26/09/2012 § 1 Comment
Though we basically covered the majority of this information in class today, this blogpost will outline what happens during the four different stages of mitosis. As a review, mitosis is nuclear division. An organism’s DNA is duplicated not during mitosis but during the S-stage of the Interphase. Mitosis divides the nucleus and usually follows with cytokinesis, which is the division of the cytoplasm.
The first stage of mitosis is the prophase. In the prophase, chromosomes become short and fat and they do this by supercoiling. The nuclear membrane breaks down near the end of the stage. Vital structures called microtubules grow from opposite poles of the cell via the MTOC (microtubule organizing centre) and reach out to the chromosomes. The microtubules are called the mitotic spindles because they form a spindle-shaped structure.
The next stage of mitosis is the metaphase. In the metaphase, the microtubules attach to the centromeres of the chromosomes. These chromosomes align themselves in the equator of the cell, or the middle of the cell. Microtubules from both poles will attach themselves to one sister chromatid of each chromosome.
The third stage of mitosis is the anaphase. In the anaphase, the microtubule fibers start to pull the sisters chromatids away from each other and towards opposite poles of the cell. Mitosis can create two genetically identical nuclei because of the genetically identical sister chromatids that get pulled to the opposite sides of the cell – the microtubules’ connection on the centromere makes sure of that. The entire time that the chromatids are moving towards the poles of the cell is part of the anaphase stage.
The last stage of mitosis is the telophase. In the telophase, a nuclear membrane starts to form around the new chromosomes (previously the chromatids). A cleavage furrow forms on the cell’s surface and the cell begins to divide.
The cell divides afterward through cytokinesis and the two new daughter cells can restart the cycle and enter interphase.
Touched upon briefly when we discussed tumors last time, cancer is a huge concern when it comes to cell division. Once again, when a cell doesn’t stop dividing, creating an excessive amount of new cells that create a tumor. These tumors are caused by carcinogens, viruses, and overexposure to ultraviolet light. Malignant (uncontrollable) cancers can detach and move to the different parts of the body and start new tumors.
DATA BASED QUESTIONS
Page 40 centromeres and telomeres
1. Deduce the stage of mitosis that the cell was in, giving reasons for your answer.
The squashed cell in the image was in the metaphase stage of mitosis. This is because we can already see that the chromosomes have coiled to become shorter and fatter. The deciding factor however was that the chromosomes are all starting to fall into place and position themselves in a line along what we can assume is the equator of the cell.
2. The cell has an even number of chromosomes.
(a) State how many chromosomes there are in this cell.
(b) Explain the reason for body cells in plants and animals having an even number of chromosomes.
This is because in eukaryotes (animals and plants), the resulting cell that comes from sex is given its set of DNA by two parents, which will give the resulting cell 2n chromosomes – an even number.
(c) In the micrograph of a cell in interphase, the centromeres are on one side of the nucleus and the telomeres are on the other side. Suggest reasons for this.
The positioning of the centromeres and telomeres on opposite sides of the nucleus have to do with making sure that later, each sister chromatid of a chromosome will be able to get to each pole of the cell. Also, the centromeres and telomeres have different polarities, which is why the centromeres stick to a chromosome’s middle while the telomeres stick to the ends. This polarity is what repels them from each other.
(d) An enzyme called telomerase lengthens the telomeres, by adding many short repeating base sequences of DNA. This enzyme is only active in the germ cells that are used to produce gametes. When DNA is replicated during the cell cycle in body cells, the end of the telomere cannot be replicated, so the telomere becomes shorter. Predict the consequences for a plant or animal of the shortening of telomeres.
The coiled chromosomes will not be capped and protected at the ends and might risk uncoiling. This might damage the DNA and interrupt the mitotic process. But that’s just my opinion.
My question now is… what is the purpose of a telomere and why have we not studied them along with centromeres?