Centrosome/Centrioles

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Centrosome

Microtubule organizing center; Consists of centriole pair, PCM & gamma-tubulin complexes; Permanent organelle w/ the property of self-replication; Eliminates lag phase; nucleate MTs @ (-) end; Structural platform to organize signaling cascades; Rapid changes in functional & structural attributes; Sperm provides centrosome to egg during fertilization.

Theodor Boveri

First described and named the centrosome and centriole; Using Ascaris he showed that the centrosome is introduced into the egg at fertilization by the sperm;

Centrosome and Fertilization

Centrioles are eliminated during oogenesis (inmost animals), while the cytoplasm remains enriched w/ centrosome precursor proteins. During spermatogenesis the sperm centrosome is reduced to its core components- a pair of centrioles- & at fertilization the sperm delivers this centrosome and a haploid nucleus to the egg. In the zygote, the sperm centrioles undergo maturation by recruiting the egg proteins to form the PCM (pericentriole material) of the zygote centrosome.

Centrioles & Basal Bodies

Centrioles become the basal bodies of cilia & flagella in motile cells; Meves saw in the moth that flagella grow out from the centrioles while they are situated @ the poles of the first mitotic spindle; Henneguy & Lenhossek proposed that the basal bodies of cilia might also be identical to centrioles.

Basal Bodies

organelle formed from the centriole & a short cylindrical array of microtubules; Found at the base of cilia/flagella and serves as the nucleation site for the growth of the axoneme MTs

Centrioles

Arranged orthogonal to one another; Organize centrosome matrix; nucleating center; serve as basal bodies; Anchor PCM; Duplicate once each cell cycle; organize mitotic spindle poles; Contain mother & daughter

Pericentriole Matrix

PCM; lattice of coiled-coil proteins; Structural context to organize signaling cascades; nucleates MTs; extends outward from centrioles in an irregular fashion

gamma-tubulin complexes

Nucleate & anchor cytoplasmic & mitotic MTs

Centriole Replication

In G1, the 2 centrioles separate (disenage); Durinf S phase, a daughter centriole begins to grow near the base (bud) of each mother centriole; By G2 the elongation of the daughter centriole is completed (pro-centriole); At the beginning of M phase the complex splits & the two halves begin to separate; Each centrosome now nucleates its own MTs.

Mother Centriole

Unique ultrastructural modifications; contain molecules not found in daughter centriole, such as Odf2 in transition zone; guides daughter cell to proper subcellular location; Contains distal end w/ appendages (transition zone, 9 MT doublets & membrane attachment), Central Barrel (9 triplet MTs) & Proximal end (cartwheel & PCM)

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