Stages of the Cell Cycle - Mitosis (Metaphase, Anaphase and Telophase)
The Mitotic Cell Cycle
Metaphase, Anaphase and Telophase
Part one of this series looked at the cycles within cycles that make up the existence of a cell. Whilst taking up such a small percentage of the overall cell cycle, mitosis is one of the most important series of events in the life of a cell. We shall now examine how the condensed sister chromatids (when chromosomes look like a big X they are called sister chromatids - the part where the arms of the x 'join' is known as the centromere) align along the metaphase plate, separate and are repackaged into nuclei
Metaphase sees the chromosomes line up along the metaphase plate. Here, the spindle fibres attach to the centromeres of the sister chromatids. These fibres act as tow cables to separate the sister chromatids.
Metaphase is one of the most easily-recognisable phases of the cell cycle, and is also the location of a key cell cycle checkpoint - the mitotic spindle checkpoint. Anti-cancer compounds such as taxanes (docetaxel and paclitaxel(Taxol)) work by inhibiting the destruction of the spindle fibres, thus preventing movement through the mitotic spindle checkpoint. This results in cell death
The major event of Anaphase is the sister chromatids moving to opposite poles of the cells, due to the action of the condensing spindle fibres. The chromatids only start separating when the pressure is sufficient to split the centromere. At this point, each chromatid effectively becomes a chromosome. The moving sister chromatids form a V shape as they move through the cytoplasm. This is because the centromeres are pulled by the spindle fibres, and lead the rest of the chromatid.
Telophase basically describesthe series of events that sees new nuclear envelopes form around each set of sister chromatids - now located at the poles of the cell. Also:
- The spindle breaks down
- Chromosomes unravel and so are invisible under light microscopy
The cell is now preparing for the final stage in the cell cycle, cytokinesis
Cytokinesis is technically a separate set of events to mitosis. It describes the series of events seen when the cell splits into two. This process starts as a cleavage furrow between the cells, making it look like the figure 8. This furrow extends until the whole cell has split. With identical genetic information, each cell can perform the same task as the parent cell.
A couple of details need clarification about the differences between plant and animal mitosis:
- Plant cells do not have centrioles. The fibres that make up the spindle are synthesised directly in the cytoplasm
- All animal cells are capable of mitosis and cytokinesis. Plant cells focus their growth to specialised regions called meristems - found at the root tips and shoot tips.
- As plant cells need to synthesis the cell wall during cytokinesis, they do not create a cleavage furrow 'from the outside in.' Cytokinesis starts where the spindle equator was, and continues with new cell membrane and new cell wall material being laid down along this plate.
Stages of Mitosis (narrated)
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Where Next? Mitosis
Another excellent resource from John Kyrk. An excellent A level resource or revision for undergrads. Well worth a look!
- The Cell Cycle & Mitosis Tutorial
Fabulously detailed but still accessible. A thoroughly useful resource for school students, undergrads and anyone who is interested in learning more about the cell cycle. Thoroughly recommended
- Mitosis: An Interactive Animation
Interactive animation showing the stages of animal cell mitosis.
- The Stages of Mitosis - YouTube
A video with a fly through of cells preparing to undergo mitosis. 3D and interesting, while lacking depth. Still, a good starter or hook into a mitosis lesson.
- Cell Division: Stages of Mitosis | Learn Science at Scitable
Fully understanding the mechanisms of mitosis remains one of the greatest challenges facing modern biologists. A Nature review of Mitosis