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Purposes of Mitosis
Purposes of Mitosis
Mitosis is the process by which one cell splits into two identical cells that are clones of the original cell. This is a form a asexual reproduction, and can have very diverse applications in a multicellular organism. The primary purposes of mitosis in a multicellular organism are for growth and repair.
When your were young and smaller, it's not that all of your cells were smaller, but, rather, you had far fewer cells. How did you go from a single cell as a zygote to trillions of cells today? Via mitosis. One cell becomes two, two becomes four, four becomes eight, and so on.
The other primary purpose of mitosis, repair, comes into play anytime a multicellular organism loses some of its cells and needs to replace them. This is often showcased with a simple cut or other injury - you can track the formation of new cells to restore those that were lost.
Stages of Mitosis
Stages of Mitosis
Mitosis, like all complicated topics in this course, is comprised of multiple steps. These steps are prophase, metaphase, anaphase, telophase, and cytokinesis. There is often considered to be a prometaphase, and intermediary phase between prophase and metaphase (hence the name). We will not include that phase for this course.
A. Prophase
A. Prophase
Prophase is the beginning of mitosis (pro- means 'before'). Prophase consists of the following:
Chromosomes supercoil and become visible
Centrioles move to opposite poles
Microtubule spindles form between the centrioles
Nucleolus and nuclear membrane start breaking down so the DNA can be accessed
One nice thing about the stages of mitosis is that they are all just logical steps if you know the inputs and outputs of mitosis. We know we have one cell to begin with and we need to end up with two identical cells. So everything must be doubled! The DNA has already been doubled during the S phase of interphase, but you will see, by the end of mitosis, two nuclei, for instance.
B. Metaphase
B. Metaphase
Metaphase is typically remembered because it starts with 'm', as does middle. This is relevant because the chromosomes will be moved to the middle of the cell by the spindle microtubules.
Sister chromatid pairs (the x-shaped chromosomes) will align with each other along the equator of the cell.
The orientation and placement of the chromosomes is random - this isn't super important right now, but it will be when we get to meiosis.
To summarize metaphase:
Spindle microtubules fully develop and attach to the centromeres
Sister chromatid pairs (aka chromosomes) align along the equator of the cell
The orientation of each pair is random
C. Anaphase
C. Anaphase
Anaphase involves the separation of the sister chromatid pairs into separate chromosomes. This is where the language gets a little confusing. The X-shaped structure (two sister chromatids connected at a centromere) was a single chromosome.
Once these chromatids are separated, they each become their own chromosome! So the cell in the bottom image actually has twice as many chromosomes as the top cell!
To summarize anaphase:
Microtubules contract and split the sister chromatids at the centromere
Each chromatid is now considered a chromosome
Microtubules pull the chromosomes (what used to be chromatids) to opposite poles of the cell.
D. Telophase
D. Telophase
Telophase is often considered to be the final stage of mitosis. Now that our sister chromatids have separated into chromosomes, they will move to opposite sides of the cell, elongating it until it looks like two cells stuck together.
The cell will also reform some nuclear membranes around the chromosomes and they will start to uncoil some. This is technically the end of mitosis!
To summarize telophase:
Chromosomes arrive at each of the poles and microtubules break down
Nuclear membranes begin to reform around the chromosomes
Chromosomes uncoil and become invisible through light microscope
E. Cytokinesis
E. Cytokinesis
Cytokinesis is the last bit of the mitotic processes that you must know. This is essentially when the cells separate and officially become two cells. These cells have identical DNA to one another, so are considered to be identical daughter cells to the original 'parent' cell. A cleavage furrow forms as part of cytokinesis, forming that figure-8 shape of the two cells connected in the top image.
An easy way to image this occurring at a scale with which we are comfortable could be twisting a balloon. If you get a water balloon made of the right material, you can actually fill it and twist it along its middle over and over and over again until it eventually separates into two water balloons. The plasma membranes of the cells, just like the rubber of the balloon, forms a seal around what would be a hole!
Cancer - Unregulated Cell Growth
Cancer - Unregulated Cell Growth
Cancer is the ultimate example of this process gone wrong. Mutations, caused by a variety of possibilities, lead to this cell division continuing over and over again without proper halting for checkpoints. This is how a tumor is formed - one cell goes rogue, duplicates itself (and its DNA) when it shouldn't. Its identical offspring do the same, and so on until a large mass of cells (the tumor) exists.
"That one of the most elemental diseases in human history happens to arise from the corruption of the two most elemental processes in biology is not a co-incidence: cancer co-opts the logic of both evolution and heredity; it is a pathological convergence of Mendel and Darwin. Cancer cells arise via mutation, survival, natural selection, and growth. And they transmit the instructions for malignant growth to their daughter cells via their genes. As biologists realized in the early 1980s, cancer, then, was a “new” kind of genetic disease—the result of heredity, evolution, environment, and chance all mixed together." (The Gene: An Intimate History by Siddhartha Mukherjee).
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