lecture 10 for BGYA01

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Biological Sciences
Clare Hasenkampf

BGYA01 L ECTURE 10 O COTBER 16, 2007 Last class we looked at the cell cycle and at the different levels of chromsome packaging that occur at the different times in the cell cycle. We also discussed some of the terminology for Genetics and chromosomes Today we want to consider cell division and get into the details of nuclear division. THE PROCESS BY WHICH ONE CELL DIVIDES TO GIVE RISE TO TWO DAUGHTER CELLS IS KNOWN AS CELL DIVISION . Cell division is divided into two parts- division of the nucleus (either mitosis or meiosis) and division of the cytoplasm (which is known as cytokinesis) THERE ARE TWO TYPES OF M PHASES- M ITOSIS AND MEIOSIS. WE LL LOOK AT MITOSIS FIRST. We divide mitosis into four phases PROPHASE , M ETAPHASE , ANAPHASE AND TELOPHASE . IGURE 9.10,PAGE 190-191. PROPHASE THERE ARE 6KEY EVENTS IN PROPHASE th th 1. The chromosomes begin to thicken and shorten, as they accomplish the 4 & 5level of packaging. Transcription stops. 2. The nucleolus (a small round structure within the nucleus) disassembles. Why? It is because RNA is such a major component of the nucleolus that without RNA synthesis it falls apart. 3. Sisters chromatids become distinct and separate, except in the region of the centromere. The pinched in region of the chromosome is the centromere region. Figure 9.11 (page 191) 4. Each sister chromatid organizes a structure known as the kinetochore at the centromere region. The kinetochore can be seen it in figure 9.9, page 189. The kinetochore is a set of proteins that form around the centromere region, and later will attract microtubules. 5. During prophase a region called the Microtubule Organizing center MTOC duplicates and the two MTOCs move to opposite sides of the cell. Microtubules grow outward from the MTOCs. [MTOC stands for MicroTubule Organizing Center] Note: In animal cells you can see centrioles at the MTOC, but in plants the MTOCs have no obvious visible structure. 6. At the end of prophase the nuclear envelope breaks up into vesicles. The nuclear envelopes break-up signals the end of prophase. www.notesolution.comMETAPHASE T HE NEXT STAGE IS KNOWN AS METAPHASE . By the end of metaphase, the chromosomes will be lined up in the middle of the cell (called the metaphase plate, Figure 9.10, panel 4, page 191), with sister chromatids facing opposite poles. Lets see how this is accomplished. When the nuclear envelope breaks down this allows the mitotic spindle to form. The mitotic spindle is an organized collection of microtubules. Microtubules start out from the MTOCs (which are at opposite poles) and grow towards the middle of the cell. Microtubules that meet other microtubules in the mid-region stabilize each other and become the polar microtubules. (note figure 9.9 is inaccurate, they dont show any microtubules overlapping each other.) The polar microtubules interact to form an elaborate support structure. Other microtubules also starts out from the poles but get captured by the kinetochore region of the chromosomes. We call these kinetochore microtubules. Each sister chromatid has a kinetochore and each kinetochore will attach to microtubules Figure 9.9. When one chromatids kinetochore captures a microtubule, the chromosome begins to move to the pole, because the kinetochore has a motor protein (kinesin) on it that will try to walk to the pole. But the other chromatid (of the same chromosome) now faces the opposite pole and so attaches to the opposite pole and tries to walk to the opposite pole. Since sister chromatids are attached to each other, and are trying to move in opposite directions, the forces are balanced. Eventually all sister chromatids are attached to microtubules from opposite poles, and so face opposite poles. figure 9.8 metaphase. How do the chromosomes get to the center, to the metaphase plate? Other microtubules that are pushing out from the poles, but do not meet other microtubules nor kinetochores, become polar wind microtubules. This generates a force from the poles to the mid-region (a region we call the metaphase plate). We call this pushing to the center force, the polar wind. www.notesolution.com
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