Cell Cycle and Cell Division · Lesson 3 of 9
Mitosis: Prophase, Metaphase, Anaphase and Telophase
“Follow the causal sequence of chromosome condensation, kinetochore attachment, sister separation and nuclear reformation.”
• Describe characteristic events of all four stages of mitotic karyokinesis. • Explain the relationship among duplicated centrosomes, asters, spindle fibres, kinetochores and centromeres. • Identify chromosomes and their motion in the four phases using diagrams. • Track chromosome and chromatid counts during mitosis. • Distinguish division of the nucleus from later division of the cytoplasm.
What must mitosis accomplish?
Mitosis distributes the two sister copies of each replicated chromosome into separate daughter nuclei. Its fundamental challenge is mechanical: very long DNA molecules must be compacted, located, attached to a bipolar system and moved into two sets without losing entire chromosomes. A cell ordinarily enters this period after S phase and G2. The amount of DNA in a typical diploid dividing cell is already 4C, and its chromosomes have sister chromatids. Mitosis does not begin by manufacturing another copy of the genome; it begins the distribution of copies made earlier.
Division of the cell nucleus into daughter nuclei through segregation of duplicated chromosomes.
The four familiar stage names are prophase, metaphase, anaphase and telophase. These labels describe recognisable events in a process that progresses continuously. Cells do not necessarily pause at a sharply drawn line between phases. For example, breakdown of the nuclear envelope is often described in modern cell biology as part of prometaphase, while introductory accounts include it in late prophase or the transition into metaphase. This difference in stage naming does not alter the sequence of biological events.
Prophase: package DNA and build the pulling apparatus
Chromatin begins to condense into compact mitotic chromosomes, each containing two sister chromatids joined around its centromeric region. DNA is extremely long compared with the nucleus and would be hard to transport as a loose network. Condensation makes chromosomes comparatively short and mechanically tractable. Their appearance as separate objects under a light microscope increases as condensation proceeds. The chromosome number is unchanged: an initially diploid cell remains 2n chromosomes and 4C DNA at this point.
In the typical animal cell, the centrosome duplicated during the preceding cycle begins to separate, forming two organising sites that move toward opposite poles. Microtubules grow out to form the spindle apparatus; star-like arrays around centrosomes are called asters. The mitotic apparatus must later connect chromosomes to opposite poles. The nucleolus disappears and nuclear-envelope breakdown occurs by late prophase or early prometaphase. Golgi and endoplasmic-reticulum organization also changes rather than simply being permanently destroyed.
An organised array of microtubules and associated proteins that positions and moves chromosomes during cell division.
Star-shaped arrangement of microtubules around an animal-cell centrosome during mitosis.
Metaphase: each chromosome must face two poles
When the nuclear envelope has broken down, spindle microtubules can access the chromosomes. Small protein assemblies at the centromere form kinetochores, the attachment sites for kinetochore microtubules. For each duplicated chromosome, the sister kinetochores ordinarily connect to opposite spindle poles. This bipolar orientation is important: if both sisters are attached to the same pole, they could move together and leave one daughter nucleus without that chromosome.
A protein complex assembled at the centromere region that provides a mechanical connection between a chromosome and spindle microtubules.
The imaginary plane across the middle of the mitotic spindle where chromosomes become aligned in metaphase.
By metaphase, highly condensed chromosomes are positioned near the equator of the cell. This plate is not a solid physical plate, and the chromosomes do not lie on a newly built wall. It is a reference plane indicating equal positioning between spindle poles. At this point each visible X-like chromosome consists of two sisters; the chromatids have not yet separated. The highly condensed metaphase state is useful for studying chromosome morphology under a microscope.
Anaphase: sister chromatids become daughter chromosomes
At anaphase onset, the connection retaining sister chromatids is removed and each chromatid becomes an independent daughter chromosome. Kinetochore-connected spindle microtubules and spindle mechanics move these chromosomes toward opposite poles. The centromeric region leads while arms often trail, producing V-like or J-like appearances. Both poles should receive one copy of every chromosome. The textbook describes separation of centromeres; the modern mechanism also involves controlled dissolution of sister-chromatid cohesion.
