodies, which exhibit the same structural form as
centrioles, are present
• These assemblies are located, however, near the cell surface at the base of each cilium or
flagellum, rather than in the centrosome near the nucleus
• Basal bodies are anchored in their cytoplasmic locations by a rootlet system in the cell
• In some organisms, such as the unicellular Chlamydomonas, basal bodies change their location
and are functionally converted to centrioles before the mitotic process
• Mother centriole functions as a MTOC for the cell and later transforms into basal body giving rise
to the cilium
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• Centriole transforms to basal body
• Transformation includes
• Daughter centriole first matures to a mother centriole
• Produces basal body accessory structures
• Migrates to plasma membrane
• Dock to the plasma membrane

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