PRESENTED BY- SWATI BAYYANA

M.Sc( I SEM) SCHOOL OF LIFE SCIENCES

CENTROSOMES AND CENTRIOLES

The centrosome, also called the "microtubule organizing center“ MTOC , is an area in the cell where microtubles are produced.

They are located in the cytoplasm near the nucleus and are responsible for cell division, cytokinesis, formation of cytoskeleton ,cell signalling pathways.

Present only in animal cells not in plant cells.

Within an animal cell centrosome there is a pair of small organelles, the centrioles, each made up of a ring of nine groups of microtubules.

Edward van Benden and Theoder Boveri first observed and identify centrioles in 1883 and 1888.

Centrioles are sub-cellular structure typically consisting of nine triplet microtubules arranged in a hollow cylinder in a 9+3 pattern.

The pattern is so named because a ring of nine microtubule "triplets" are arranged at right angles to one another.

They are found in animal eukaryotic cells and absent in higher plants and fungi.

They play an important part in cell division, cell organization, and formation of cilia and flagella.

STRUCTURE A centriole is made up of nine (9) sets of triplet

microtubules.

A triplet contains one complete microtubule fused to two incomplete microtubules.

The two centrioles are arranged such that one is perpendicular to the other.

The centriole is a short cylinder-like structure or a hollow cylinder.

Centrioles organise a cloud of protein material around themselves,this dense material is called as Pericentrioler matrix (PCM), both constitute the all important centrosome.

CARTWHEEL STRUCTURE

CENTROSOME DUPLICATION Centrosome duplication is coupled with centriole

replication.

Centrosome duplication is first visible during early S- phase, when a new procentriole assembles next to the proximal end of each parental centriole.

As the cell progresses through S and G phases, daughter centrioles elongate until they reach the length of their parents.

At the G -M phase transition the two centrosomes, each containing a parent and a daughter centriole,separate at prophase to form mitotic poles.

Centriole Biogenesis Centriole duplication is triggered at a defined site on a

parent centriole, which acts as a scaffold to concentrate key regulatory molecules required for centriole biogenesis(Self assembly process).

The core machinery for centriole biogenesis rely on a surprisingly small number of proteins, prominently they are PLK4(responsible for formation of procentriole adjacent to mother centriole) and SAS6(assembly of microtubules in cartwheel structure)

PLK4 is the key regulatory for centriole duplication and to maintain the number of centrioles in the cell.

Centrosome Duplication Centriole Replication

FUNCTIONS…. In Cell Division- Centrioles are involved in the

organization of the mitotic spindle and in the completion of cytokinesis

Cellular Organization- Centrioles are a very important part of centrosomes, which are involved in organizing

microtubules in the cytoplasm . The position of the centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of cell organelles.

Continued…. Formation of Cilia and Flagella – In organisms

with flagella and cilia, the position of these organelles is determined by the mother centriole, which becomes the basal body.

Vincristine, a drug found in the Madagascar periwinkle (a wildflower), binds to tubulin dimers preventing the assembly of microtubules. This halts cells inmetaphase of mitosis.

Taxol®, a drug found in the bark of the Pacific yew, prevents depolymerization of the microtubules of the spindle fiber. This, in turn, stops chromosome movement, and thus prevents the completion of mitosis.

Both Vincristine and Taxol are used as anticancer drugs.

CONCLUSION

Centrioles occurs as paired organelles along with PCM in the centrosome.

Centrioles are constructed of microtubules.

They are involved in microtubule assembly and nucleation of cilia and flagella.

Recent studies…

1) It shows that role of centrioles in mitotic spindle assembly during cell division is unclear.

2) Centrioles numerical aberrations is linked to tumour formation.

REFERENCES

Cooper & Hausman; The Cell : A Molecular Approach

Bruce Alberts ; Molecular Biology of the Cell

Uzbekov R. and Alieva I. Centrosome book 2013

Tracing the origins of centrioles, cilia, and flagella; Zita

Carvalho-Santos1, Juliette Azimzadeh2, José. B. Pereira-

Leal1, and Mónica Bettencourt-Dias; JCB Home > 2011

Archive > 25 July > Carvalho-Santos et al. 194 (2): 165

web.stanford.edu_group_stearnslab_papers_Nigg_Stearns

_NCB

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