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Introduction to Mitosis 1 of 29 © Boardworks Ltd 2004 How many cells? An adult human is made up of about 100 trillion cells. That’s 100 000 000 000 000 cells! Everyone started out as just one single cell. How does one cell become 100 trillion? 2 of 29 © Boardworks Ltd 2004 By cell division – called….. MITOSIS Parent cell Compare to first cell –DNA is copied nucleus Nucleus divides Cytoplasm divides 3 of 29 This animation will be explained in more detail in the slides to come. For now, just sit back and watch it. © Boardworks Ltd 2004 Mitosis Why is mitosis important? a) For growth – How else would you go from being a single fertilized egg cell to the young person you are today? 4 of 29 © Boardworks Ltd 2004 b) To replace old/ damaged cells – - did you know that we shed our skin cells about every 35 days. But don't worry, we do not shed all our skin cells at the same time like snakes do. In human beings, only the skin cells that are old are shed, others are not. 5 of 29 © Boardworks Ltd 2004 c) To repair damaged tissue -when you cut yourself, new skin cells will grow to seal the wound. These new cells come from preexisting cells found near the wound that divided many, many times. How? BY MITOSIS, of course! 6 of 29 © Boardworks Ltd 2004 Cell division and genetic information Cells don’t just split in half when they divide. If they did, important information would be lost. Think back to the “house building” anology. Carpenters, bricklayers, electricians all need the house drawings (blueprints). This information needs to be copied. Your cells have special molecules called DNA that carry the cells’ building instructions. Your DNA needs to be copied before it is transferred to the new cells. 7 of 29 © Boardworks Ltd 2004 DNA molecules can only be seen in The real thing a microscope when they are looks like this: supercoiled (condensed) – like when you take 100 feet of yarn and wrap it around and around and Copied DNA strand (chromosome) around so that it forms a small ball (the ball of yarn is in a condensed form). A DNA molecule that has been copied and has supercoiled 2 identical DNA strands are held (condensed) is called a together by a centromere chromosome. Original DNA strand (chromosome) 8 of 29 © Boardworks Ltd 2004 Find the model of a chromosome provided by your teacher. Notice the coiling? Imagine you could uncoil that molecule into one long strand. What does the elastic in your model represent? 9 of 29 © Boardworks Ltd 2004 Cell division and chromosomes Chromosomes must be accurately copied and passed on during cell division. This is important to make sure that no genetic information is lost. Compare the DNA in the nucleus of this cell with the DNA of a cell in interphase. Do you notice how the DNA molecules are shorter and fatter (more visible). We call them chromosomes now. Did you notice that not all chromosomes are the same size? 10 of 29 How many chromosomes would you expect to count if this was a human skin cell? © Boardworks Ltd 2004 What is mitosis? Mitosis begins with a single cell. How many chromosomes does this cell contain? (answer: 4) original cell First the cell makes a copy of each chromosome… …then it divides. cell division Each new cell has a full set of chromosomes and is identical to the original cell. 2 new cells 11 of 29 © Boardworks Ltd 2004