Mitosis is the process in which a cell divides to produce two new cells that are genetically identical. This allows multicellular organisms to grow and damaged cells to be replaced.
The original cell is often referred to as the mother and the two new cells as the daughters.
The process of mitosis can be broken down into four stages: prophase, metaphase, anaphase and telophase. Before these steps take place the cell prepares to divide in a process called interphase.
For more information on interphase and the cell cycle click here.
Once interphase is complete the cell has a copy of its DNA along with a copy of its centrosome. Centrosomes are organelles that are made of centrioles and complex proteins.
The cell begins mitosis with prophase:
Prophase
The chromosomes condense and the centrosomes begin to move towards opposite ends of the cell. As the centrosomes move they form a spindle – which is a network of protein fibres that helps to organise and move the chromosomes.
The opposite sides of the cell, each with a centrosome, are referred to as the poles of the cell.
The nuclear envelope that surrounds the nucleus breaks down so that the chromosomes are free in the cytoplasm. The chromosomes (which each have two chromatids called sister chromatids) then begin to bind to the spindle at their centromere.
Metaphase
As metaphase begins all of the chromosomes are now attached to the spindle which lines them up in the middle of the cell. Each sister chromatid is attached to the spindle by fibres from opposite centrosomes in the cell.
Anaphase
Anaphase is the process in which the sister chromatids are separated, so that they each become a single chromatid (two genetically identical chromosomes). The spindle contracts to pull the separated chromosomes to opposite ends of the cell. During this phase the cell begins to elongate.
Telophase
Once the chromatids have reached opposite sides of the cell they begin to decondense (lengthen) and return to their original size. This leaves two identical sets of chromosomes and a nuclear envelopes forms around each set to form two nuclei.
As this happens the cytoplasm divides – a process called cytokinesis. This leaves two ‘daughter’ cells that are genetically identical to the original cell (as well as to each other).
Mitosis is now complete and depending on the type of cell they both begin interphase (learn more on the cell cycle here).
The diagram below shows all phases of mitosis in sequence:
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