Mitosis – Cell Division Ep 2

Mitosis is a method of cell division used by multicellular organisms for growth and repair of tissues. Some plants and fungi (and rarely some animals) also use it for asexual reproduction – producing genetically identical offspring from one parent. Mitosis results in two daughter cells which are genetically identical to the parent cell. In this article we will go through each individual stage of mitosis.

Interphase

In the last article we looked at the cell cycle and DNA replication. Interphase is the period before mitosis which incorporates gap phase 1, the synthesis phase, and gap phase 2. Head over to the last article to learn more detail about interphase. If you were to look at a cell under a microscope at the end of interphase, you would see that the DNA is unravelled and messy. You would not be able to distinguish the chromosomes.

Mitosis: prophase, metaphase, anaphase, and telophase

These are the four stages of mitosis. Let’s take a look at the sequence of events:

1. Prophase – the chromosomes (made of chromatin) become condensed and compact. They are now ‘x’ shaped because there are two sister chromatids joined together by the centromere. Sister chromatids are identical copies of the same chromosome. The nuclear envelope breaks down, and the centrioles begin to move to the opposite poles of the cell.
2. Metaphase – the centrioles form the spindle fibres across the cell. The chromosomes (still consisting of two sister chromatids) anchor themselves to the spindle fibres at the centromeres, and they line up along the centre of the cell. Note that there is a checkpoint in metaphase – the cell checks that all the chromosomes are attached to the spindle before carrying on.
3. Anaphase – the chromosomes divide at the centromere, so now the sister chromatids are separated. The spindle fibres contract to pull one of each to the opposite poles of the cell. The chromatids (which now go back to being called chromosomes) appear ‘v’ shaped as they are pulled.
4. Telophase – the nuclear envelope forms around each set of chromosomes so there are two nuclei visible. The chromosomes decondense and the spindle fibres disappear.

Cytokinesis

Cytokinesis can start in telophase or anaphase. A contractile ring forms around the centre of the cell, which contracts to form a cleavage furrow. Eventually, this process divides the cytoplasm into two genetically identical daughter cells, each with a full set of chromosomes. Note that in plants this process is slightly different, but you only need to know the animal cell version.

If you want to see some microscope images of cells in different stages of mitosis, take a look here.

Mitotic index

The mitotic index is the proportion of cells any stage of mitosis at any one time. A higher mitotic index suggests that the tissue is growing more quickly than a tissue with a lower mitotic index. It can be calculated as follows:

Mitotic index = no. of cells with visible chromosomes / total no. of cells

Summary

• Mitosis is used for growth, repair, and asexual reproduction.
• It produces two genetically identical daughter cells.
• The four phases are prophase, metaphase, anaphase and telophase. These are preceded by interphase, and cytokinesis spreads across anaphase and telophase.
• Each copy of a chromosome is called a sister chromatid when they are joined at the centromere.

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