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Transcript
Lab. 2 Cell Division 1. Mitosis Division Main topics: • Cell cycle definition • What’s Mitosis? • karyokinesis division • Cytokinesis division The Cell Cycle definition: Each eukaryotic cell has a repeating set of events that make up the life of every cell, called the cell cycle. Although they vary in length depending upon the cell's function, the cell cycle for all cells can be described in five steps. The first three steps where the cell grows, matures, and carries out its life function are collectively called interphase, followed by mitosis, and cytokinesis. Figure (2): Cell cycle 1- Interphase: The cell is engaged in metabolic activity and performing it’s prepare for mitosis. Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules. It is divided into three phases that lead up to and include nuclear division, and it will explain below: Principles of Genetics 1 • The G1 stage A lot of protein synthesis happens in order to increase the amount of cytosol in the cell. Cytosol is the liquid inside the cell, but outside the organelles, that contains the cell's proteins. Proteins are the molecular machines that sustain the cell’s day-to-day activities. The increase in cell size happens not just because more proteins are being made, but also because the cell absorbs nutrients. • The S stage Stands for "Synthesis". (“Synthesis”) is the time when the DNA is replicated, when the chromosome goes from having one chromatid to having 2 chromatids held together at the centromere. • The G2 stage The cell prepares to enter mitosis. The DNA has already been duplicated during the S phase, so the G2 phase is when the organelles of the cell need to duplicate. Not only will the duplicated DNA be evenly divided during cell division, but so will the organelles. Some organelles, such as mitochondria and chloroplasts, are discrete units that do not bud off from larger organelles. Discrete organelles increase in number by undergoing their own division during G2. • Checkpoints The advantage of having three phases in interphase is that it allows time to check that things are happening as they should. Three checkpoints exist during interphase, during which the cell makes sure that everything has gone as planned and, if needed, fixes errors. The G1-S checkpoint at the end of the G1 phase makes sure that the DNA is intact and that the cell has enough energy to enter the S phase. The S phase checkpoint makes sure that DNA is replicated correctly without any breakages. The G2M checkpoint at the end of the G2 phase is another safeguard in case something happens to the DNA or cell before it undergoes the massive task of dividing. Principles of Genetics 2 2- Mitosis A process that takes place in the nucleus of a dividing cell, involves typically a series of steps consisting of prophase, metaphase, anaphase, and telophase, and results in the formation of two new nuclei each having the same number of chromosomes as the parent nucleus. Figure (3): Mitosis Phases. Principles of Genetics 3 Phases of mitosis division Phase Drawing 1. Prophase: Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. 2. Metaphase: Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome. 3. Anaphase: The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules. Principles of Genetics 4 Microscope 4. Telophase: Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage. Cytokinesis division: Cytokinesis division in Animal cells In animal Cytokinesis division in plant cells cells, In plant cells, the results rigid wall requires when a fiber ring that a cell plate be composed synthesized cytokinesis of a protein called actin between around the center of daughter cells. the cell contracts pinching the cell into two daughter cells, each with one nucleus Principles of Genetics 5 the two References: http://biothemes.wikifoundry.com/page/Cytokinesis+-+Plant+vs.+Animal+Cell http://commons.wikimedia.org/wiki/File:Mitosis_%28261_14%29_Pressed;_root_meristem_of_onion_%28cells_in_prophase,_anaphase%29.jpg http://classroom.synonym.com/list-3-steps-occur-during-interphase-17577.html http://www.merriam-webster.com/dictionary/mitosis http://www.infoplease.com/cig/biology/cell-cycle-interphase-mitosis-cytokinesis.html http://w3.marietta.edu/~biol/introlab/Onion%20root%20mitosis.pdf Principles of Genetics 6 0 Training Preparing the root tip squash. 1. Transfer a root to the center of a clean microscope slide and add a drop of water. 2. Using a razor blade cut off most of the unstained part of the root, and discard it. 3. Cover the root tip with a cover slip, and then carefully push down on the cover slide with the wooden end of a dissecting probe. Push hard, but do not twist or push the cover slide sideways. The root tip should spread out to a diameter about 0.5 – 1 cm. Onion root tip squash 1. Find and draw a cell showing each stage of mitosis Prophase Principles of Genetics Metaphase Anaphase 7 Telophase Exercise 2 1- Describe all of the phases: 1- ........................................ 2- ........................................ 3- ........................................ 4- ........................................ 5- ........................................ 6- ......................................... 2- Choose the right answer: 1. What is the name of the microtubule fibers that pull the sister chromatids apart? A. Centromeres B. Membranes C. Ligands D. Spindles 2. What is the name of the structures responsible for generating the spindle fibers? A. Centromeres B. Membranes C. The funky hot chocolates D. Centrioles 3. The pinching off of the cell membrane that creates two new cells (after mitosis) is called A. Interphase B. Metaphase C. Anaphase D. Cytokinesis 4. What is the name of the attachment point between sister chromatids in a chromosome? A. Centrioles B. Centromeres C. Spindles D. Membranes Principles of Genetics 8