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CELL DIVISION: MITOSIS AND MEIOSIS I. As we know living cells must reproduce themselves to maintain life. A. In unicellular organisms, cell division acts to increase the number of individuals present, while in multicellular organisms, cell division acts to replace dead or damaged cells. B. Reproduction- C. In cell division, each newly made cell must contain the genetic information that is needed to run the activities of the cell. 1. Each new cell must receive a copy of the parental cell’s DNA. 2. The new cells must also inherit the organelles and polymers needed for the cell to function properly. D. Living cells can reproduce themselves by dividing. All living cells divide by one of two possible processes. The types of cell division include: II. SOME BASIC GENETICS OF CELL DIVISION A. Chromosomes-organelles located in a cell’s nucleus. 1. These structures are composed of 2. Chromosomes contain a cell’s genetic information. 3. Genes-sequences of DNA found on chromosomes. These are responsible for giving cells their characteristics/traits. Genes are also involved in the production of proteins and enzymes in cells. 4. Chromosomes are only visible during cell division. a. Chromatin-this material contains the DNA of a non-dividing cell. These condense to form chromosomes prior to cell division. 5. In most cells, chromosomes occur in pairs known as Homologous Chromosomes. a. Each member of the pair is known as a homolog. 6. Replication- a. This process ensures that a newly produced cell will have a copy of the parental cell DNA. b. During replication, chromosomes appear as 2 separate but connected bodies. These extra copies of the chromosomes are referred to as sister chromatids. The sister chromatids are held together by a centromere. B. Chromosome Number 1. Different species of plants and animals contain different numbers of chromosomes in their cells. How many pairs of chromosomes do humans contain? 2. One thing to remember is that all healthy members of a species have the exact number of chromosomes that is required for that species Fewer or extra chromosomes causes genetic disorders. 3. Terms That Refer to Chromosome Number a. Diploid- 1) Diploid cells contain homologous chromosomes. These are abbreviated 2N. 2) Most living cells are diploid. b. Haploid- 1) These are abbreviated 2) Gametes are the only cells that are haploid. What are gametes? a) Gametes fuse to produce a structure known as a zygote. III. THE CELL CYCLE A. Steps in The Cell Cycle 1. The G-1 Step (Gap 1) a. What happens in the G-1 Step? 2. The S Step (Synthesis) a. In this step, the cell’s DNA replicates or doubles itself. Why is this important in cell division? 3. The G-2 Step (Gap 2) a. In this step, a cell’s organelles double themselves. Why is this important? b. The enzymes that are needed for cell division to occur are also produced in this step. c. Interphase- 4. Mitosis-cell division, including division of the chromosomes. IV. MITOSIS A. General Features of Mitosis: 1. Mitosis occurs in somatic cells 2. In mitosis, one cell divides to produce two genetically identical cells. 3. Mitosis is a quick process that proceeds through four distinct phases. B. The Phases of Mitosis 1. Prophase-Events/Steps a. Chromatin condenses to form distinct homologous chromosomes. At this point the chromosomes become visible in the cell. b. 2 pairs of centrioles appear on the outside of the cell’s nucleus. Centrioles are hollow tubes that are composed of microtubules. c. Next, the centrioles begin moving towards opposite ends of the cell. As they migrate, the centrioles begin producing the Spindle Apparatus (Fibers). 1. The homologous chromosomes will attach to the spindle fibers as mitosis continues. d. Eventually, the spindle fibers will fill the entire cell. e. Once the centrioles reach opposite ends of the cell, asters develop. 1) What are asters and what is their function? f. At the end of prophase, the nuclear membrane disappears. The homologous chromosomes are then free to attach to the spindle fibers. 2. Metaphase-Events/Steps a. During this phase, the chromosomes line up in the center of the dividing cell. The chromosomes line up at a region known as the metaphase plate. b. In this phase, the cell is preparing to pull the chromosomes apart and then divide to produce two new cells. 