Mitosis – Cell Division Ep 2

Mitosis is a method of nuclear 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, however they are still randomly arranged because they are not yet attached to the spindle. The nuclear envelope breaks down, and the centrioles move to the opposite poles of the cell to form the spindle.
2. Metaphase – the chromosomes (consisting of two sister chromatids joined at the centromere) anchor themselves to the spindle at the centromeres, and they line up along the equator 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 contracts 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 breaks down.

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.

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 in mitosis at a specific 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.

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