Mitosis (AQA A-Level Biology): Revision Notes

Mitosis

The cell cycle

Mitosis is a type of cell division that produces two genetically identical daughter cells from a single parent cell. This process is essential for growth in multicellular organisms, starting from a single fertilised cell that divides repeatedly to form all the cells in the adult organism.

Cells undergo a cell cycle that includes both growth phases and the division process itself. The cell cycle consists of:

• Interphase - the longest phase where cells carry out their normal activities including producing cytoplasm and organelles, respiration, and protein synthesis
• Mitosis - the actual division phase

Interphase is further divided into three stages:

• G₁ phase - initial growth phase
• S phase - DNA replication occurs, creating identical copies of each chromosome
• G₂ phase - further growth phase

Chromosome structure

After DNA replication in the S phase, each chromosome contains two identical DNA molecules called chromatids. These chromatids remain joined together at a region called the centromere. This structure is vital for ensuring that genetic material is distributed equally between daughter cells during division.

The stages of mitosis

Mitosis consists of four main stages, followed by cytokinesis:

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Prophase

During prophase, several important changes occur:

• Chromosomes condense and become visible under a microscope
• The centrioles duplicate and begin moving towards opposite poles of the cell
• The spindle apparatus starts to form
• Centriole pairs separate and move to opposite ends of the cell

Metaphase

The key events of metaphase include:

• The nuclear envelope disappears completely
• Centriole pairs reach the opposite poles of the cell
• The spindle is now fully formed
• Chromosomes continue to condense and become more compact
• Microtubules from the spindle attach to the centromeres of chromosomes

Anaphase

Anaphase is characterised by:

• The connections between sister chromatids break
• Individual chromatids (now separate chromosomes) move apart
• Microtubules pull the chromatids towards opposite poles of the cell
• The centromeres of sister chromatids separate, pulled by spindle microtubules

Telophase

During telophase:

• Individual chromatids (now effectively separate chromosomes) reach the opposite poles of the cell
• Chromosomes decondense and return to their extended form
• Nuclear envelopes reform around each group of chromosomes at opposite poles
• The spindle apparatus disappears

Cytokinesis

Cytokinesis is the final stage where:

• The cell divides into two separate daughter cells
• In animal cells, this occurs by infolding of the plasma membrane
• In plant cells, a new cell wall and plasma membrane form between the two nuclei
• Each daughter cell receives approximately half the organelles from the parent cell

Cell division and differentiation

As multicellular organisms develop, many cells become differentiated - they specialise for specific functions. When cells differentiate, they typically lose their ability to divide by mitosis. Most adult human cells have lost the capacity for division, though some cells (such as skin cells) can still divide a limited number of times before they die.

Mitosis is a tightly controlled process. When the control mechanisms that regulate cell division break down, cells may begin dividing repeatedly in an uncontrolled manner.