Question
Describe the stages of mitosis from prophase to telophase, including chromosome behaviour at each stage.
Solution — Step by Step
flowchart LR
A[Interphase - DNA replication 2n to 4n DNA] --> B[Prophase]
B --> C[Metaphase]
C --> D[Anaphase]
D --> E[Telophase]
E --> F[Cytokinesis]
F --> G[Two daughter cells - each 2n]Chromatin condenses into visible chromosomes (each consisting of two sister chromatids joined at the centromere). The nucleolus disappears. The nuclear envelope begins to break down. In animal cells, centrioles move to opposite poles and form the spindle apparatus. This is the longest phase of mitosis.
The nuclear envelope is completely dissolved. Spindle fibres from both poles attach to the kinetochores (protein structures on the centromere of each chromosome). Chromosomes are pulled to the metaphase plate (cell equator) and align in a single row. This is the best stage to study chromosome morphology and count chromosome number.
The centromeres split, and sister chromatids are pulled apart to opposite poles by shortening spindle fibres. Each chromatid is now an independent chromosome. The cell elongates. This is the shortest phase of mitosis. At the end, each pole has a complete set of chromosomes (2n).
Chromosomes decondense back into chromatin. The nuclear envelope reforms around each set of chromosomes. The nucleolus reappears. Spindle fibres disassemble. The cell now has two nuclei. Telophase is essentially the reverse of prophase.
The cytoplasm divides. In animal cells: a cleavage furrow forms and pinches the cell in two (centripetal — from outside inward). In plant cells: a cell plate forms at the centre and grows outward (centrifugal), eventually forming a new cell wall. Result: two genetically identical daughter cells, each with 2n chromosomes.
Why This Works
Mitosis ensures that each daughter cell receives an exact copy of the parent cell’s DNA. The key events are: DNA replication during S phase (before mitosis), chromosome condensation (to prevent tangling), alignment at the metaphase plate (to ensure equal distribution), and separation of sister chromatids (so each daughter gets one copy of each chromosome).
Common Mistake
Students confuse chromosome number and DNA content. Before S phase: 2n chromosomes, 2C DNA. After S phase (before mitosis): still 2n chromosomes, but 4C DNA (each chromosome has two chromatids). After mitosis: 2n chromosomes, 2C DNA. The chromosome number stays 2n throughout mitosis — what changes is the DNA content per cell.
Metaphase is the best stage for karyotyping because chromosomes are maximally condensed and aligned neatly at the equator. Colchicine is used to arrest cells at metaphase (it prevents spindle formation) for chromosome studies.