Like any other living organism, a cell exists to perform the primary duties of growing and reproducing. The stages in the life cycle where these duties are executed can be broken down into two main categories… Interphase and Mitosis. While the marvel of cell division and duplication that occurs in Mitosis gets most of the attention, it only takes up about 25% of the total life cycle. The rest of the time, the cell progresses through “everyday life,” growing and performing its duties, as well as preparing itself for another round of reproduction. This stage, called Interphase, contains four “sub” phases within itself which perform all the functions of healthy cell life… G0 phase, G1 phase, S phase, and G2 phase.
In G1 (Gap1) phase, the cell will synthesize protein for DNA, produce organelles, and perform its own unique functions. Sometimes, its growth may be limited by factors such as insufficient nutrients or inappropriate temperature. This phase also determines whether a cell commits to division or to leaving the cell cycle.
There are times when a cell will remain undivided, in which case it will leave the G1 phase, not go to S phase, and enter a phase called G0. Cells enter G0 for two reasons… to remain dormant, or because they have been found to be aging or deteriorating. In G0, there is no growth or cell duplication, and cells that have damaged DNA will not re-enter the cell cycle, but are filtered out so as not to corrupt the duplication of daughter cells. Dormant cells, however, may re-enter the cycle when they are needed and proceed on to S phase.
The preparation for Mitosis happens in S phase, where cell duplication begins. In this phase, the cell duplicates the DNA instructions, then assembles the newly replicated DNA into a complex of very large macromolecules called chromatin. The job of chromatin is to prepare DNA for Mitosis, protect the DNA from damage, and to control the replication of DNA.
DNA is replicated when it is copied in the chromatin, then spiraled up to form two chromatids. The 2 copies of each chromatid are then joined together by a centromere to form a chromosome in the shape of an X. So, in a replicated chromosome, two chromatids are joined together in the middle by one centromere, and the amount of DNA in the cell has effectively doubled.
Every cell has a centrosome, with two centrioles, which is an organelle near the nucleus of a cell from which the spindle fibers, called microtubules, develop during cell division. In S phase, the centrosome replicates, becoming a microtubule organizing center. These microtubules will be further synthesized in the final stage of Interphase which is the G2 phase. A cell in G2 continues to grow and prepare for entry into Mitosis.
During Interphase, a cell will go through transitions, or checkpoints, and the process of every transition will be intensely monitored and regulated. This is necessary, because every event depends on the successful completion of earlier events. Major transitions occur at the end of G1, before entering S phase, and the end of G2, before entering Mitosis. As the cell passes through these checkpoints, the integrity of the DNA will be assessed for improperly or partially replicated DNA. It will also be checked for mutated cells and uncontrolled cell division that may lead to diseases like cancer. If any of this occurs, the cell cycle will pause until the problem is fixed.
In the Mitosis stage of the cell cycle, the mother cell divides into two identical daughter cells. Mitosis contains four main “sub” phases called Prophase, Metaphase, Anaphase, and Telophase. The final separation of the new cells is called Cytokinesis.
During prophase, the chromatin in the chromosomes packages more tightly and condenses into shapes that are visible through very high magnification. The centrosomes that were duplicated in S phase migrate to opposite poles of the cell and begin to form a microtubule spindle apparatus. The spindle formed between the centrosomes pushes them outward toward the opposite poles of the cell. At this point, the membrane around the cell nucleus dissolves to release the chromosomes.
In Metaphase, the centrosomes are at opposite ends of the cell, with their spindle fibers attaching to the centromeres of each chromosome. The spindle fibers neatly line the chromosomes up end to end along the center line of the cell. This action is initiated and controlled by kinetochores, which form the interface between the chromosomes and the microtubules of the mitotic spindle. A kinetochore is a structure that forms on the centromere during late prophase that binds to the microtubules.
The mitotic spindle consists of kinetochore microtubules and polar microtubules. Kinetochore microtubules connect to the centromeres of the chromosomes in order to separate the chromosome’s two chromatids. Polar microtubules from each centrosome connect to each other to exert a pushing force that elongates the cell for future separation.
The two chromatids that form a chromosome are called sister chromatids. During Anaphase, the sister chromatids are pulled apart as the kinetochore microtubules attached to the centromere draw one chromatid to one pole and the other chromatid to the opposite pole.
In telophase, the polar microtubules continue to lengthen, and a full set of chromosomes gather together at each pole, with their own centrosome. A new membrane forms around each set of separated daughter chromosomes and the nucleolus reappears. However, the membrane does not enclose the centrosomes. Both sets of chromosomes are now surrounded by a new nuclear membrane, and the stage of cell division, or Mitosis, is complete.
A final stage called Cytokinesis completes the process with a cleavage furrow that pinches the cell in the middle to form two separate and identical daughter cells.
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