Cell Division — Mitosis

Chromosomes

Chromosomes are found in the nucleus of every cell. Before division, each chromosome is copied, producing two identical sister chromatids joined together at a point called the centromere. Once the chromatids separate during division, each becomes its own chromosome.

The Cell Cycle

The cell cycle is the continuous process a cell goes through between one division and the next. It has two main parts:

• Interphase — the "resting" phase (though the cell is very active): the cell grows and copies its DNA, ready for division.
• Mitosis — the nucleus divides, producing two identical nuclei, followed by cytokinesis — the division of the cytoplasm to form two separate daughter cells.

The phases of mitosis

Mitosis produces two identical daughter cells, each with the same number of chromosomes as the original parent cell.

Cytokinesis — plant vs animal

After telophase, the cytoplasm itself divides — but plant and animal cells do this differently:

• Animal cells — the cell membrane pinches inward from the outside, forming a "cleavage furrow" that eventually splits the cell in two.
• Plant cells — because they have a rigid cell wall, they cannot pinch inward. Instead, a new cell plate forms across the middle of the cell and develops into a new cell wall, dividing it into two.

Why mitosis matters

• Growth — an organism grows by producing more cells through mitosis.
• Repair — damaged or worn-out cells are replaced.
• Reproduction — some simple organisms reproduce asexually using mitosis.

When mitosis goes wrong: cancer

Cancer is uncontrolled cell division and growth. Normally, the cell cycle is carefully regulated — but if this control breaks down, cells divide uncontrollably, forming a tumour. Treatments include surgery, radiotherapy, and chemotherapy, which aim to destroy the rapidly-dividing cancer cells.