Mitosis & Cytokinesis
14/10/2010 § 1 Comment
In class, we went over the final stages of the cell cycle, finishing the sixth chapter. Today, we learned about Mitosis and Cytokinesis. We started off by reviewing chromosomes. They are made of one chromatid (before synthesis) which are made of chromatin, which are the little coils that hold nucleotides, DNA and/or information. During the G1 phase and before synthesis, the chromatin—in its coils—spread out so that it can function and make proteins for the cell and the body. To express the number of chromosomes before synthesis, we express the number with the common variable x. After synthesis, however, we know that the number of chromosomes are copied once, therefore the number is double: 2x. After mitosis, cytokinesis and telophase—after the cycle has finished, because the two chromatids of the chromosomes have split, a chromosome in either of the two daughter cells has only one chromatid each. Therefore, the quantity of DNA in a cell is divided by two (after the cell was divided by two) and the number is expressed with only an x.
The next phase after the entire interphase is mitosis. It was repeated in class, so I guess I should also repeat it on the blog. Mitosis does divide and it divides something very important to the cell: the nucleus. In mitosis, the cell itself is not divided and the two daughter cells have to wait one more phase before they can separate. During mitosis (and leading into cytokinesis), there are four mini phases that separate chromatids in the chromosomes. The first phase is the prophase. It is the longest phase of mitosis with five different steps or stages.
The first step, the chromosomes condense. Before they condensed, they were still coiled up and functioned to make proteins and through a microscope, look stringy and wispy. After they condense, the chromatids are concentrated a little, preparing for the next steps. This step is kind of like actors tensing for a performance. They become visible as they coil up.
In step two, the nucleolus disappears. This part of mitosis is described as if the lights are going out and the curtains are going up during a performance. The main act is about to start. The nucleolus disappears from around the chromosomes.
The third step dissolves the nuclear membrane of the cell, freeing the chromosomes completely.
During the fourth step, the spindle apparatus forms. Spindles are cellular structures made out of both microtubule fibers and centrioles that play a large role in moving the chromosomes. The spindle fibers grab the chromatids later and help separate them from their centromeres and 2x chromosomes.
The last step is when the centrosomes, proteins that formed during the cell’s interphase, while it was preparing for mitosis and cytokinesis. The centrosomes (in an animal cell’s case, the centrioles—which are in the centrosomes, according the textbook, p. 128) move to opposite poles of the cell, situation themselves for the next step.
Afterwards is the metaphase. The chromosomes line up along the equator (the middle) of the cell.
Next, the anaphase is the time when the chromosomes move towards opposite poles of the cell. The spindle fibers have taken hold of the centromere and pull one chromatid from the 2x chromosome to each side of the cell.
Finally, the telophase occurs as the reverse of the prophase. The cell de-condenses and kind of ‘relaxes’ to move around and spread its organelles out again. The spindle fibers disintegrates and the nucleus membrane and nucleolus reforms again. Here, the curtain goes down and lights come back on because the show has ended. What’s a little difficult to understand is when telophase starts and stops because cytokinesis also occurs at the same time.
Cytokinesis divides the cell itself and the cytoplasm. Afterwards, two identical daughter cells have been produced. Cells divide differently depending on what kind of cell they are. In an animal cell, because it grew during interphase and is large enough, the membrane pinches inward and releases to form two daughter cells. Plant cells, however, have a cell wall and can’t pinch into the membrane, therefore a new cell wall must be formed between the two daughter cells from Golgi apparatus vesicles. They form a cell plate to separate the two daughter cells.
I think today was when we really finished the cell cycle unit. If I go over this topic a little bit more and understand it better, then I can choose between topics to answer Why sex? for the essay question.