Question
Describe the stages of mitosis from prophase to telophase. What happens to the chromosomes at each stage? How does cytokinesis differ in plant and animal cells?
Solution — Step by Step
Chromatin condenses into visible chromosomes (each with two sister chromatids joined at the centromere). The centrioles migrate to opposite poles and begin forming the spindle apparatus. The nucleolus disappears. By late prophase, the nuclear envelope starts breaking down.
Chromosomes are at their most condensed (best stage for karyotyping). They align at the metaphase plate (equatorial plane), attached to spindle fibres at their centromeres via kinetochores. Each sister chromatid is connected to a spindle fibre from opposite poles.
The centromere splits. Sister chromatids are pulled to opposite poles by shortening of spindle fibres. Each chromatid is now an independent chromosome. Anaphase is the shortest phase of mitosis. The cell elongates.
Chromosomes reach the poles and begin to decondense back into chromatin. The nuclear envelope reforms around each set of chromosomes. The nucleolus reappears. Two daughter nuclei are formed, each with the same chromosome number as the parent.
In animal cells: a cleavage furrow forms (constriction of the cell membrane from outside inward). In plant cells: a cell plate forms at the centre (vesicles from the Golgi fuse to build a new cell wall from inside outward).
flowchart LR
A[Prophase] --> B[Metaphase]
B --> C[Anaphase]
C --> D[Telophase]
D --> E[Cytokinesis]
A --> A1[Chromosomes condense]
B --> B1[Align at equator]
C --> C1[Chromatids separate to poles]
D --> D1[Nuclear envelope reforms]
E --> E1[2 daughter cells]Why This Works
Mitosis ensures that each daughter cell receives an exact copy of the parent cell’s chromosomes. The key is the S phase (before mitosis) where DNA replicates, producing sister chromatids. Mitosis then separates these chromatids equally.
Common Mistake
Students say “chromosome number doubles during mitosis.” No — the DNA replicates during S phase of interphase (before mitosis begins). During mitosis, the chromosome number stays constant. A cell with 2n chromosomes produces two daughter cells, each with 2n chromosomes.
For counting: if a human cell (2n = 46) enters mitosis, at metaphase it has 46 chromosomes (each with 2 chromatids = 92 chromatids total). After anaphase, there are 92 chromosomes (chromatids now counted as separate chromosomes), split into 46 at each pole.