Module 5 Cell Division

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Before we begin, it is worth mentioning that the process you are about to study is exactly that - a process. Cell division should be viewed as having a definite beginning and end. In between divisions, that is, when a cell is not undergoing cell division, a cell performs normal vital activities such as respiration, photosynthesis, protein synthesis, absorption, secretion, and excretion. These processes take up most of a cell's life. Perhaps the most important process is the replication of genetic material in the nucleus. This must be completed before a cell divides so that each daughter cell receives the same genetic complement as the parent cell. Once the replication is complete, the cell is ready to divide.

 

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Cell Cycle

Our bodies are made up of an amazing array of specialized cells that keep us healthy and able to maintain an internal balance in an environment that seems bent on changing. During our daily combat with the environment, cells must be replaced as they wear out. Exactly how fast this occurs depends mostly on the role those cells perform. Skin, which is constantly being scratched, rubbed or cut, replaces itself very quickly. However, muscle or nerve cells may remain healthy for most of our life and therefore may not need to divide to replace themselves. Some cells, such as red blood cells, which lack a nucleus and genetic material, or gametes, which only have one of each chromosome, will never reproduce.

 

The typical life cycle of a cell can be broken up into two main phases; Interphase and M phase. Interphase is the much longer phase, and is the phase where the cell will carry out its intended function in the body. M phase, or Mitosis phase, is shorter and moves the cell through a complex sequence of steps that are carried out in order to divide the cell’s genetic material equally. This stage ends with cytokinesis, which is the physical division of the cell into two daughter cells. Read pp. 553 – 555 in your textbook to preview the stages of the cell cycle discussed below.

 

Interphase is further broken down into three phases; G1 phase, S phase, and G2 phase.

During G1, or Growth 1, the cell is growing rapidly, producing proteins and carrying out its intended function. In the case of muscle or nerve cells, they may remain healthy and functioning at this stage for so long that they may be referred to as being “stuck” in G1 or, as it has more recently been refered to, in G0 phase. If, however, this is a regular cell, it will reach a point where it moves on to the S phase.

 

The G1 stage is critical if the cell is to divide properly later on. In S phase, or synthesis phase, the cell will duplicate its DNA exactly. Each single chromosome makes a copy of itself and holds on to the copy. These doubled chromosomes do not contain any new genetic material. Rather, they are identical copies of each other. While together, they are known as sister chromatids, and are joined together by a regional structure called the centromere.

 

After the DNA has been successfully doubled, the cell will enter G2 or Growth 2. Here the cell continues to carry out its role in the body. Nearing the end of Interphase, the cell will get ready for M phase by storing energy and building proteins and other structures needed for cell division.

 

During M phase or Mitosis phase, the cell will divide each doubled chromosome into two separate single chromosomes. We will consider this process in detail in the next lesson. At this point it should be clear to you that the cell goes through an orderly set of steps necessary to ensure that its genetic material is correctly divided into two complete and equal sets. Following this division, the cell will go through cytokinesis in order to physically divide the cell into two.

 

Watch and Listen

The animation above introduced the checkpoints within the cell that regulate division. These are of great interest when considering how cells ensure that they are growing and are in fact capable of division.

 

There are three major checkpoints of the cell cycle. One is at the end of G1. Here, the cell evaluates if it is large enough and strong enough to continue with the division process. The second checkpoint is at the end of G2. This is a very important checkpoint for the cell. At this time, the cell must evaluate if it has properly duplicated all of its chromosomes. If it has not, it may attempt to carry on with the division, or it may simply self-destruct. The last major checkpoint occurs during M phase. At this checkpoint the cell evaluates whether the spindle apparatus has properly attached itself to each of the chromosomes, and whether the rest of the cell is ready for cytokenisis, or physical cell division. If something is wrong at this stage, the cell will often simply die. 

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Reasons and Limits for Cell Growth

There are many factors that may lead cells to reproduce. The most common stimulation is the need for replacement due to damage or age. Cells work very hard, and many simply become too brittle or build up toxins too fast to continue to function. These cells are broken down by the body and their raw materials are re-used.

 

Once the call for cell reproduction is given, how do cells know when to turn off? Let’s consider skin cells as a working example. Your skin is one of the most active areas on your body for cell reproduction. Skin cells use two common clues to determine when to start or stop reproduction. The first is density-dependent inhibition.

 

For density-dependent inhibition to work, cells must pay attention to their neighbors. First off, regular skin cells will not divide if alone. Next, if a gap is created in a layer of skin cells,then those that remain will automatically start to divide until the gap is covered. Also, once skin cells bump up against their neighbors, they stop dividing.

 

Another clue that cues cell growth is anchorage dependence. Using skin cells again as the example, skin will grow naturally if anchored to a substratum of tissue. Conversely, they will not grow if simply free floating in a nutrient bath.

 

Cancer

Cancer is a broad group of diseases characterized by a rapid, uncontrolled division of cells. Cancer cells appear to ignore all the regulations in place for cell growth. They do not wait or stop at any of the check points. They do not appear to be density-dependent, nor are they anchorage dependent. Cancer cells grow and reproduce constantly.

 

Module 5: Lesson 2 Assignment

Knowing that certain chemicals interfere with the process of cell division, researchers set out to find drugs that would help in curing cancer. This has led to a cancer treatment method called chemotherapy, or treatment by chemical drugs. Since cancer cells divide rapidly and continually, any chemical which blocks cell division or kills cells while they are dividing will have a much greater effect on cancerous cells than on normal cells. However, these drugs will also destroy other fast-growing cells in the body, such as hair follicles. This explains the loss of hair by cancer patients on chemotherapy.

 

There are now more than two dozen different anticancer drugs which can be used to treat cancer. One drug used in chemotherapy is methotrexate, which attaches to certain enzymes involved in chromosome (DNA) replication and prevents these enzyme from doing their job. Without these enzymes, new molecules of DNA cannot be synthesized. If cell division does not take place among these drug-damaged cells, none of the newly formed cells will survive. Methotrexate is generally quite successful at first, but like other similar drugs, it loses its effectiveness over time. Studies show that the cancer cells become resistant to these anticancer drugs. Researchers believe that resistance to methotrexate occurs because the drug-treated cancer cells produce multiple copies of the specific gene that is affected by the drug. Methotrexate alters the DNA molecules in cancer cells so that some genes begin to multiply uncontrollably. One of these genes directs the synthesis of the DNA-replicating enzyme, the exact enzyme that the drug inhibits. Multiple copies of this gene cause a pronounced increase in the production of the DNA-replicating enzyme, which in turn causes a dramatic increase in the rate of DNA replication within the cancer cells. This leads to an increase in the rate of cell division. Daughter cells from these altered cells also show multiple genes and a more rapid rate of cell division. Ironically, the very drug that stops cancer cells from dividing also has the effect of making these cells more resistant. Eventually, the chemical's inhibition of cell division in cancerous cells becomes ineffective and essentially useless.

 

Two other drugs used in the treatment of certain cancers are vinblastine and vincristine. These two drugs were discovered in the Madagascar periwinkle plant, Catharanthus roseus. Vincristine is very effective in the treatment of leukemia, and vinblastine in the treatment of Hodgkin's disease. Vinblastine doubles the chance of surviving Hodgkin's disease. Currently the only practical source of the two drugs is from this plant. However, to produce 5.0 g of vincristine, an expensive and laborious process requiring 1000 kg of periwinkle stems is used. Chemists have successfully synthesized the substances, but this is even more expensive. At present, new methods of culturing the plants are currently being developed to speed up the production of these drugs. The medical potential of Madagascar periwinkle is a good example of why conserving plant diversity is so important.