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Mitosis

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Biology

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.

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 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.

Mitosis mitosis in the cell cycle StudySmarterFigure 1. Mitosis in the cell cycle. Source: mysid, Public domain, via WIkimedia Commons.

The interphase of a cell will take about 18 to 20 hours, and mitosis will only take about two!

Important definitions in mitosis

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.

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.

Mitosis an overview of mitosis StudySmarterFigure 2. An overview of mitosis. Source: LadyofHats, CC0, via Wikimedia Commons.

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.

Often students will use onion cells to observe different stages of cell cycle. An example in Figure 3!

Mitosis [+] mitosis in onion cells [+] StudySmarterFigure 3. Mitosis in onion cells. Source: Pixelmaniac pictures (leave a reply), CC BY-SA 3.0, via Wikimedia Commons

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.

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.

Mitosis [+] types of mitosis in different organism groups [+] StudySmarterFigure 4. Types of mitosis in different organism groups. Source: Baba68, CC BY-SA 4.0, via WIkimedia Commons.

Cell division calculations

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.

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.

Mitosis a summary of differences between mitosis and meiosis StudySmarter

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).

Mitosis - Key takeaways

  • Mitosis is the process of cell division or reproduction that produces clone daughter cells. There are four main stages: prophase, metaphase, anaphase and telophase. Before mitosis, the cell will grow, replicate its DNA and prepare for mitosis; this is interphase.
  • Plants cells do not have centrioles and do not form a cleavage before the final cytokinesis. Plant cells use microtubule clusters instead of centrioles and form a cell wall plate during telophase.
  • Binary fission, similarly to mitosis, produces two clone cells. Binary fission occurs in prokaryotes.
  • Cancerous cells develop when mitosis becomes uncontrolled. They will no longer respond to the cellular signals and do not undergo programmed cell death regulated by the mitochondria.


(1) PMT education (2016). AQA Biology A-Level. Required Practical 2.

Mitosis

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.

Final Mitosis Quiz

Question

What are the four phases of mitosis?

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Answer

Prophase, metaphase, anaphase and telophase.

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Question

What is cytokinesis and when does it occur?

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Answer

Cytokinesis is the physical split of the cell, when the cytoplasm separates. It will occur following telophase.

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Question

What happens before mitosis?


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Answer

The cell grows, replicates its DNA and gets ready for mitosis.

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Question

What is the difference between the centromere and the kinetochore?


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Answer

Centromere refers to the constricted region that holds the two chromatids in the chromosome together. Kinetochore is the protein structure associated with the centromere to which spindle fibres will attach during mitosis.

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Question

Interphase is the first stage of mitosis. True or False?


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Answer

False.

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Question

Fill in the blanks about metaphase. ________ fibres have fully arrived at the _________ poles of the cell. They are organised by the ______________ when they arrive. _________ line up at the ___________. ____________ fibres are attached to the ___________ which are at the __________ of the chromosome.

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Answer

Spindle fibres have fully arrived at the opposite poles of the cell. They are organised by the centrosomes when they arrive. Chromosomes line up at the equator/metaphase plate. Spindle fibres are attached to the kinetochores which are at the centromere of the chromosome.

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Question

The centromeres of the chromosome are pulled apart by the spindle apparatus. True or False?

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Answer

True.

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Question

Telophase and cytokinesis is all about the split of the cell. How are they different?


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Answer

During telophase the nucleus splits into two and the nuclear membrane will form around each new nucleus. Cytokinesis is the final split of the cytoplasm.

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Question

During what stage of the cell cycle are chromatids replicated?


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Answer

Interphase.

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Question

What are the two differences between plant and animal mitosis?


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Answer

The differences are:

  1. Plants do not have centrioles.
  2. Plant cell does not create a cleavage (like in an animal cell) and instead a cell wall plate will form at the equator.


Show question

Question

To observe mitosis in the plant cells, the practical requires the tissue to be squashed. Why is this?


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Answer

To produce a layer that is only one cell thick and can be observed under the microscope.

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Question

What is binary fission?


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Answer

Equivalent to mitosis in eukaryotic cells, binary fission occurs in the prokaryotic cells. Two clone daughter cells are produced. During binary fission, replication of DNA and plasmids and the division of the cytoplasm will occur.

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Question

A cell (2^0) takes five minutes to divide. How many times will it divide in one hour?


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Answer

4096 cells per hour produced.

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Question

Calculate the mitotic index (in the ratio form) in the 100 cells. 70 of the cells were observed to be in the interphase.


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Answer

7:3 (undividing:dividing)

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Question

During meiosis, four genetically identical diploid cells are produced. True or False?

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Answer

False.

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Question

Fill in the blanks about uncontrolled mitosis. Cell division is carefully controlled by the _______. These can _________ and become _________ to the cellular signals. These cells become _________. They will not undergo __________ __________ __________ (PCD). PCD is regulated by the ___________.


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Answer

Cell division is carefully controlled by the genes. These can mutate and become unresponsive to the cellular signals. These cells become cancerous. They will not undergo programmed cell death (PCD). PCD is regulated by the mitochondria.

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