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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…
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Jetzt kostenlos anmeldenDid 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.
Mitosis is when chromosomes are duplicated in a cell and result in two genetically identical daughter cells.
Although a continuous process, mitosis can be divided into four main stages - prophase, metaphase, anaphase and telophase (and cytokinesis).
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.
The interphase of a cell will take about 18 to 20 hours, and mitosis will only take about two!
Below are some definitions that will make it easier to understand and follow the path of cell mitosis.
Centriole: A cylindrical structure composed of microtubules found near the nuclear membrane.
Centromere: A constricted region that holds the two chromatids in the chromosome together.
Centrosome: Also known as the primary microtubule-organising centre (MTOC) in animal cells. Centrosomes facilitate spindle alignment at the poles.
Kinetochore: A protein structure associated with the centromere, to which the spindle fibres attach.
Metaphase plate (equator) is a region at an equal (approximately) distance from each opposite cell pole, hence the equator. The chromosomes will line up at the equator during metaphase.
Chromatin: An unravelled, condensed substance within a chromosome, consisting of DNA structures and proteins.
Chromatid: A chromosome consists of two chromatids that have identical genetic information.
Spindle fibres: Protein structures that divide genetic material.
Spindle apparatus: A term that refers to a collection of spindle fibres.
Aster: A star-shaped structure that holds two centrioles at the two poles.
There are two phases in the prophase - early and late phases.
The early phase:
The late phase/prometaphase:
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.
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.
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.
This section is based on the AQA Biology A-Level practical (1).
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.
Often students will use onion cells to observe different stages of cell cycle. An example in Figure 3!
You can use a mitotic index to calculate the ratio between the cells undergoing mitosis and ones that are not. The formula is below:
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:
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.
Mitosis can slightly differ between different organism groups. In some groups, unlike the animals, the nulear membrane will remain intact during mitosis. If you remember, the nulear membrane will break down in animal cell mitosis. In Figure 4 you will find some examples.
Figure 4
The number of cells produced during mitosis at a given time can be estimated using the following formula:
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 number of divisions will rarely be completely accurate. Hence, estimations! Divisions will not be constant due to environmental factors.
Let’s look at a comparison of mitosis and meiosis (Table 1).
Table 1. A summary of differences between mitosis and meiosis.
Mitosis | |
One division, two identical diploid daughter cells | Two divisions, four genetically different haploid (chromosomes halved) daughter cells |
In all organism except viruses | |
Somatic (body) cells | Production of gametes only (sperm and egg) |
Shorter prophase | Prophase takes longer |
No chromosomal crossing or recombination over during prophase | Crossing over and recombination of the chromosomes to exchange genetic information |
During metaphase, chromatid pairs line up at the metaphase plate | Pairs of chromosomes line up at the metaphase plate |
During anaphase, sister chromatids are pulled to the opposite poles | Sister chromatids will move together to the same pole and then are separated to the opposite poles |
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).
(1) PMT education (2016). AQA Biology A-Level. Required Practical 2.
Interphase. This is because things go wrong during the DNA replication stage.
Mitosis still occurs but it is not controlled by the cellular signals.
Cell division becomes uncontrolled. The cell that would normally age and die will keep dividing and does not undergo apoptosis.
Mitosis is important in producing somatic cells which include replacement of cells and development.
Two daughter cells.
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