Mitosis

Did you know that, on average, cells in your body are replaced every seven to ten years? Different cells in you will have different life spans; for example, white blood cells will only live about 13 days, while cells in the middle of your eye lens will last your whole lifetime! So, how are dead cells replaced? Parent cells undergo a process called mitosis to make new cells.

Although a continuous process, mitosis can be divided into four main stages - prophase, metaphase, anaphase and telophase (and cytokinesis).

Mitosis in the cell cycle

Mitosis is an essential stage within the cell cycle, but it will only occupy a small portion. The interphase will occupy the rest. Interphase within the cell cycle is about preparation for cell division (mitosis), cell growth and chromosome replication. Following mitosis, the cell will physically split into two by cytokinesis.

Fig. 1 - Mitosis in the cell cycle

Important definitions in mitosis

Below are some definitions that will make it easier to understand and follow the path of cell mitosis.

Mitotic stages

Prophase

There are two phases in the prophase - early and late phases.

The early phase:

1. Chromosomes in the cell’s nucleus will condense, and chromatin coils up.
2. Short rods will appear and become visible.
3. The centrioles will divide.

The late phase/prometaphase:

1. The nuclear membrane breaks down, and the nucleolus forms parts of several chromosomes.
2. The centrioles are fully developed and begin to move to opposite cell poles.
3. Centrioles appear as asters in the microtubules form (spindle fibres). The spindle apparatus is attached to the chromosomes and the centrioles.

Metaphase

1. Spindle fibres have fully arrived at the opposite poles of the cell (also mentioned in prometaphase).
2. Centrosomes are there to organise them at the poles.
3. Chromosomes line up at the metaphase plate.
4. Spindle fibres are attached to the centromeres of the chromosomes. Each centromere consists of two kinetochores separated at the anaphase stage.

Anaphase

The centromeres of the chromosomes split, and sister chromatids are pulled apart by the spindle apparatus. Chromatids are pulled towards the opposite poles, centromeres (and kinetochores) first.

Telophase and Cytokinesis

During telophase, chromatids reach the poles and will start uncoiling. The nucleus splits, and the nuclear membrane reforms around the chromatids. The spindle apparatus starts to break down.

Cytokinesis is the physical split of the cell - the cytoplasm is separated. The cell “pinches”, forming a cleavage on each side, with the help of Myosin II and actin filaments. Sister chromatids are identical; therefore, the two new cells are clones of each other. Chromatids will replicate themselves during the interphase before the cell divides again at the next mitosis.

Fig. 2 - An overview of mitosis

How does plant cell mitosis differ from animal cell mitosis?

Plant cell mitosis differs from animal cell mitosis in two different ways. Most plants will have microtubule clusters instead of centrioles, which help the division of the cytoplasm (cytokinesis). Plants still develop a spindle apparatus.

The plant cell will not produce a “cleavage” during cytokinesis before the final split. A cell wall plate will be formed instead during the plant cell division.

Mitosis in stained plant cells

This section is based on the AQA Biology A-Level practical (1).

Equipment:

• Optical microscope
• Microscope slides and coverslips
• Water bath
• 1 mol/dm3 hydrochloric acid (HCl)
• Toluidine blue O stain (selectively stains acidic tissue)/ or other relevant stains
• Distilled water
• Scalpel
• Forceps
• 100 ml beaker
• Root tip

Methodology:

1. A small sample is cut from the plant root tip (where plant mitosis occurs).
2. 1 mol/dm3 hydrochloric acid (HCL) is heated at 60℃ in a water bath and the root tip is incubated for five minutes.
3. The sample is washed and the very tip is removed with a scalpel.
4. The tip is placed under a microscope slide and a few drops of the stain is applied to make the chromosomes visible. The tip is covered with a slide and is squashed gently (to produce a layer that is one cell thick).
5. The objective lens of an optical microscope is set to the lowest magnification. The lens is carefully lowered towards the object and the focus re-adjusted. Higher magnification can also be used if needed.

A stained cell will indicate the stage of the cell cycle taking place. For example, during metaphase, chromosomes will line up at the equator, and during anaphase, they will be pulled towards the opposite poles. Refer back to the mitotic division summary to familiarise yourself with the indicators.

Fig. 3 - Mitosis in onion cells

Mitotic Index

You can use a mitotic index to calculate the ratio between the cells undergoing mitosis and ones that are not. The formula is below:

Binary fission in prokaryotic cells

Like eukaryotic cells, bacteria cells can also divide to produce identical cells. Bacterial cells will reproduce asexually by binary fission.

During binary fission, the following two processes occur:

• Replication of the cellular DNA and plasmids
• Division of the cytoplasm

How do viruses divide if they are non-living? A virus will infect a cell by altering its DNA. This altered DNA will then undergo mitosis.

Cell division calculations

The number of cells produced during mitosis at a given time can be estimated using the following formula:

${\mathbf{2}}^{\mathbf{n}}\mathbf{,}\mathbf{}\mathit{w}\mathit{h}\mathit{e}\mathit{r}\mathit{e}\mathbf{}\mathbf{n}\mathbf{=}\mathit{t}\mathit{h}\mathit{e}\mathbf{}\mathit{n}\mathit{u}\mathit{m}\mathit{b}\mathit{e}\mathit{r}\mathbf{}\mathit{o}\mathit{f}\mathbf{}\mathit{m}\mathit{i}\mathit{t}\mathit{o}\mathit{t}\mathit{i}\mathit{c}\mathbf{}\mathit{c}\mathit{e}\mathit{l}\mathit{l}\mathbf{}\mathit{d}\mathit{i}\mathit{v}\mathit{i}\mathit{s}\mathit{i}\mathit{o}\mathit{n}\mathit{s}$

Let’s take a look at an example; imagine you have a cell that divides ten times. This is 2^10 = 1024 cells.

Let’s say that you want to know the hourly rate. This is calculated by dividing 60 minutes by the time taken (in minutes). For example, you know that a cell will divide every 10 minutes. So, the rate is: 60/10 = 6 divisions/per hour. Hence, 2^6 = 64 cells.

The difference between mitosis and meiosis

Let’s look at a comparison of mitosis and meiosis (Table 1).

Table 1. A summary of differences between mitosis and meiosis.

Uncontrolled mitosis

The genes carefully control cell division. When things go wrong, and genes in the cell mutate, the cell becomes unresponsive to the signals controlling its cellular growth and death. They become cancer cells. Cancer cells become almost independent of the other cells’ signals and start to evade programmed cell death (apoptosis or autophagy). The mitochondria regulate programmed cell death (PCD).