Mitosis-- Movie Narrative (Advanced Look)
Cell division is required for an organism to grow, mature, and maintain tissues. During the mitotic phase, a cell will undergo mitosis to form two new nuclei and then divide to form two new individual cells during cytokinesis.
Mitosis is the process of dividing the duplicated DNA of a cell into two new nuclei. Mitosis is split into distinct stages. The first stage is prophase; the DNA condenses, organizes, and the classic chromosome structure appears. Next comes prometaphase where microtubules attach to the chromosomes. This step is followed by metaphase where the chromosomes align. Metaphase is followed by anaphase where the chromosomes separate. Finally, during telophase nuclear membranes reappear around the two sets of chromosomes. Mitosis is now complete. After mitosis two new cells are formed by a process called cytokinesis.
Mitosis is only one part of what is called the cell cycle. For many eukaryotic cells, a cell is duplicated every 24 hours. Most of the life of a cell is spent in interphase. Interphase consists of three stages called G1, S, and G2.
G1 (or Gap 1) is the first growth stage of interphase. In G1, the cell grows to nearly its full size and performs many of its specific biochemical functions that aid the organism.
Next is the S (or synthesis) phase. This is an important stage, because it is during the S phase that DNA in the nucleus is replicated.
The cell next enters another growth stage called G2 (or Gap 2). It is during G2 that the cell finishes growing. Once the cell has duplicated DNA in the nucleus, and two centrosomes have appeared in the cytoplasm, mitosis can begin. For a typical eukaryotic cell this will last about 80 minutes.
During the first stage of mitosis, called prophase, we first see the classic chromosome structure. This occurs through a condensation process. At the same time, protein strands called microtubules appear from the centrosomes in animals. Finally, a structure found within the nucleus, the nucleolus, disappears.
Next, prometaphase begins when the nuclear membrane is broken down. At the same time, microtubule strands, or spindle fibers, are growing from the centrosomes. These strands attach to a protein structure called the kinetochore. One kinetochore is attached to the centromere of each sister chromatid.
Next comes metaphase. During this stage the sister chromatids align along the center of the cell so that both chromatids face toward opposite poles of the cell.
Now the sister chromatids are ready to be separated. This occurs during anaphase through a shortening of the microtubules attached to the kinetochores. Additionally, the poles of the cell move farther apart and cause increased separation of sister chromatids. At the end of anaphase, the sister chromatids have moved to the two ends of the cell.
Telophase is the final stage of mitosis. It is here the components of the new cells begin to appear. At this point the spindle fibers are broken up. A new nuclear membrane surrounds the chromosomes at the end of each cell. And the chromosomes uncoil and return to an uncondensed state. Mitosis is now complete. The formation of two cells is all that remains.
Following mitosis, the cell undergoes a process called cytokinesis. First the cell is compressed by a contractile ring that divides the cell in nearly equal halves. By now the organelles in the cell have been replicated, and are now divided between the two halves of the cell. This includes mitochondria, golgi bodies, and the rough ER. Plant cells also have chloroplasts. Once split, the two new cells are now fully in the G1 stage of interphase and ready again to begin their growth.
Let’s watch the process one more time. Mitosis begins with prophase. Notice the DNA condensing into chromosomes during this stage. Microtubules appear during prometaphase, and the nuclear membrane breaks down. Metaphase occurs when the chromosomes are aligned at the center of the cell. During anaphase the chromosomes are moving apart. The telophase stage is marked by the appearance of new nuclear membranes. This is the end of mitosis.
Finally, the splitting of the cell occurs during cytokinesis. The two new cells are now ready to grow and perform their specialized functions.