Mitosis and Cytokinesis

Mitosis is the process that segregates duplicated chromosomes equally into two nuclei, and cytokinesis divides the cytoplasm to complete the formation of two daughter cells.

Definition

Mitosis is the division of a duplicated nucleus into two genetically identical nuclei; cytokinesis is the partitioning of the cytoplasm that yields two separate daughter cells.

Scope

This topic covers the stages of mitosis from prophase through telophase, the assembly and dynamics of the mitotic spindle, the attachment and movement of chromosomes, the spindle assembly checkpoint, and the mechanics of cytokinesis in animal and plant cells.

Core questions

• What happens in each stage of mitosis?
• How is the mitotic spindle built and how does it capture chromosomes?
• How does the spindle assembly checkpoint prevent missegregation?
• How is the cytoplasm divided during cytokinesis?

Key theories

Dynamic instability of microtubules

Spindle microtubules switch stochastically between growth and rapid shrinkage, a dynamic behavior that lets the spindle search for and capture chromosomes and generate forces for their movement.

Mechanisms

After chromosome condensation in prophase, the nuclear envelope breaks down and the bipolar spindle forms from dynamically unstable microtubules nucleated at the centrosomes. Kinetochores on each chromosome capture microtubules so sister chromatids attach to opposite poles, and the chromosomes align at the metaphase plate. The spindle assembly checkpoint blocks anaphase until attachment is complete; then cohesion is cleaved and sisters move to opposite poles. Cytokinesis follows, with a contractile actomyosin ring pinching animal cells and a cell plate forming in plant cells.

Clinical relevance

Mitosis and cytokinesis ensure faithful inheritance of chromosomes and underlie growth and tissue renewal, making them central to cell biology. The treatment here is descriptive and non-prescriptive.

History

Flemming's nineteenth-century observations named and described mitosis; the discovery of microtubule dynamic instability by Mitchison and Kirschner in 1984 explained how the spindle assembles and captures chromosomes, grounding the mechanics of division.

Key figures

• Walther Flemming
• Tim Mitchison
• Marc Kirschner

Seminal works

• mitchison1984
• alberts2014

Frequently asked questions

What is the mitotic spindle?

It is a bipolar array of microtubules that attaches to chromosomes and pulls the duplicated copies apart toward opposite ends of the cell.

How does cytokinesis differ between animal and plant cells?

Animal cells pinch in two using a contractile ring of actin and myosin, while plant cells, which have rigid walls, build a new cell plate that grows outward to separate the daughters.