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18.18: Distribution of Cytoplasmic Content
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Molecular Biology

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Distribution of Cytoplasmic Content
 
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18.18: Distribution of Cytoplasmic Content

Cytokinesis segregates a cell’s chromosomes and organelles into its daughter cells. Organelles divide and grow prior to cell division but cannot be synthesized de novo; therefore, cells must receive at least one copy of each organelle to survive. Currently, many of the details of how the organelles are distributed are not yet fully elucidated.

Distribution of cytoplasmic determinants

The cytoplasm contains various organelles, as well as salts, proteins, and water. The distribution of small organelles like peroxisomes and lysosomes can be explained by simple diffusion. Since each cell has multiple copies of each organelle, daughter cells have a high probability of receiving at least one copy of the organelle. However, the distribution of large organelles like the ER or the chloroplast requires more complex evolutionarily conserved mechanisms.

Recent advances in the distribution of Golgi bodies

The Golgi apparatus breaks down as the cells enter mitosis. Two possible models may explain the distribution of the Golgi apparatus into daughter cells. According to the first model, during prophase, the Golgi tubules are cleaved, and small stacks of Golgi are released. These are further fragmented into vesicles and tubules. These mitotic Golgi clusters (MGC) disperse into the cytoplasm and segregate into the dividing cells during anaphase. Spindle microtubules guide the distribution of MGC in an ordered manner. The second model hypothesizes that the Golgi proteins and membranes are reabsorbed in the ER at the onset of mitosis, allowing the Golgi to partition with ER.

Recent advances in the distribution of mitochondria

Mitochondria are critical organelles for cell survival and growth. During mitosis, mitochondria are divided into smaller fragments which can then segregate between the daughter cells. A conserved protein DRP1 constricts the outer mitochondrial membrane and brings about mitochondrial fission. Cyclin B-CDK1 and Aurora-A, two mitotic kinases, regulate the activation of DRP1.

Recent advances in the distribution of endoplasmic reticulum

The interphase endoplasmic reticulum (ER) is a continuous membrane that synthesizes the membranes for Golgi bodies, endolysosomes, vesicles, and the plasma membrane. During mitosis, the ER may undergo fragmentation or may remain as cisternae. In HeLa cells, the ER is maintained as cisternae and is restricted to the cell cortex. The ER is non-uniformly distributed, and the central cisternae are associated with the spindle. The ER may be tethered to the walls of the mitotic cell cortex before being distributed to the daughter cells.


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