The spindle apparatus is a crucial structure in mitosis, responsible for the accurate segregation of chromosomes into daughter cells. It is composed of microtubules, which are protein filaments that polymerize and depolymerize to form the spindle. The spindle originates from the centrosomes, which are microtubule-organizing centers located near the poles of the cell.

Structure of the Spindle: The spindle consists of several types of microtubules:

• Kinetochore microtubules: These microtubules attach to the kinetochores of the chromosomes.
• Polar microtubules: These microtubules overlap in the middle of the cell and help to maintain cell polarity.
• Astral microtubules: These microtubules radiate outwards from the centrosomes and attach to the cell cortex.

Chromosome Segregation: The spindle ensures accurate chromosome segregation through a complex interplay of forces. Kinetochore microtubules exert pulling forces on the chromosomes, moving them towards the poles. Polar microtubules push the poles apart, elongating the cell. Astral microtubules help to position the spindle correctly and maintain cell stability. The spindle checkpoint ensures that all chromosomes are correctly attached to the spindle before anaphase begins, preventing errors in chromosome segregation.

Differences between Animal and Plant Spindles:

During mitosis, the primary goal is to accurately divide the duplicated chromosomes into two identical sets, ensuring each daughter cell receives a complete genetic complement. The mitotic cell cycle is a continuous process, divided into distinct phases: prophase, metaphase, anaphase, and telophase.

Prophase: This is the initial phase where the duplicated chromosomes condense into visible, distinct structures. Each chromosome consists of two identical sister chromatids joined at the centromere. In animal cells, the nuclear envelope begins to break down into small vesicles. The mitotic spindle, composed of microtubules, begins to form from the centrosomes, which migrate towards opposite poles of the cell. In plant cells, the nuclear envelope doesn't break down; instead, the spindle forms within the nucleus.

Prometaphase: The nuclear envelope completely disappears. Spindle microtubules attach to the kinetochores, protein structures located at the centromere of each sister chromatid. Chromosomes begin to move towards the metaphase plate, an imaginary plane equidistant from the two poles of the cell.

Metaphase: The chromosomes align along the metaphase plate, with each sister chromatid attached to spindle microtubules originating from opposite poles. This alignment ensures that each daughter cell receives a complete set of chromosomes. The checkpoint mechanism ensures that all chromosomes are correctly attached before proceeding to anaphase.

Anaphase: The sister chromatids separate at the centromere and are pulled towards opposite poles of the cell by the shortening spindle microtubules. Each separated chromatid is now considered an individual chromosome. In animal cells, the spindle poles move further apart, elongating the cell. In plant cells, cytokinesis (the division of the cytoplasm) occurs later, resulting in a phragmoplast, a structure made of vesicles that guides the formation of a new cell wall between the daughter cells.

Telophase: The chromosomes arrive at the poles of the cell and begin to decondense. The nuclear envelope reforms around each set of chromosomes, creating two distinct nuclei. The spindle microtubules disappear. Cytokinesis, the division of the cytoplasm, completes the process, resulting in two separate daughter cells. In plant cells, a new cell wall is formed between the daughter cells.

Differences between plant and animal cells: The most significant difference lies in cytokinesis. Animal cells divide by cleavage, where the cell membrane pinches inward. Plant cells, however, must build a new cell wall between the daughter cells, using vesicles derived from the Golgi apparatus and guided by a phragmoplast.

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.