Mitosis, a fundamental cellular process in eukaryotic organisms, is essential for growth, tissue repair, and asexual reproduction. It ensures accurate replication of DNA and distribution of chromosomes into two daughter cells, each genetically identical to the parent cell. Understanding mitosis also underpins our knowledge of cell differentiation, which is crucial for the specialised functions cells eventually undertake.

The Cell Cycle

Before discussing the stages of mitosis, it's necessary to understand where mitosis fits into the broader context of the cell cycle, which comprises the period from one cell division to the next. The cell cycle consists of interphase (comprising G1, S, and G2 phases, where the cell grows and DNA is replicated) and M phase, in which mitosis and cytokinesis occur. The process of cell division is vital for the continuous renewal and growth of organisms.

Prophase

Prophase marks the beginning of mitosis. This phase is characterised by drastic changes within the cell to prepare for chromosome segregation.

• The cell's chromatin, a relaxed form of DNA, condenses into tightly coiled structures known as chromosomes. Each chromosome is comprised of two genetically identical strands, or sister chromatids, which are bound at the centromere.
• As the chromosomes condense, the nucleolus, a structure within the nucleus responsible for ribosome synthesis, dissipates, and the nuclear envelope begins to disintegrate.
• At the same time, the mitotic spindle starts to form. This structure, made up of microtubules, is essential for chromosome movement during mitosis. The microtubules originate from centrosomes, organelles that act as the main microtubule organising centres in animal cells. The spindle fibres push the centrosomes to opposite ends of the cell.

Prometaphase

Following prophase, the cell transitions into prometaphase, which is often considered part of prophase but is distinct due to several key events.

• The nuclear envelope finishes breaking down, allowing the spindle fibres to access the chromosomes.
• Each chromosome features a kinetochore, a protein structure located at the centromere. During prometaphase, spindle fibres attach to the kinetochores on each chromosome.

Metaphase

Metaphase follows prometaphase, marked by the alignment of chromosomes along the middle of the cell, known as the metaphase plate.

• This alignment is facilitated by the spindle fibres attached to the kinetochores of each chromosome. These fibres exert equal but opposite forces on the sister chromatids, aligning all the chromosomes at the cell's equator.

Anaphase

Anaphase is initiated when the sister chromatids of each chromosome separate and are pulled towards opposite ends of the cell.

• The separation of sister chromatids is triggered by the cleavage of cohesin, the protein complex holding the chromatids together. This process is closely linked to the replication of DNA, ensuring that each new cell receives an exact copy of the parent cell's genetic material.
• As the spindle fibres connected to the kinetochores shorten, the sister chromatids are pulled apart and moved towards the cell poles.
• By the end of anaphase, each pole of the cell has an equal and complete set of chromosomes.

Telophase

The final stage of mitosis is telophase, during which the cell prepares for division into two new cells.

• A new nuclear envelope, formed from fragments of the parent cell's nuclear membrane and components of the endoplasmic reticulum, assembles around each set of chromosomes.
• The chromosomes begin to uncoil and return to their relaxed chromatin state, allowing for gene transcription to resume. The importance of gel electrophoresis in analysing gene transcription underscores the complexity of genetic regulation during and after mitosis.
• The nucleolus reappears within each new nucleus, and the mitotic spindle disassembles.
• Telophase ends with the establishment of two distinct nuclei within the cell, each containing an identical set of genetic material. The role of mitosis in plant growth highlights its fundamental importance across all eukaryotic life.

Cytokinesis

Following telophase, the process of cytokinesis divides the cytoplasm to form two separate cells. Although not part of mitosis itself, cytokinesis is a crucial step in the cell cycle and often overlaps with the late stages of mitosis.

Significance of Mitosis in Maintaining Chromosome Number

The highly regulated process of mitosis is crucial for maintaining genetic continuity across generations of cells. By ensuring each daughter cell receives an exact copy of the parent cell's DNA, mitosis maintains the organism's chromosome number.

• In humans, for example, each somatic cell (i.e., non-sex cell) contains 46 chromosomes. Mitosis ensures that each new cell also receives 46 chromosomes, retaining the species' chromosomal number and genetic identity.
• The accurate segregation of chromosomes is also essential for cellular function. Each chromosome carries various genes necessary for cellular processes; thus, each daughter cell must have a complete set of chromosomes to function correctly.