In Metaphase, the centrosomes reach opposite poles and the chromosomes line up along the metaphase plate (the cell's equator). This alignment ensures that each new nucleus will receive exactly one copy of each chromosome.
Spindle fibers attach to the kinetochores located at the centromere of each sister chromatid. Each chromatid is connected to a fiber originating from an opposite pole.
Anaphase begins when the centromeres divide, allowing the sister chromatids to separate. Once separated, each chromatid is considered an individual chromosome.
The spindle fibers shorten, pulling the newly independent chromosomes toward opposite poles. The chromosomes move "centromere first," often appearing in a V-shape as they are dragged through the cytosol.
Telophase is essentially the reverse of prophase. The chromosomes reach the poles and begin to decondense back into a less tightly coiled chromatin state.
New nuclear envelopes reform around each set of chromosomes, and the nucleoli reappear within the developing nuclei. The spindle apparatus is dismantled as its protein subunits are recycled.
Cytokinesis usually occurs concurrently with telophase. It involves the physical division of the cytoplasm into two separate cells.
In animal cells, a cleavage furrow forms as the cell membrane pinches inward, while in plant cells, a cell plate is constructed to form a new cell wall between the daughter nuclei.
| Feature | Centromere | Centrosome |
|---|---|---|
| Definition | Region of DNA where sister chromatids are joined | Organelle that serves as the main microtubule organizing center |
| Function | Attachment point for spindle fibers | Produces and organizes the spindle apparatus |
| Location | Part of the chromosome structure | Located at the poles of the cell during mitosis |
The PMAT Mnemonic: Always remember the sequence Prophase, Metaphase, Anaphase, Telophase to correctly order the events of nuclear division.
Counting DNA vs. Chromosomes: In a human cell, there are 46 chromosomes throughout mitosis. However, from the end of S phase until the end of metaphase, there are 92 DNA molecules because each chromosome is made of two chromatids.
Micrograph Identification: Look for specific visual cues. If chromosomes are scattered, it is Prophase. If they are in a single line, it is Metaphase. If they are in two distinct V-shaped groups moving apart, it is Anaphase.
Check for Cytokinesis: Do not confuse mitosis (nuclear division) with cytokinesis (cytoplasmic division). Mitosis is strictly about the movement and separation of genetic material.
