How Does the Cell Cycle Work? Mitosis Explained
Learn how the cell cycle works in AQA A Level Biology: DNA replication in interphase, the four stages of mitosis, cytokinesis and a common exam mistake.
The cell cycle is the sequence in which a cell grows, copies its DNA and divides. Mitosis is the part that separates the copied chromosomes into two nuclei. Cytokinesis then divides the cytoplasm, producing two genetically identical daughter cells. This process is important for growth and replacing damaged cells.
Interphase: preparation
The cell spends much of the cycle in interphase. It grows, makes proteins and organelles, and replicates its DNA. After replication, each chromosome consists of two identical sister chromatids joined at a centromere. A chromatid is one of the two copies of a replicated chromosome.
Interphase is preparation, not a stage of mitosis. DNA replication supplies two copies of each chromosome, ready for distribution to the new cells. At this point, chromosome number is counted by centromeres, not by chromatids.
Mitosis: separating the chromosomes
- Prophase: Chromosomes condense so they become visible. The nuclear envelope breaks down and spindle fibres form.
- Metaphase: Chromosomes line up at the cell’s equator. Spindle fibres attach to their centromeres.
- Anaphase: The centromeres divide and spindle fibres pull sister chromatids to opposite poles. Each separated chromatid is now a chromosome.
- Telophase: Nuclear envelopes form around the chromosomes at each pole. The chromosomes uncoil.
Spindle fibres move the chromosomes. Their attachment at the centromeres helps separate sister chromatids into different nuclei, so each daughter cell receives a complete set of genetic information.
Cytokinesis follows: the cytoplasm divides to form two separate cells. Each daughter cell has the same chromosome number as the original cell.
For example, a human body cell starts with 464646 chromosomes. Replication gives each chromosome two sister chromatids; it does not give the cell 929292 chromosomes at this stage. Mitosis separates those chromatids, so each daughter cell receives 464646 chromosomes.
Exam mistake to avoid: DNA replication happens during interphase, before mitosis, not during prophase. Mitosis separates the copies; it does not make them.
