Every living
organism is built out of the smallest fundemental units of life,
cells. These cells, while very resiliant, do wear out over a period of time (differing for each cell) and others are destroyed by
bacteria,
fungi, and other
pathogens. To keep a constant supply of fresh cells, the cells that are already present must undergo a process known as the
Mitotic Cell Cycle.
The Mitotic Cell Cycle is broken into two large phases, Interphase and The Mitotic Phase, which are then broken down further into smaller subsidiary groups. But first there are a few questions that must be answered to help further understand Mitosis.
Q: What is Mitosis?
A: Mitosis is one of two processes used by cells to reproduce themsevles. (The other is called Meiosis)
Q: What does Mitosis do?
A: In a nutshell, the purpose of Mitosis is for a cell to duplicate itself, making a copy of it's DNA, organelles, protiens, and cytoplasm.
Q: What cells go through Mitosis?
A: Almost all of them. All Somatic cells go through this process. A Somatic cell is almost any cell in an organism, excluding sperm and eggs and the cells that generate them.
Interphase
The first stage of the Mitotic Cell Cycle is Interphase. Interphase is the time spent between the time a cell is actually dividing, and the cell spends most of it's time in this phase (99%). Interphase is therefor broken down into three steps that are planely visible in their destict qualities: G1, S Phase, and G2.
G1 is short of "First Gap". During this phase the cell grows in size due to the duplication of organelles and cellular protiens that will be needed during the S Phase and later Mitotic Phase.
The S Phase is the most important part of Interphase, because it is during this period that the cell actually synthesizes its DNA, which is of course the main purpose of the Mitotic Cell Cycle. This process begins by the unwinding of the DNA Strand, which exists within the cell nucleus as a helical strand. As the DNA unwinds, the hydrogen bonds that are holding the nucleotide bases together brake. A very important Mitotic Protien, called DNA polymerase, decyphers the DNA bases on each strand and joins the corresponding bases (Adenine to Thymine, and Guanine to Cytosine). When this process has finished there are two new identical DNA strands which conform to the Double Helix shape to which they are most chemically accustomed to. These structures of DNA are called Chromosomes.
Chromosomes are long and linear structures that are made out of DNA. When a cell is not splitting the chromosomal material exists as a long, thin fiber with no definite shape. Only during the final stages of Mitosis do they take on the little "winged" shape that you might be accustomed to seeing in your highschool biology books.
The final stage of Interphase is the G2, or "Second Gap". During this phase organelles continue to replicate and are prepared to be distributed between daughter cells, or the cells that will be created after cell division. Protiens and enyzmes that are unique to and govern cell division are also produced. Once all of these preparations are complete, the final stages of the Mitotic Cell Cycle begin; these take place in the Mitotic Phase.
Mitotic Phase
The Mitotic Phase is separated into five parts: Prophase, Prometaphase, Metaphase, Anaphase, and Telophase. Another part of the Mitotic Phase is Cytokenesis, although this stage is often explained as independant of the other five. Also, the five stages are somewhat disputed amongst scientists, due to the fact that, unlike interphase, these stages have no definite ending or beginning. Once the Mitotic Phase begins it doesn't stop.
During Prophase the nucleoli of the cell, which normally produces protiens, disappears. Also, Mitotic Spendles form and move to either end of the cell, which become known as poles. Microtubules, which usually make up the cytoskeleton form and span the distance of the poles.
This is the beginning of Prometaphase. Some of these fibers attach to the Mitotic Spendles on opposite ends of the cell, the others attach to the chromosomes at their centromeres, attaching to a site known as the kinetochore.
Metaphase begins as the microtubules move the chromosomes towards the center of the cells. The chromosomes, pulled tight by the microtubules, align upon a plane known as the metaphase plate.
Anaphase, the fourth part of the cycle, begins when the chomosomes are pulled apart by the microtubules, creating what are known as sister chromotids. The sister chromotids are pulled to the poles of the cells, each poll now sharing the same number of identical chromosomes as the other.
Telophase, the final step, occurs when the two daughter cells (the two cells being created by mitosis) begin to form. Daughter Nuclei form around the chromatic material, and all of the changes that occured during prophase are reversed. The nucleoli reappears, one for each daughter cell. The cellular protiens that were created during Interphase are separated and divided evenly amongst the two daughter cells, and they also split the organelles.
Cytokinesis signals the end of the Mitotic Cell Cycle. The remaining cytoplasm is split between the two daughter cells, and the cellular membrane(or cell wall for plants) closes off to literally divide the cell in half. The result of this are two identical cells, each containing the same genetic material as the mother cell from which they were made.