The start of mitosis
Mitosis is usually described as four successive stages: prophase, metaphase, anaphase and telophase. By the start of prophase, the DNA has already been replicated during interphase.
The stage diagrams in this note use four chromosomes for clarity. This is only a simplified example; chromosome number varies between organisms.
During prophase, the chromosomes become shorter and more condensed, making them visible when the cell is appropriately stained.
- Each replicated chromosome consists of two identical sister chromatids joined at a centromere.
- The chromosomes condense from their less compact interphase state.
- Spindle fibres, made from microtubules, begin to form.
- The nuclear envelope breaks down, allowing the chromosomes to interact with the spindle.
- In an animal-cell model, the centrosomes move towards opposite poles as the spindle develops.
Exam Tip: To justify that a cell is in prophase, refer to visible condensed chromosomes and, where it can be seen, the breakdown of the nuclear envelope.