As we have been looking at the nucleus for the past couple of weeks, I realized that so many of the functions of the various parts of the nucleus are involved in cell division. Therefore, before we move on to other parts of the nucleus, I thought it best to cover the processes of cell division before moving on.
There are two types of cell division: mitosis and meiosis. This week we will take a simplistic look at mitosis and next week we will cover meiosis.
Mitosis occurs in both prokaryote (without a nucleus) and eukaryote (with a nucleus) cells. Since we have been discussing the nucleus of the cell, we will only look at mitosis involving eukaryote cells. In mitosis, the original cell, often referred to as the mother cell, creates a duplicate set of chromosomes and then separates the chromosomes, nucleus, cell organelles, cytoplasm and cell membrane to form two identical daughter cells.
Prior to the onset of mitosis, the cell undergoes a pre-mitotic stage known as interphase. Once interphase has ended mitosis begins. The process of mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase and telophase.
During the interphase stage, the DNA, which is still contained in the loosely coiled chromatin, replicates itself. The cell expands as it produces more proteins and organelles in preparation for cell division. .
In the first stage of cell division the chromatin with the replicated DNA is now visible under a high power microscope as it becomes more condensed and organized as it forms into chromosomes. The centrosome, located outside the nucleus, replicates and the two new centrosomes begin to move towards opposite ends of the cell. At the same time, the centrosome send out a network of fibers made up of microtubules, which form the mitotic spindle.
In this stage of cell division, the spindle fibers extend into the nuclear area as the nuclear membrane disintegrates. A protein structure called a kinetochore forms at the centromere. The spindle fibers begin to attach to the kinetochores.
The chromosomes arrange themselves in the center of the cell forming the metaphase plate. The metaphase plate is an imaginary line equidistant between the two centrosomes. Unattached kinetochores send out a signal that blocks the onset of anaphase. Once the last kinetochore has been attached, the signal stops and anaphase begins.
In this stage the two chromatids that are bound together are separated. The spindle fibers attached to the kinetochores begin to shorten, pulling back towards the centrosome. Then other spindle fibers pull the centrosomes and their attached chromosomes apart and towards opposite ends of the cell.
In the final stage of cell division, the spindles that pulled the chromosomes apart and away from the center of the cell disappear and the nuclear membrane forms around each new set of chromosomes. As the two new nuclei pull away from each other, the rest of the cell is pinched in two and separates forming two new daughter cells.
Some sources list a process called cytokinesis which is suppose to be separate from mitosis yet occurs simultaneously with telophase. Other sources list cytokinesis as part of telophase. Cytokinesis in animal cells involves the formation of a contractile ring around the area of the cell where the metaphase plate had been. The contractile ring continues to contract until it pinches off the membranes and completely separating the two daughter cells.
In conclusion on mitosis, I just want to ask one simple question. How did the first cell evolve this complex orchestrated process of mitosis from random chance processes in order to duplicate itself before it died trying?