Cell Division: Phases of Mitosis | A Level Biology | OCR, AQA, Edexcel

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This week we are revising Cell Division: Phases of Mitosis
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Now that we have familiarised ourselves with chromosomes and their structure, we can move on to the steps involved in mitosis. There are five phases of mitosis and they are briefly outlined in this section.
Prophase
Chromosomes become visible. In prophase the chromosomes condense. This is often complicated for many students. What does condense mean? Very simply, they get shorter and thicker. This makes them visible as two sister chromatids joined at the centromere.
Centrioles split and move to opposite ends of the cell. Tiny organelles known as centrioles begin to move to opposite ends of the cell. The centrioles produce special fibres made of special protein which we call the mitotic spindle. The spindle extends across the cell.
The nuclear envelope disintegrates. The end of prophase is marked by the breakdown of the nuclear envelope. The chromosomes are freed into the cytoplasm.
Metaphase
At the end of prophase, we have visible chromosomes, a spindle formed, and a near broken nuclear envelope.
The nuclear envelope is completely gone. There is no visible sign of the nuclear membrane.
The spindle fibres attach to the chromosomes. The spindle fibres attach at the centromeres of each chromosome.
The chromosomes line up at the equator. The spindle fibres pull the chromosomes, making them line up in the centre of the cell (at the metaphase plate).
Anaphase
At the end of metaphase, the chromosomes are lined up in the centre of the cell. They are still visible as two sister chromatids joined at the centromere.
The chromosomes break apart at the centromere. The spindle fibres begin pulling the sister chromatids away from each other to opposite poles of the cell. This splits the chromosome into two V-shaped sister chromatid structures.
Telophase
The chromatids reach opposite poles of the cell. The spindle fibres eventually pull the chromatids completely to opposite poles of the cell. Now, the chromatids uncoil and become long and thin again and can be called chromosomes again.
The nuclear envelope reforms around the chromosomes. A nuclear envelope forms around each set of chromosomes - one on each pole of the cell. Therefore the cell now has two nuclei, each with a complete set of identical DNA.
The cell prepares to split the cytoplasm and its contents. At this point, the cell will begin to form a structure called a cleavage furrow. The cleavage furrow is the point at which the cytoplasm and cell membrane will divide.
Cytokinesis
Cytokinesis is the final step of mitosis. In this step, the cell will completely divide in to two daughter cells as the cytoplasm and cell membrane separates.
Each daughter cell will be identical to each other and their parent cell. Each daughter will have one nucleus, a complete set of DNA, and a set of organelles and the same amount of cytoplasm as their twin sister cell.
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