3. Anaphase-Events/Steps a. In this phase, the spindle fibers shorten and begin to pull on the homologous chromosomes. b. Eventually, the chromosomes are pulled apart. Furthermore, they are pulled towards opposite ends of the cell. c. At the same time of chromosome separation, the cell is elongating. d. Anaphase is complete when the chromosomes have reached opposite ends of the dividing cell. 4. Telophase-Events/Steps a. By this phase, the chromosomes have reached opposite ends of the dividing cell. b. Cytokinesis occurs-this is division of the cytoplasm. Why is this important? c. New nuclear membranes from in the center of the two new cells. d. At this point, the chromosomes lose their identity and develop into chromatin again. e. Now, the two new cells begin the G-1 Step of the Cell Cycle. C. Mitosis is similar in plant and animal cells; however, there are some differences between cell division in plants and animals. Some of these differences include: D. What regulates and controls Mitosis? E. Cells must stop mitosis at some point. Failure to stop cell division, at least for a brief period of time, can lead to the development of cancer. What is cancer? 1. What stops mitosis in living cells? 2. Oncogenes- 3. Carcinogenesis- 4. Mutations lead to the formation of many forms of cancer. F. Variations on Mitosis: 1. Binary Fission- 2. Budding- G. Generally, mitosis is associated with cells that reproduce asexually (without the formation of gametes). Meiosis, a second type of cell division, is associated with sexual reproduction. V. SEXUAL REPRODUCTION-involves the production of gametes-what are these? A. In this process, an individual receives genetic information from two parents. B. In sexual reproduction, haploid gametes fuse to form a diploid zygote which develops into a new individual. C. Meiosis is a type of cell division that produces haploid gametes for living organisms. 1. Both plants and animals can produce haploid gametes. 2. Without meiosis, living organisms would not be able to produce haploid gametes. VI. MEIOSIS-process through which haploid gametes arise from diploid cells. A. Characteristics/Features of Meiosis 1. It occurs in germ cells- a. Where are germ cells stored in humans? 2. Since diploid germ cells undergo meiosis to produce haploid gametes, then meiosis is referred to as a reduction division. What does this mean? 3. Meiosis requires 2 separate divisions to occur. Due to this, meiosis produces 4 genetically different haploid gametes. B. The Events/Phases of Meiosis 1. Meiosis I-Events/Phases a. Prophase I-Events/Steps 1) Chromatin condenses to form homologous chromosomes. 2) Next, the homologous chromosomes fuse together through a process known as synapsis. a) This is a key process in meiosis. It does not occur in mitosis. b) In synapsis, pairs of homologous chromosomes attach or fuse together. 3) While these pairs of chromosomes are attached to each other, a process known as crossing over occurs. a) What is crossing over? b) Why is crossing over important? c) Why is genetic recombination useful to living organisms? 4) Centrioles appear and form the Spindle Apparatus. 5) The now unattached pairs of homologous chromosomes attach to the spindle fibers. b. Metaphase I-Steps/Events 1) What major event occurs in this phase of Meiosis? c. Anaphase I-Steps/Events 1) Homologous chromosomes separate and move towards the poles of the dividing cell. d. Telophase I-Steps/Events 1) The chromosomes reach opposite ends of the cell. 2) Cytokinesis occurs. What is this? 3) The cell partially divides to form 2 new cells. 4) The 2 cells formed at this point are haploid; therefore, reduction occurs in Meiosis I. 2. Interkinesis-brief period of time that resembles interphase in mitotic cells. DNA replication does not occur during this period. 3. Meiosis II-is identical to mitosis in every way. VII. COMPARISON OF MITOSIS AND MEIOSIS A. What types of cells does each process occur in? B. How many division occur in mitosis? In meiosis? C. What are the end results of each process? D. Define the following: synapsis, crossing over. Where do these two processes occur? E. Why is genetic recombination important to sexually reproducing organisms?
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