Mitosis and Meiosis

Another key process especially to do with cancer is mitosis. Mitosis is the process cells use in order to replicate themselves. This is how humans grow and how they repair damage. Mitosis is a well documented and easily observed process. A regular microscope with the correct dyes are able to watch cells actively replicate. This concept can be explored in as little or as much depth as a reader would like. Conceptually mitosis is a very simple process, the cell will make duplicates of its organelles, it will then begin to expand and eventually it will constrict and separate into 2 separate cells with their own membranes.

To go into further detail would be to cover the stages of mitosis that could be referred to in other literature. Though this information is largely unnecessary for the understanding covered in this paper, some studies or medicines may rely on specifics related to this process. 

Before going into mitosis, and to a lesser extent meiosis, it is important to understand chromosomes. Chromosomes are bundles of DNA. A healthy human’s entire DNA sequence is made of 23 pairs of chromosomes, forming 46 in total. Each pair of chromosomes have one from each parent. Sex chromosomes carry information regarding the biological sex of the person, inheriting a Y and X chromosome causes a person to develop into a male, while 2 X chromosomes causes the person to develop into a female. 

Fig 2: All human chromatids.

Like many things in biology people cannot agree upon the way with which to organise the process of mitosis. There is an explanation of some of the variations at the end of the segment so the reader is able to understand how other sources may classify them slightly differently. 

Fig 3: A simple diagram illustrating one interpretation of the process of mitosis.
  1. Interphase is marked as the phase between replication, generally cells spend most of their life in interphase. Interphase can be separated into further phases once again. G1 and G2 are often referred to as the phases before (G1) and after (G2) the DNA is replicated. More on the specifics of DNA replication here. S phase is the phase where DNA is replicated this can get confusing as there are now 2 copies of each pair of chromosomes, these 2 copies can remain joined for some phases of replication forming the classic X shape, each half of these copied chromosomes are called chromatids, 2 copies of each other can be referred to as sister chromatids. Fig.1 and Fig.2 show chromosomes organised in both single strand and double stranded forms respectively. 
Fig 1: All human chromosomes.
Fig 2: All human chromatids.

 A cell spends most of its life in interphase so most regular cellular function happens in interphase.  

  1. In prophase the DNA has officially formed chromosomes and is now organised and clear within the nucleus as the nuclear membrane dissolves. Mitotic spindles also begin to develop in prophase. 
  2. In metaphase mitotic spindles attach to the kinetochore of each chromosome and apply equal force in each direction bringing the chromosomes into the centre of the cell. 
  3. In anaphase the sister chromatids are separated at the centromere. They are then pulled to each pole of the cell. 
  4. In telophase new nuclear membranes form around the two clusters of chromatids. Some consider cytokinesis a separate phase. Regardless of the characterization, in cytokinesis the space between the 2 nuclear membranes constricts, eventually forming 2 separating membranes.

Some people may say there are 4 stages to mitosis as they do not count interphase as part of the process, they do not disagree that this happens, but simply do not consider it a part of the process of mitosis. Others may say there are 6 or even 7 stages by breaking down prophase into prometaphase and prophase, and/or telophase into telophase and cytokinesis. 

This process is important to understand as it highlights not only the mechanism cancer cells have disrupted, but also how they pass on these mutations. In interphase these cells make an exact copy of the DNA, as this is an exact copy it contains the exact same information and mutations the parent had. More on DNA replication here. 

Meiosis is a slightly more complex process but will be covered in less detail as it is only mentioned for context. 

Meiosis is the process specialised cells undergo to create cells used for sexual reproduction. These cells are called gametes or sex cells. Meiosis is very similar to mitosis however instead of 1 cell forming 2 separate cells with complete sets of chromosomes it forms 4 cells each with half the chromosomes (cells with only half the amount of chromosomes necessary are called haploid). Then when gametes combine with others they can form a full set of chromosomes, leading to a diploid cell (in humans specifically a sperm and ovum must meet). 

There is also a process these cells undergo called crossing over. Simply put, each of the pairs of chromosomes pass small sections of their DNA to each other, thus promoting genetic diversity and ensuring that part of each chromosome is passed on to the next generation. 

More information can be found here: https://byjus.com/biology/mitosis/

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It is important to note whenever doing independent research, especially in medical fields, that regardless of your intelligence, confidence, or effort, your research has been influenced by websites, personal interpretation and biases. Although this is also true for medical professionals they have had their understanding repeatedly checked, peer reviewed and marked. Listen to medical professionals over any research you have done. The aim of websites such as this is for you to have a baseline of knowledge so you can have productive discussions with medical staff or to discern the merit of other online sources.

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