35.17: 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.

A cell’s cytoplasm contains many different organelles. Each organelle differs in its structure, function, and copy number; therefore, a cell has to use distinct strategies to separate each organelle type properly into its daughter cells.

The nuclear envelope and endoplasmic reticulum are directly connected, and therefore their distribution to daughter cells is coupled.

In prophase, the nuclear envelope and associated proteins merge with the endoplasmic reticulum.  The sheet-like endoplasmic reticulum transforms into a tubular structure and reorganizes itself near the cell cortex during metaphase.

After chromosome segregation, the endoplasmic reticulum encircles sister chromatids and reforms the nuclear envelope.

In mitosis, mitochondria undergo fission to produce fragmented mitochondria that spread throughout the cell.

During cell division, these fragmented mitochondria are believed to be distributed to the daughter cells through a stochastic or random process and then undergo fusion to regain their shape and size.

A single copy of the Golgi body is present in the cell, and its equal distribution is vital for cell survival. The Golgi body disassembles stepwise before segregating into the daughter cells.

The Golgi ribbon containing stacked cisternae breaks down into multiple mini stacks, which are then further disassembled into vesicles.

In anaphase, the Golgi vesicles undergo fusion to form cisternae, which then get restacked. Then, small and large-sized twin Golgis form near the midbody and the centrosome, respectively, in telophase.

Finally, during cytokinesis, the smaller twin Golgi bodies merge with the larger ones to form the single Golgi body in each daughter cell.

Symmetrical division occurs in most cells where a cell produces two identical daughter cells with the same fate.

In some cells, asymmetrical division occurs where there is an unequal inheritance of cell fate determinants, resulting in the production of non-identical daughter cells.

For example, stem cells can undergo asymmetric cell division to produce one daughter cell that behaves like the parent stem cell, while the other daughter cell can follow a different cell lineage.