Biology – Chromosome behaviour in mitosis | e-Consult

The nuclear envelope, cell surface membrane, and spindle apparatus work in concert to orchestrate the complex events of mitosis, ensuring accurate chromosome segregation and the formation of two genetically identical daughter cells. Their behaviour changes dynamically throughout the different phases of the mitotic cell cycle, with coordinated interactions essential for proper progression.

Nuclear Envelope: The nuclear envelope plays a crucial role in separating the chromosomes and protecting the genetic material. In prophase, the nuclear envelope breaks down, allowing the spindle microtubules to access the chromosomes. It reforms in telophase, creating two new nuclei. The breakdown and reformation are tightly regulated by phosphorylation events.

Cell Surface Membrane: The cell surface membrane is involved in cytokinesis, the physical division of the cytoplasm. In animal cells, cytokinesis occurs through cleavage furrow formation, where the membrane pinches inward. In plant cells, a phragmoplast, a structure derived from Golgi vesicles, guides the formation of a new cell wall between the daughter cells. The cell surface membrane is also involved in signaling pathways that regulate the cell cycle, including checkpoints.

Spindle Apparatus: As described previously, the spindle apparatus is responsible for chromosome segregation. Its behaviour changes throughout mitosis: it forms in prophase, attaches to the chromosomes in prometaphase, aligns the chromosomes at the metaphase plate in metaphase, pulls the sister chromatids apart in anaphase, and disassembles in telophase. The spindle's function is tightly regulated by the cell cycle checkpoints.

Checkpoints: Cell cycle checkpoints are crucial control mechanisms that ensure the fidelity of mitosis. These checkpoints monitor the progress of the cell cycle and halt progression if errors are detected.

• G1 Checkpoint: Ensures the cell has sufficient resources and DNA integrity before entering S phase.
• G2 Checkpoint: Ensures DNA replication is complete and DNA damage is repaired before entering mitosis.
• Spindle Assembly Checkpoint (SAC): Ensures that all chromosomes are correctly attached to the spindle microtubules before anaphase begins. This checkpoint prevents anaphase from starting if chromosomes are not properly attached, preventing aneuploidy (an abnormal number of chromosomes).

The coordinated behaviour of the nuclear envelope, cell surface membrane, and spindle apparatus, regulated by checkpoints, is essential for accurate and timely progression through the mitotic cell cycle, ensuring the faithful transmission of genetic information to daughter cells.