Lecture 22 Mitosis Notes

Lecture 22 Mitosis Notes - Lecture 22 Cell Reproduction;...

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Lecture 22 Cell Reproduction; Mitosis Campbell, Chapter 12, 5 th Ed, pp. 206-216 6 th Ed, pp. 215-224 7 th Ed, pp. 218-227 8 th Ed, pp. 228-237 LEARN THE MITOSIS FIGURE: Fig. 12.5 (5 th th Ed); Fig. 12.6 (7 th th Eds.)!! _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Introduction Reproduction of cells is one of the most important processes in biology. This is true in our bodies, where certain cells must continuously replicate to replace others that die. It’s also important for unicellular organisms, where the organism reproduces itself by cell reproduction. Cells reproduce themselves by splitting in two, in a process called cell division, that gives rise to 2 daughter cells. It doesn’t matter if the cytoplasm and organelles are split exactly in half between the two daughter cells. That is, if one daughter cell gets 5 lysosomes and the other gets 7, it’s OK. The most important thing is that each daughter cell gets the same DNA as the parental cell. This is because DNA is the genetic material; it’s the blueprint that tells the cell how to make all its proteins. If an E. coli wants to divide into 2 other cells that can do all the things that E. coli needs to do, it needs to make sure that each daughter cell ends up with EXACTLY the same DNA as in the parental cell. For this reason, the first thing that has to happen before any cell divides is that the DNA is replicated. After this happens, the cell has 2 identical copies of its genome, or total complement of DNA. One copy of the genome will end up in each daughter cell after cell division. This is true in E. coli and also in our cells. All organisms face a problem in this regard. DNA is a very long, thin molecule. For instance, each of our cells contains a total of 2-3 meters of DNA. Yet each cell is only at most a few microns in diameter. The problem of how to fit all that DNA into a cell is daunting enough. But trying to figure out how to separate the two copies of the genome, after DNA replication, so that each can be packaged into a daughter cell, is really hard. Precisely dividing the DNA between daughter cells during cell division turns out to be one of the biggest problems in biology. Bacteria solve the problem in a relatively simple way. We’ll talk briefly about how cell division occurs in bacteria. Cell division in bacteria A chromosome, in any organism, contains ONE molecule of DNA with associated proteins. Bacteria contain only 1 chromosome, so all their DNA is in one molecule. This chromosome is attached to the bacterial membrane. When the DNA replicates, each of the two identical new molecules (with associated proteins) is attached to the bacterial membrane. The bacterium then simply depends on the process of cell growth to separate the 2 chromosomes. As the bacterial cell grows, new plasma membrane and new cell wall components are inserted into the existing cell, in between the sites of membrane attachment of the 2 chromosomes. Thus, growth pushes the 2 chromosomes apart. Finally, when the cell is twice as big as its original size, it divides in half.
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Lecture 22 Mitosis Notes - Lecture 22 Cell Reproduction;...

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