The metaphase checkpoint, also known as the spindle checkpoint, is a critical regulatory point in the cell cycle that ensures chromosomes are properly attached to the spindle microtubules and aligned at the metaphase plate before anaphase begins. This checkpoint prevents the separation of sister chromatids until all chromosomes are correctly bi-oriented, with kinetochores attached to spindle fibres from opposite poles. The significance of the metaphase checkpoint lies in its role in maintaining genetic stability. If a chromosome is not properly attached, it can lead to unequal distribution of genetic material, resulting in cells with the wrong number of chromosomes (aneuploidy). Aneuploidy can cause developmental disorders and is a common feature in many types of cancer.

The mitotic spindle is a structure composed of microtubules, which are dynamic filaments made of tubulin protein. Its formation begins in prophase, with the replication and migration of centrosomes to opposite poles of the cell. These centrosomes, also known as spindle poles, act as microtubule-organising centres. During spindle assembly, microtubules radiate out from these centrosomes, intermingling in the middle of the cell. The primary function of the mitotic spindle is to segregate chromosomes into two daughter cells during mitosis. It achieves this by attaching to chromosomes via the kinetochores and exerting forces that move chromosomes to align them at the metaphase plate, and later, to separate the sister chromatids during anaphase.

Kinetochores are complex protein structures that form on the centromere of each chromosome during cell division. They play a pivotal role in mitosis, particularly in the segregation of chromosomes. Kinetochores are the attachment sites for spindle fibres, the microtubules that pull chromosomes apart during anaphase. They are essential for the movement and alignment of chromosomes. During metaphase, kinetochores ensure that each chromosome is properly attached to spindle fibres from opposite poles, a condition known as bi-orientation. This is crucial for equal segregation of chromosomes, as incorrect attachment can lead to unequal distribution of genetic material, resulting in cells with abnormal numbers of chromosomes, a condition known as aneuploidy, which can have serious implications including developmental disorders and cancer.

Errors in mitosis can have profound consequences on the cell and the organism. One of the most significant outcomes is the production of cells with an abnormal number of chromosomes, a condition known as aneuploidy. This can occur due to non-disjunction, where sister chromatids fail to separate properly during anaphase. Aneuploidy can lead to various developmental disorders and diseases, including Down syndrome, which is caused by an extra copy of chromosome 21. Additionally, errors in mitosis can contribute to cancer development. Mutations that disrupt the normal regulation of the cell cycle can lead to uncontrolled cell division, producing a mass of abnormal cells known as a tumour. Moreover, mitotic errors can lead to genetic instability, increasing the likelihood of further mutations and accelerating tumour progression.

In both plant and animal cells, mitosis proceeds through the same stages: prophase, metaphase, anaphase, and telophase. However, there are key differences in the mechanisms, particularly during cytokinesis. In animal cells, cytokinesis occurs through the formation of a cleavage furrow. This furrow is created by a contractile ring of actin and myosin filaments that pinch the cell membrane inward, eventually dividing the cell into two. In contrast, plant cells, due to their rigid cell walls, cannot form a cleavage furrow. Instead, during telophase, a cell plate forms at the location of the former metaphase plate. This cell plate grows outward, aided by vesicles from the Golgi apparatus, until it reaches and fuses with the cell wall, thus dividing the cell into two.