Mitosis: The Process of Eukaryotic Cell Division

In the complex world of eukaryotic cells, growth and repair depend on a precise mechanism of duplication known as mitosis. Mitosis is a specialized part of the cell cycle where replicated chromosomes are separated into two new nuclei. This process is an equational division, meaning it produces two genetically identical daughter cells that maintain the same total number of chromosomes as the original mother cell.

To ensure genetic stability across generations, mitosis is carefully timed. It is preceded by the S phase of interphase, during which DNA replication occurs. Following the nuclear division, the cell undergoes telophase and cytokinesis to divide the cytoplasm, organelles, and cell membrane. Together, these stages define the mitotic phase (M phase).

: Mitosis in the animal cell cycle (phases ordered counter-clockwise).

Key Facts

• Genetic Identity: Mitosis results in two daughter cells that are genetically identical to the parent cell.
• Chromosome Maintenance: The total number of chromosomes is preserved throughout the division.
• Prerequisite: DNA must be replicated during the S phase of interphase before mitosis can begin.
• Outcome: The process concludes with cytokinesis, splitting one cell into two.

The Stages of Mitosis

Mitosis is divided into distinct stages based on the cellular activities occurring at each step. While the general process is consistent, some variations exist, such as the preprophase stage found specifically in plant cells.

Prophase and Prometaphase

During prophase, the duplicated chromosomes condense, making them visible under a microscope. They attach to spindle fibers, which are protein structures that will eventually pull the genetic material apart. This is followed by prometaphase, where the nuclear envelope breaks down, allowing the spindle fibers to access the chromosomes.

: Interphase nucleus (left), condensing chromosomes (middle) and condensed chromosomes (right)

: Prophase during mitosis

Metaphase

In metaphase, the spindle fibers align the chromosomes along the center of the cell, known as the metaphase plate. This alignment ensures that when the chromosomes separate, each new nucleus will receive exactly one copy of each chromosome.

: Metaphase during mitosis

: A cell in late metaphase. All chromosomes (blue) but one have arrived at the metaphase plate.

Anaphase

During anaphase, the sister chromatids (the two identical halves of a duplicated chromosome) are pulled apart toward opposite poles of the cell. This movement is driven by the shortening of the spindle fibers.

: Anaphase during mitosis

Telophase and Cytokinesis

Telophase is the final stage of nuclear division, where two new nuclei form around the separated sets of chromosomes. However, the cell is not yet fully divided. Cytokinesis follows, dividing the cytoplasm and cell membrane to create two independent daughter cells.

: Telophase during mitosis

: Cytokinesis illustration

: Ciliate undergoing cytokinesis, with the cleavage furrow being clearly visible

Cellular Dynamics and Variations

Mitosis involves significant physical changes to the cell. In many animal cells, mitotic cell rounding occurs; the cell changes shape to facilitate the assembly of the spindle. This process involves the actomyosin cortex and is essential for efficient division.

: Cell shape changes through mitosis for a typical animal cell cultured on a flat surface. The cell undergoes mitotic cell rounding during spindle assembly and then divides via cytokinesis. The actomyosin cortex is depicted in red, DNA/chromosomes purple, microtubules green, and membrane and retraction fibers in black. Rounding also occurs in live tissue, as described in the text.

Forms and Errors

Depending on the organism, mitosis can take different forms, such as open orthomitosis or closed intranuclear pleuromitosis. When mitosis fails, errors can occur. For example, tripolar mitosis is an abnormal division that can be seen in precancerous lesions, such as those in the stomach or breast cancer tissues.

: An abnormal (tripolar) mitosis (12 o'clock position) in a precancerous lesion of the stomach (H&E stain)

: Mitosis appearances in breast cancer

Summary of Mitotic Process

Stage	Primary Action	Key Outcome
Prophase	Chromosome condensation	Visible chromosomes and spindle formation
Metaphase	Alignment at the center	Chromosomes positioned at the metaphase plate
Anaphase	Separation of chromatids	Identical genetic sets move to opposite poles
Telophase	Nuclear envelope reformation	Two distinct daughter nuclei
Cytokinesis	Cytoplasmic division	Two separate, genetically identical cells

: Mitosis and meiosis differences

: Some types of cell division in prokaryotes and eukaryotes

Frequently Asked Questions

How does mitosis differ from meiosis?

Mitosis produces two genetically identical daughter cells for growth and repair, whereas meiosis is used to produce gametes (sex cells) with half the original number of chromosomes.

What happens during the S phase before mitosis?

The S phase is the period of interphase where DNA replication occurs, ensuring that there are two complete sets of chromosomes available to be divided during mitosis.

What is the role of the spindle fibers?

Spindle fibers attach to the chromosomes and physically pull the sister chromatids apart toward opposite ends of the cell during anaphase.

What is cytokinesis?

Cytokinesis is the physical process of dividing the cell's cytoplasm, organelles, and membrane, which officially separates the mother cell into two distinct daughter cells.

Can mitosis go wrong?

Yes. Errors such as tripolar mitosis can occur, where chromosomes are pulled in three directions instead of two. Such abnormalities are often associated with precancerous lesions and cancer.