In a person’s life, many cells that make up the human body age and die. In order to maintain optimal body function, these cells must be replaced. The reasons these cells will die and must be replaced are as follows:
- Epithelial cell shedding (such as those on the skin and the surface of the intestine): ageing, tired cells on the surface of the tissue are often replaced. The replacement of uterine cells by menstruation before menstruation is a special case.
- Wound healing requires cell division at the wound to compensate for those missing cells. Organs damaged by viral diseases such as hepatitis also need to make up for lost cells.
- Replacement of cells that make up the blood: Red blood cells transport oxygen to the tissue. White blood cells (such as B lymphocytes and T lymphocytes) are part of the body’s immune system and help fight off infections. However, most blood cells have only a short survival time and must be constantly replaced. The precursors (both stem cells) of these cells are located in the bone marrow, and they must be replicated at a high frequency to maintain a sufficient number of blood cells.
The process by which cells replicate to form two identical copies of themselves is called mitosis. The purpose of mitosis is to divide two mother cells into two identical cells, which are called daughter cells. In order for mitosis to occur, the following points must occur:
- Gene information (DNA in the chromosome) is faithfully replicated, a process known as gene duplication.
- Organelles (such as mitochondria) must be distributed so that both daughter cells have enough organelles to function properly.
- The cytoplasm must be separated into two different cells.
We will see that many of the characteristics of cancer cells are caused by defects in the genes that control cell division. Cell division is an ordered process involving four stages. These four processes are collectively referred to as the cell cycle. The following pages detail the cell cycle.
This page includes:
- Normal cell division
- Cancer cell division
- Summary of cell division
Normal cell division
There are multiple safety mechanisms in the process of cell division. Such a safety mechanism ensures that cells can only divide when they have completed a normal replication process and when the cell’s living environment favours cell division. In addition, there are a number of mechanisms that can be used to determine the following:
- Is DNA completely replicated?
- Is DNA damaged?
- Is there enough nutrients to support cell growth?
If these inspection mechanisms do not function properly, normal cells will stop dividing until the inspection mechanism returns to normal. Tumour cells do not follow this pattern, they will continue to grow and divide.
Now that we have discussed the cell cycle, we can briefly mention the way to initiate cell division.
Most cells in the human body do not actively split. They perform their own functions, such as the production of enzymes to help digestion, assisting the movement of the arms or legs. Only a few cells will undergo the above-mentioned division process.
When an external signal “notifies” cells to enter the cell cycle, the cells will split . These signals may be estrogens (Estrogens is present) or proteins (Proteins) (such as platelet derived growth factor) and the like. These signal molecules are represented in the animation below as x-shaped molecules that bind to target cells and signal to the nucleus. The result is that the genes that control cell division are activated and the cells begin to divide. For example, a wound on the skin induced by certain blood cells (i.e., platelets) to produce a growth factor (Growth factor) to stimulate the proliferation of skin cells to fill the wound. Cell division is the normal process of replacing dead cells.
These signals as the binding molecule with the target cell, the signal is passed to the nucleus (Nucleus). As a result, genes responsible for cell division are activated.
Normal cell division II
What is the signal that stops the cell from dividing?
Lack of positive external signals can cause cells to stop dividing.
The cells are able to feel the surrounding environment and react to changes in the surrounding environment. For example, if a cell feels that it is surrounded by cells, it stops dividing. In this way, the cells will split when they need to split; when they reach the goal, they stop dividing. Still taking wounds as an example, the cells will divide and proliferate, filling the wounds left by the trauma; however, when the wound heals, the cells will stop dividing. However, cancer cells do not exhibit “contact inhibition.” Even if they are surrounded by cells that form clumps, they will continue to grow. The response of normal cells (top panel) and tumour cells (bottom panel) to “contact inhibition” is shown.
The circular container used to culture the cells in the animation is called a Petri dish (with a Petri dish). In the laboratory, cells are usually placed in such culture dishes and immersed in a nutrient-rich liquid.
Most cells seem to have an upper limit on the number of pre-defined cell divisions. Interestingly, this cap seems to depend in part on the ability of cells to maintain DNA integrity. Called telomerase (telomerase enzyme) capable of maintaining the integrity of chromosomes (Chromosomes) tip. In adults, most cells in our body cannot use telomerase, so this part of the cell will eventually die. In malignant cells, telomerase is often active, causing cells to continue to divide without restriction. For more information on telomerase, see Cancer Genes.
In terms of cell division, malignant cells break almost all the laws!
- Cancer cells can still divide without an external signal. This is like a car that can still be exercised without the need for a throttle. For example, the growth of breast cancer cells does not require estrogens (a normal growth factor). Some breast cancer cells have actually lost their ability to respond to estrogens because the expression of estrogens receptors in the cells is turned off. These cells continue to grow by not requiring an external growth signal. These cells no longer need to be stimulated by external growth signals to divide.
- Cancer cells do not exhibit contact inhibition. Although most cells are aware of whether they are “squeezed” by surrounding cells, malignant cells no longer respond to this stop signal. As indicated above, this continued growth results in the accumulation of cells and the formation of tumour masses.
- Malignant tumour cells can still divide without receiving a “release” signal. Normal cells stop dividing when the gene (DNA) is damaged; while malignant cells continue to divide in this case. As a result, “child” cells contain abnormal DNA and even chromosomes. These mutant cells appear to be more abnormal than “mother” cells. In this way, malignant tumour cells will evolve more and more abnormally.
The continued division of the cells leads to tumour formation. Genetic variation caused by abnormal division is a cause of resistance in many cancers. Mutations in specific genes alter the behavioural patterns of cells, leading to faster evolution and progression of Tudors. The next chapter will discuss some of the most thoroughly studied genes.
More information can be found in Chapter 8 of Cancer Biology by Dr. Robert A. Weinberg.
Summary of cell division
Cell division control
- Cell division is a normal process.
- Some functions ensure that DNA is correctly replicated in an environment suitable for cell division, and errors in the replication process can be corrected.
- When the gene is damaged or the condition is not conducive to growth, the normal cells stop dividing; and the cancer cells continue to divide under inappropriate conditions.
Cell division signal
- Most cells in the human body do not automatically divide, they perform their normal functions.
- Cell division is in response to an external signal (protein or steroid growth factor).
- The cell stops dividing for several reasons:
- Lack of positive external signals
- Cells perceive to be surrounded by other cells – contact-dependent (density-dependent) inhibition
- Most cells have pre-set the upper limit of the number of divisions.
Cell division in cancer cells
- Cancer cells can divide without proper external signals.
- Cancer cells have no contact inhibition mechanisms.
- Cancer cells continue to divide in the event of genetic damage.
- Uncontrolled, persistent division of damaged cells results in tumour formation.
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