7.17
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 sy…
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.
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Q1: How does the endoplasmic reticulum distribute to daughter cells during mitosis?
The nuclear envelope and endoplasmic reticulum are directly connected, so their distribution is coupled. During prophase, they merge together, and the sheet-like endoplasmic reticulum transforms into a tubular structure. It reorganizes near the cell cortex during metaphase, then encircles sister chromatids and reforms the nuclear envelope after chromosome segregation, ensuring both structures reach daughter cells.
Q2: What happens to mitochondria during cell division?
During mitosis, mitochondria undergo fission to produce fragmented mitochondria that spread throughout the cell. These fragments are distributed to daughter cells through a stochastic or random process. After cell division, the fragmented mitochondria undergo fusion to regain their original shape and size, ensuring each daughter cell receives functional mitochondria.
Q3: How does the Golgi body divide and distribute during cell division?
The single Golgi body disassembles stepwise before segregating. The Golgi ribbon containing stacked cisternae breaks into mini stacks, then further disassembles into vesicles. In anaphase, vesicles fuse to form cisternae that restack. Small and large twin Golgis form near the midbody and centrosome in telophase, then merge during cytokinesis to form one Golgi body in each daughter cell.
Q4: Why do cells need distinct strategies to distribute different organelles?
Each organelle differs in structure, function, and copy number, requiring tailored distribution strategies. Small organelles like peroxisomes and lysosomes distribute through simple diffusion since cells have multiple copies. Large organelles like the endoplasmic reticulum require complex evolutionarily conserved mechanisms to ensure proper segregation and that each daughter cell receives at least one functional copy for survival.
Q5: What is the difference between symmetrical and asymmetrical cell division?
Symmetrical division produces two identical daughter cells with the same fate, occurring in most cells. Asymmetrical division results in unequal inheritance of cell fate determinants, producing non-identical daughter cells. Stem cells exemplify asymmetric division, producing one daughter cell that behaves like the parent stem cell while the other follows a different cell lineage.
Q6: How is mitochondrial fission regulated during cell division?
The conserved protein DRP1 constricts the outer mitochondrial membrane to bring about mitochondrial fission. Two mitotic kinases, Cyclin B-Cdk1 and Aurora-A, regulate the activation of DRP1. This regulated fission process ensures mitochondria fragment properly during mitosis so they can be distributed evenly between daughter cells.
Q7: How is the endoplasmic reticulum organized and maintained during mitosis?
The interphase endoplasmic reticulum is a continuous membrane that synthesizes membranes for various organelles. During mitosis, it may undergo fragmentation or remain as cisternae. In HeLa cells, the endoplasmic reticulum is maintained as cisternae and restricted to the cell cortex. The central cisternae associate with spindle microtubules, and the endoplasmic reticulum may be tethered to mitotic cell cortex walls before distribution.