Counts require care. A cell with 2n = 4 at metaphase has four replicated chromosomes and eight sister chromatids. Immediately after separation it temporarily has eight independent daughter chromosomes in one still-undivided cell, four moving to each pole. Its total DNA content remains 4C during this stage. Division into two cells later creates daughters with four chromosomes and 2C DNA each. Students who say that DNA replication takes place at anaphase are confusing duplication with segregation.
Telophase: return from transport form to functional nuclei
At telophase, the separated chromosome sets reach their respective poles and begin to decondense. Nuclear envelopes re-form around the two groups, nucleoli reappear and normal nuclear organisation resumes. The endoplasmic reticulum and Golgi system are reorganised as daughter cells emerge. Each newly formed nucleus contains one complete copy of the chromosome complement. Nuclear reformation is not yet equivalent to completion of cell division because the shared cytoplasm still needs to be partitioned during cytokinesis.
| Stage | Chromosomes | Spindle or nuclear change | What students should identify |
|---|---|---|---|
| Prophase | Two chromatids per condensed chromosome | Centrosomes separate; spindle begins; nuclear envelope breaks down late | Discrete condensed chromosomes and developing apparatus |
| Metaphase | Two sisters still attached per chromosome | Kinetochore attachment and alignment | Chromosomes at equatorial plate |
| Anaphase | Sisters now independent daughter chromosomes | Movement towards opposite poles | Two separating groups, centromeres leading |
| Telophase | One complete set in each new nucleus | Nuclear envelopes and nucleoli reform | Two daughter nuclei, decondensing chromatin |
Structure, position and function are interconnected
Condensation reduces entanglement, bipolar attachment directs sister copies to opposite poles, and nuclear reassembly restores separate compartments for gene expression. Each event solves a problem created by the preceding state. This is why the stages should be taught in causal order. If chromosomes failed to attach correctly at metaphase, anaphase segregation could distribute the wrong number of chromosomes, producing aneuploid daughter cells. Spindle attachment and cohesion are therefore not decorative textbook terms; they are mechanisms supporting genome fidelity.
Problem
A cell displays condensed X-like chromosomes at the centre; fibres extend to opposite poles. Has sister separation begun?
- 1.The chromosomes are condensed and aligned at an equatorial plane, identifying metaphase.
- 2.Each X-like structure still represents one chromosome made of two sister chromatids.
- 3.Sister separation defines the onset of anaphase, so it has not yet occurred.
Metaphase shows chromosomes at the middle of the spindle, but 'middle' by itself is not sufficient: a metaphase cell also has kinetochore attachments to opposite poles, and sister chromatids remain joined.
Quick Quiz
Quiz
What best identifies prophase?
At which stage are chromosomes aligned at the metaphase plate?
The kinetochore primarily serves as:
What initiates mitotic anaphase?
During metaphase of a cell with 2n = 6, the number of chromosomes is:
During anaphase of the same cell before cytokinesis, the whole cell temporarily contains:
Which events occur in telophase?
Which structure commonly forms asters in animal cells?
Practice Problems
- Describe the major changes of prophase, metaphase, anaphase and telophase in order.
- Explain why kinetochore attachment to opposite poles is essential.
- Draw a labelled four-panel mitotic sequence for 2n = 2 and show centromeres and spindle poles.
- A cell is in early anaphase, has 4n daughter chromosomes but still contains 4C total DNA. Explain this apparent contradiction.
- Contrast prometaphase and metaphase in the detailed interpretation of the textbook stages.
Prophase: duplicated chromosomes condense and spindle forms; late nuclear-envelope breakdown. Metaphase: sister kinetochores connect to opposite poles and chromosomes align at the equator. Anaphase: sisters separate and migrate to opposite poles. Telophase: daughter chromosome sets decondense, nuclear envelopes and nucleoli reform.
Key Takeaways
• Prophase condenses duplicated chromosomes and constructs the spindle. • Metaphase requires bipolar kinetochore attachment and equatorial alignment. • Anaphase separates sister chromatids, producing independent daughter chromosomes. • Telophase forms two daughter nuclei as chromosomes decondense. • Mitosis distributes copies of chromosomes; cytokinesis completes cell separation.