GED Science Practice Test: Mitosis
Eukaryotic cells do not have a single, circular chromosome, instead, they often have pairs of chromosomes. For example, humans have 46 chromosomes (23 pairs), pigeons have 80 chromosomes (40 pairs), and fruit flies have 8 chromosomes (4 pairs). Because eukaryotic cells have two of each kind of chromosome, they are referred to as diploid.
Additionally, eukaryotic cells have their chromosomes contained within a membrane-bound nucleus. These differences between prokaryotic and eukaryotic cells mean that eukaryotic cells cannot divide through binary fission. Instead, eukaryotic cells divide through a more complex process called mitosis. Mitosis occurs as part of the cell cycle, the overall cycle through which a cell grows, replicates its DNA in preparation for division, grows some more, and then finally, divides. The cell cycle has checkpoints after growth and DNA replication stages that help to ensure that the cell has properly completed all parts of this step prior to moving on to the next step. Sometimes, all of the stages in the cell cycle except mitosis are called interphase. During mitosis, the chromosomes duplicate in number, the nucleus divides, and the organelles and structures in the cytoplasm divide to form two daughter cells. The following image shows the cell cycle:
Before mitosis occurs, the DNA in a eukaryotic cell is duplicated. For example, humans have 46 chromosomes in most of the cells in their bodies. After DNA synthesis (see “S phase” in the cell cycle diagram above), humans have 92 chromosomes, 4 copies of each individual chromosome. Mitosis is the complex process that divides those chromosomes up properly so that each daughter cell receives exactly 2 copies of each of the 23 chromosomes. When the cell is not undergoing mitosis, however, the chromosomes are not visible as separate structures. Instead, the chromosomes are all stretched out. When the genetic material is stretched out like this, and not visible as separate structures, it is called chromatin.
The process of mitosis proceeds through a series of stages. During the first stage, prophase, the chromatin condenses into the visible chromosomes, the nuclear membrane disappears, and a spindle forms, which is made of microtubules. Remember that microtubules help move substances within a cell. Microtubules attach to the chromosomes during prophase. The spindle will later serve to pull the duplicated chromosomes apart.
During the second stage, metaphase, the chromosomes line up along the center of the cell in preparation to be pulled apart. In anaphase, 2 copies of each chromosome begin to move to opposite ends of the cell. In the last stage, telophase, the chromosomes relax back into chromatin, nuclear membranes reform, and the cell beings to pinch in to split into two cells. This final process of physically splitting into two cells is called cytokinesis. The following diagram shows the stages of the cell cycle, including interphase (which is not a part of mitosis), as well as the four stages in mitosis.
Like binary fission for prokaryotic cells, mitosis results in daughter cells that are genetically identical to each other, as well as to the parent cell. Mitosis can be used to replace aging cells with genetically identical new cells, but it can also be used as a method for whole organisms to produce genetically identical offspring, or clones. The production of genetically identical offspring is called asexual reproduction, because it only requires only one parent. One such example of this is when plants bud or form new plants through runners, which grow underground. The resulting plants are identical to the original plant. Binary fission is also considered asexual reproduction, because it produces genetically identical offspring. Bacteria providing through binary fission produce genetically identical offspring bacteria.
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Cancerous tumors can result when cells in multicellular organisms begin dividing too quickly, resulting in daughter cells that have abnormal numbers of chromosomes and other atypical structural features. What is the most likely explanation for how cancerous tumors can sometimes form?