Mitosis is the type of cell division that produces two daughter cells with the same number of chromosomes as the parent cell. It is used for growth, repair, and replacement of somatic cells.
Mitosis has four main stages:
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
Cytokinesis follows telophase but is not part of mitosis itself.
Each stage has specific events. Exams test these events directly. Learn what happens at each stage and nothing else will confuse you.
Prophase — The Start of Visible Change
Prophase is the first stage of mitosis. Three things happen here.
Chromosomes condense. Chromatin threads coil and shorten. Chromosomes become thick and visible under a microscope. In interphase, chromosomes are too thin and spread out to see. Prophase is when they first become visible.
Spindle fibres start to form. Tubulin proteins begin to polymerise. The mitotic spindle starts to build from the two centrioles (in animal cells).
The nucleolus disappears. The nucleolus breaks down at the start of prophase. It will not be visible again until telophase.
The nuclear envelope is still intact at this stage. This is an important distinction. The nuclear envelope breaks down later, in prometaphase — not in prophase.
Prometaphase — The Spindle Connects to Chromosomes
Prometaphase begins when the nuclear envelope breaks down. This is the key event of prometaphase.
Why must the nuclear envelope break down?
The spindle fibres are in the cytoplasm. The chromosomes are inside the nucleus. For spindle fibres to attach to chromosomes, the nuclear envelope must first break down. This gives the spindle access to the chromosomes.
This is a direct exam point. The correct answer to “which process is required for efficient attachment of sister chromatids to the spindle” is breakdown of the nuclear membrane.
After the nuclear envelope breaks down, spindle microtubules attach to chromosomes at the kinetochore. The kinetochore is a protein complex located at the centromere of each chromosome. Each sister chromatid has one kinetochore. The kinetochore of each chromatid attaches to spindle fibres from opposite poles.
Metaphase — Chromosomes Line Up at the Centre
In metaphase, all chromosomes move to the centre of the cell. They line up along an imaginary plane called the metaphase plate or equatorial plate.
This alignment is not random. Each chromosome is pulled equally from both poles. When the tension from both sides is equal, the chromosome stops at the centre.
Why Metaphase is the Best Stage to Study Chromosomes
Chromosomes are most condensed at metaphase. They are short, thick, and clearly separate from each other. This makes it the best stage to:
- Count chromosome number
- Study chromosome shape and size
- Identify chromosome pairs
- Prepare a karyotype
Exam point: Chromosome measurements are taken during metaphase. Chromosome morphology is best studied at metaphase.
Some questions also mention metaphase and anaphase together as the best phases to study chromosome morphology. Both are acceptable answers. Metaphase is the single best stage when only one option is given.
Anaphase — Chromosomes Move to the Poles
Anaphase is the stage of chromosome separation. Three things define anaphase.
1. Centromere Divides
At the start of anaphase, the centromere of each chromosome splits. This separates the two sister chromatids. Each chromatid is now an independent chromosome. The cell now has double the chromosome number — temporarily.
Exam point: Centromere divides during anaphase.
2. Sister Chromatids Separate
Once the centromere splits, the two sister chromatids move to opposite poles. Each pole receives one chromatid from each chromosome. This ensures each daughter cell gets the correct chromosome set.
The separation of sister chromatids happens at anaphase. This is a direct MCQ answer.
3. Chromatids Move Due to Microtubule Depolymerisation
This is a high-level exam point asked in CSIR NET and GATE.
Chromatids do not move to the poles because microtubules push them. They move because kinetochore microtubules depolymerise — they shorten from the kinetochore end. As the microtubule shortens, it pulls the chromatid toward the pole.
Exam point: Chromatids move poleward due to depolymerisation of kinetochore microtubules — not polymerisation, not actin filaments, not depolymerisation of polar spindle fibres.
Telophase — The Cell Prepares to Split
Telophase is the reverse of prophase. The division machinery dismantles and the nucleus reforms.
Four events happen in telophase:
Spindle disassembles. Microtubules depolymerise and the spindle apparatus breaks down.
Chromosomes decondense. Chromosomes uncoil and return to the relaxed chromatin form seen in interphase.
Nuclear envelope reforms. A new nuclear envelope forms around each set of chromosomes. Two nuclei now exist in one cell.
Nucleolus reappears. The nucleolus reforms in each new nucleus.
Exam point: Telophase events = disassembly of spindle apparatus + decondensing of chromosomes + nuclear envelope reformation.
The option “alignment of chromosomes in the centre” describes metaphase, not telophase. Do not pick it for a telophase question.
What Does NOT Happen During Mitosis?
This question type asks you to pick the event that does NOT occur during mitosis.
DNA replication does NOT occur during mitosis.
DNA replication happens during S phase of interphase — before mitosis begins. By the time the cell enters mitosis, DNA replication is already complete.
The events that DO occur during mitosis:
- Chromosome condensation — Yes, prophase
- Spindle formation — Yes, prophase and prometaphase
- Sister chromatid separation — Yes, anaphase
- Nuclear envelope breakdown — Yes, prometaphase
The answer to “which does not occur during mitosis” is DNA replication.
Open Mitosis — What Does This Term Mean?
In most animals and plants, mitosis is “open.” This means the nuclear envelope breaks down before the chromosomes separate.
Open mitosis = nuclear envelope breaks down BEFORE chromosomes separate.
This allows the spindle, which forms in the cytoplasm, to access and attach to chromosomes directly.
Some fungi and protists have “closed mitosis.” In closed mitosis, the nuclear envelope stays intact throughout division. The spindle forms inside the nucleus.
Exam point: “Open mitosis” = nuclear envelope breaks down before chromosome separation.
Do not confuse it with “nuclear division without cytokinesis” — that is a different concept (endomitosis or coenocyte formation).
Quick Summary Table
| Stage | Key Events |
|---|---|
| Prophase | Chromosome condensation, spindle formation, nucleolus disappears |
| Prometaphase | Nuclear envelope breaks down, kinetochores attach to spindle |
| Metaphase | Chromosomes align at metaphase plate, best stage for karyotyping |
| Anaphase | Centromere divides, sister chromatids separate, chromatids move to poles |
| Telophase | Spindle disassembles, chromosomes decondense, nuclear envelope reforms |