Mitosis

Where is mitosis happening?

I had pizza last night.  And then had heartburn.  But I bit into the pizza and instantly burned the roof of my mouthThe opening of the digestive tract where food enters and mastication begins. and my lip. I killed some of the skinThe body’s largest organ, providing protection and regulation. cellsThe basic structural and functional units of life. of my lip. I also killed some of the cells in my mouth.  Now they need repair.

Part of that repair will involve replacing the damaged cells. Those replacement cells will be provided by copying some of the nearby, undamaged cells.

Luckily this is a small wound. It won’t take long to heal. However, it still involves producing thousands of new cells through cellular reproductionThe process of producing offspring., also called cell division. At my age the principle function of cell division is the repair and replacement of damaged cells.

Another function of cell division is growthAn increase in size and number of cells..  You are built from approximately 60 trillion cells. Every one of those cells can trace its origin back to a single fertilized egg. This egg formed about nine months before you were born.

Every cell in your body today was formed via cellular reproduction starting from that first cell.  Cell division ensures that the set of genetic instructions passed on to each of your cells is complete and identical.

Different kinds of cells divide at different rates depending on their role. Some human cells (like your skin cells and the lining of your digestive tract) divide every day. Others, like nerve cells, are so specialized that they never divide at all. 

This may help explain why brain injuries and nerve damage may never heal. Cells that do divide follow a sequence of steps known as the cell cycleThe sequence of events in a cell’s life, including growth, DNA replication, and division..

Beginning Mitosis

This slide emphasizes the necessity of understanding DNA and the cell cycle. This knowledge is crucial before advancing to mitosis. Recall that the cell cycle has two phases: interphaseThe phase of the cell cycle in which the cell grows, performs its functions, and replicates DNA. and a cell division phase that is mitosis or meiosis.  When a cell enters into mitosis it has already been through all three sub phases of interphase. It is required that chromosomes are in a duplicated state with the sister chromatids attached at the centromereThe region of a chromosome where sister chromatids attach and spindle fibers bind.. 

This duplication occurs in the S phaseThe DNA synthesis phase of the cell cycle where chromosomes are replicated.. It is so named because it refers to the synthesis of new DNA. It also involves the sister chromatids. We are approaching a cell division phase. This means that the chromosomes will be condensed enough to be seen with a light microscope. Remember that in interphase you cannot see the chromosomes because they are being duplicated or being used to make proteinsLarge molecules made of amino acids with various functions in the body.. Along the bottom of this slide, I have 3 cells. The first cell just has four chromosomes. They are in an unduplicated state. The middle cell has four chromosomes in their duplicated state with sister chromatids attached but uncondensed. The last cell has four chromosomes in their duplicated state and condensed ready for mitosis.

Mitosis Phases

The phases of mitosis can be split into 5 or 4 phases. It depends on your teacher’s preference and the author of your book. I recognize 4 phases and let’s just list them right here: all things metaphaseA stage of mitosis where chromosomes align at the metaphase plate. anaphaseA stage in mitosis or meiosis where sister chromatids or homologous chromosomes separate. and telophaseThe final stage of mitosis where nuclear envelopes reform around separated chromosomes.. To distinguish these phases, you can look at a few features. These include the size of the cell, the extent of the mitotic spindleThe structure made of microtubules that separates chromosomes during mitosis., and the placement of the chromosomes. When a cell approaches mitosis with its chromosomes in a condensed state it enters prophaseThe first stage of mitosis where chromatin condenses into visible chromosomes.. In prophase, the nuclear envelopeA double membrane that surrounds the nucleus and separates it from the cytoplasm. disappears. This is why you see a dotted line in this first picture. The central songs move to opposite ends of the cell. The mitotic spindle starts growing some fibers.

In the second picture here, we can see the most defining characteristic of metaphase. The chromosomes are lined up in this orientation along an imaginary line called the metaphase plateThe imaginary plane in the center of the cell where chromosomes line up during metaphase.. If alignment is not correct in metaphase, it could result in two cells. These cells may not be identical copies of each other. The third phase here is called anaphase. This phase is actually quite energyThe capacity to do work or cause change. intensive. It may look like nothing is happening when you view it under the microscope.

Anaphase is basically a tug of war period. During this phase, the spindle fibers attached to the centromeres start to break the centromere apart. They pull the sister chromatids to opposite ends of the cell. The last phase which is called telophase is basically just the opposite of prophase. The chromosomes become dispersed. The mitotic spindle starts to disappear. The nuclear envelope begins to form, indicated by the dotted lines. This structure here between the two potential cells develops. In an animal cell this is called a cleavage furrowThe indentation in the cell membrane that forms during cytokinesis in animal cells.. 

The cleavage furrow will separate these two cells entirely in a process that is called cytokinesis. Many students wrongly think that telophase and cytokinesis are the same phase.  Telophase results in the complete separation of the genetic material that is potentially within the nuclear envelope. Cytokinesis separates the contents of each cell. It involves all the stuff that is not in the nucleusThe control center of the cell that contains DNA and directs cellular activities..

Mitosis Checkpoints

Just as there are checkpoints in the cell cycle, there are also checkpoints specific to the phases of mitosis. This diagram here shows you some common issues with the metaphase plate. It also highlights the mitotic spindle. These are the two most common things resulting in mitosis errors. Something as simple as one extra centriole in your centrosomesThe region near the nucleus that contains centrioles and helps organize the cytoskeleton. can lead to the creation of multipolar spindles. The picture up top illustrates this phenomenon. A cell encountering multipolar spindles would be forced into apoptosisProgrammed cell death, an essential process for growth and development. or cell suicide before it could complete mitosis. There are pro hands that detect division errors and turn on the genes for apoptosis.

Mitosis Mistakes

Although there are checkpoints there are some very common mistakes that consistently happen in mitosis. Most of them have to do with alignment on the metaphase plate. These mistakes are called non dysfunctional mistakes. Try to think of the resulting 2 cells from each of these mitosis mistakes that you see here. Basically, these types of mistakes do not result in cells that are clones of each other. Usually, one of the cells has more genetic material than the other cell.

Cancer

Cancer is an overproduction of cells resulting in a tumor. This would be the cell cycle completely out of control.  Cells that are not meeting the checkpoint criteria are not being removed before they have the chance to complete mitosis. This results in a cancerous for precancer is cell making many copies of itself resulting in a tumor. These cells will be misshapen and they will not serve their intended purpose. As these mutated cells continue to divide unregulated they will amass into a tumor. If the tumor becomes large enough, fragments will start to split from the original mass. These fragments will enter into the bloodstream and become deposited in other tissues. This is what metastasis is.

Most cancer drugs target some aspect of mitosis and prevent it from happening. For example, many cancer drugs prevent the mitotic spindle from forming. They prevent the nuclear envelope from breaking down. They also prevent the cleavage furrow from performing cytokinesis. In all of these instances the cells that are trying to divide will then be removed by apoptosis. The problem with cancer drugs is that many of them are systemic. This means they affect all of the tissues in your body, not just the dividing tumor. While under the treatment of these drugs, new cells in the mouth will not form. New cells in the digestive system will not form. Additionally, new cells on any epithelial tissues, such as those lining the bladderA muscular organ that stores urine before excretion., will not form. Therefore systemic cancer drugs prevent all acts of mitosis in the body leaving the patient open to immune system intrusion. There are other methods of cancer treatment one being surgery the other being radiation. These methods are not systemic. However, they have just as long a cons list as some of the more general cancerous drugs.