lecture9 for BGYA01

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University of Toronto Scarborough
Biological Sciences
Clare Hasenkampf

BGYA01 Lecture 9 Last lecture we talked about prokaryotic cells with one essential large double stranded, DNA molecule replicates its DNA and divides by the process of binary fission. Since the two products of binary fission are identical and the DNA came from one parent, we call this process asexual reproduction. Now I would like to get ready to think abREPRODUCTION IN EUKARYOTIC CELLS . Here is the order of what well consider: THE EUKARYOTIC CELL CYCLE PACKAGING DNA INTO CHROMOSOMES C ELLD IVISION FOR ASEXUAL REPRODUCTION - MITOSIS THE CELL CYCLE The cell cycle refers to the cycle of activities that occur in the cell from its beginning until it divides to produce two cells. FIGURE 9.3,PAGE 184. We can think of a cells life as being divided into 4 distinct phases. These phases are called M phase G1, S phase and G2. It is during the M phase that one cell divides to give rise to two cells. Lets imagine that two cells have just been created from the division of an existing cell. Each of these daughter cells has all of the necessary DNA with each double helix consisting of one old strand and one new strand. Each of the daughter cells have gotten a share of the parental cells ribosomes, endomembrane system, important enzymes, cytoskeleton components and all the other important organelles. [The round cells shown in the picture must be animal cells because only animal cells, without a cell wall would be that round.] Two cells are the product of cell division but we will only follow one cell through its cell cycle. Lets take our first look at the cell cycle. As a cell comes out of M phase, it has its first, and usually main, period of growth. We call this the G1phase. D URING G1RNA is made in the nucleus, and the RNA leaves the nucleus and moves to the cytoplasm where it is used to direct the production of specific make proteins. So lots of RNA and proteins are being made during 1 . This means that transcription and translation are 1 www.notesolution.comoccurring. Additionally the cell is busy making the other types of molecules it needs -like the carbohydrates and lipids. G1 is a period of cell growth. Once a cell grows a certain amount it is ready for the decision of what fate it should follow. A cell can decide to divide only if it passes a checkpoint. FIRST THE CELL HAS TO CHECK ITSELF FOR DNA DAMAGE . Cells have ways of detecting damage and cells wont get the go ahead to divide if damage is present. The cell will try to repair the damage but if there is too much, the cell may make the decision to die by apoptosis. A POPTOSIS IS PROGRAMMED CELL DEATH . For apoptosis a cell initiates a programmed set of events. It chops up its own DNA, and the cell fragments into pieces. Neighboring cells can use the materials from these dying cells as sources of monomer units. [Note there is another type of cell death that occurs if a cells cytoplasm is damaged by toxins, or dperived of oxygen or other essential nutrients. This is called necrosis. ] If a cell does not have DNA damage it might be ready to divide but it receives signals from other cells of the body that influence the decision. These chemical signals that originate from neighboring cells are called growth factors. Sometimes even if a cell is healthy it might receive a signal from other cells that indicates the cell should initiate apoptosis. This is often true during development. The example the textbook gives an example from human development; early in our development there is a stage when we have webbed fingers, with the webbing made of connective tissue. Because our body plan does not call for webbed fingers after birth, the cells of the webbing receive the signal to undergo apoptosis and they die. So a cell might be damaged and have an internal signal that says undergo apoptosis, or a cell might be healthy and still get the signal to undergo apoptosis. What other fates might a cell have? Some cells once they have obtained their full growth, receive a signal that they should stay alive, but leave the cell cycle and not divide again. Such a cell will leave the cell cycle and go into a state known as G 0 They may specialize in function but they dont usually ever divide again. www.notesolution.com 2
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