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Instruction 1-3
Basic Cell Characteristics | Nucleus, Mitochondria, Chloroplasts | Mitosis and Organism Development |
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| Mitosis and Organism Development | ||||||
| CCSTD Science Grade 7 1.e.& f. | ||||||
Every organism starts out as a single cell -- and bacteria stay that way. But most life forms are multicellular, which means that they are made up of many cells. Our bodies, for example, are made up of billions of them. Cells can only arise from other cells. The process by which new cells are made is called cell division. Cell Division
There are two different kinds of cell division as shown in the diagram above – mitosis and meiosis. Mitosis is when a cell divides itself in two to create two new cells (called "daughter cells") that are often, but not always, identical to the original cell. Meiosis is a kind of cell division in which the number of chromosomes in the original cell is cut in half – with each daughter cell getting half the number of chromosomes from the original. Meiosis only occurs in eukaryotic cells in reproduction where there are different sexes involved (like in most plants and animals). That's because the chromosomes from two parents can be combined and passed on to an offspring without increasing the total number of the offspring's chromosomes. This Instruction is primarily about mitosis. We'll talk about meiosis in an upcoming Lesson. Mitosis There are six stages in mitosis. Here's what happens in each of them: A. Early Prophase A cell receives a message that it is time to divide. So it duplicates its DNA and gets ready for division. During early prophase, the cell's DNA (which usually occurs in a loosely packed form called chromatin) condenses into chromosomes. Seen under a microscope, this transformation would look like the change from a tangled mass of thin threads into shorter, thicker units. B. Prophase Since the DNA has been duplicated, there are now two identical copies of each chromosome inside the cell. These chromosomes are bound together into identical pairs by stands of protein. Toward the end of prophase, these pairs of chromosomes begin to move toward the center of the cell. C. Metaphase During metaphase, the bound-together pairs of chromosomes line up at the middle of the cell and prepare to split apart. D. Anaphase Now, during anaphase, the protein that bound the chromosome pairs together begins to break. As this happens, one of each of the formerly paired chromosomes goes to one end of the cell and the other goes to the other. At the end of anaphase, the cell has split the copies of its genetic material into two separate populations (groups) and is ready for telophase. E. Telophase Once the chromosomes have reached opposite ends of the cell, telophase can take place -- which is essentially the reverse of prophase. A new membrane now folds around each of the groups of chromosomes (at opposite ends of the cell) and two new cells are created -- identical daughter cells of the original. F. Interphase Interphase is the "holding" stage or the stage between cell divisions. Most cells spend most of their time in interphase. While the word "interphase" might sound passive, a number of processes are still going on. If viewed under a microscope, the cell may appear dormant but its biochemical activity is still quite high. What Triggers Cell Division? As we said, mitosis begins when a cell gets the message that it's time to divide. What triggers this message? Scientists have been searching for the answer for centuries. It now appears that the urge to divide is triggered by chemical stimuli -- specifically by proteins. Although we know some of the proteins and genes involved, parts of the process are still a mystery. Cell Differentiation Sometimes when a cell divides, the daughter cells are different from the original cell in that they now have different functions. This process is called cell differentiation and is most dramatic during the time an animal is growing before birth or hatching. Muscle cells, liver cells, bone cells, skin cells, neurons and so on are all examples of cells that have specialized functions and are called differentiated cells. Scientists don’t fully understand how cells “know” that they are to take on a specialized function. They believe that there are special proteins that control what the cell will become. Video Instruction
Now let's doPractice Exercise 1-3 (top).
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