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Chapter 11

Chapter 11 Textbook Notes - Mitosis

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University of Toronto Mississauga
Fiona Rawle

Notes From Reading CHAPTER 11:T HE C ELLCYCLE (PGS .234-256) Key Concepts - After chromosomes are copied, mitosis distributes one chromosome copy to each of two daughter cells. Mitosis and cytokinesis produce two cells that are identical to the parent cell. - Over their life span, eukaryotic cells go through a cycle that consists of four carefully controlled phases. - In multicellular organisms, uncontrolled cell growth leads to cancer. Different types of cancer result from different types of defects in control over the cell cycle. Introduction: Meiosis - Cells arise through the division of preexisting cells. There are two types of cell division: meiosis and mitosis. - Meiosis is a division of the genetic material in the nucleus to produce daughter cells with one- half the amount of hereditary material found in the parent cell. Meiosis is involved only in the production of gametes (eggs and sperm); it is the basis of sexual reproduction and genetic inheritance (see Chapter 12). Introduction: Mitosis - Mitosis is a division of the genetic material in the nucleus that produces daughter cells genetically identical to the parent cell. - Mitosis is usually accompanied by cytokinesis, the division of the cytoplasm into the two daughter cells. - Mitosis is the basis of asexual reproduction and is involved only in the production of somatic (body) cells. It is responsible for three key events in multicellular eukaryotes: (1) growth, (2) wound repair, and (3) cellular reproduction. 11.1 Mitosis and the Cell Cycle - Chromosomes contain a long double helix of deoxyribonucleic acid (DNA) wrapped around proteins. DNA carries the cell’s genetic information. The purpose of mitosis is to distribute this genetic material to daughter cells during cell division. - Chromosomes can be stained with dyes and observed under the light microscope. - Prior to mitosis, each chromosome is duplicated. As mitosis starts, the chromosomes condense from long, thin filaments into compact structures that can be moved around the cell efficiently. M Phase and Interphase - Growing cells cycle between a dividing phase called the mitotic (M) phase and a nondividing phase called interphase. The Discovery of the Cell Cycle - The cell cycle is the orderly sequence of events that occurs starting from the formation of a eukaryotic cell through the duplication of its chromosomes to the time it undergoes cell division. - Two key events of the cell cycle are (1) replication, in which the chromosomes are copied, and (2) partitioning of the copied chromosomes to the two daughter cells during M phase. Notes From Reading CHAPTER 11:T HE C ELLCYCLE (PGS.234-256) - Chromosome replication occurs only during interphase and not during M phase. The stage in which DNA duplication occurs is called the synthesis (S) phase. The Discovery of the Gap Phases - Interphase also includes two gap phases, during which no DNA synthesis occurs. The first gap, called G1phase, occurs before the S phase. The second gap, G phase2 occurs after the S phase and before mitosis. - It takes a cell about 24 hours to complete one cell cycle. G phase lasts 7 to 9 hours; S phase, 6 1 to 8 hours; and G 2hase, 4 to 5 hours. - During the gap phases, organelles replicate and additional cytoplasm is made in preparation for cell division. 11.2 How Does Mitosis Take Place? - Mitosis results in the division of replicated chromosomes and formation of two daughter nuclei with identical chromosomes and genes Events in Mitosis - Before mitosis, chromosomes replicate. Each of the two DNA strands in a replicated chromosome is called a chromatid. - Sister chromatids are attached to each other at the centromere and are exact copies of the same genetic information. - During mitosis, the two sister chromatids separate to form independent chromosomes, and one copy of each chromosome goes to each of the two daughter cells. - Mitosis (M phase) is a continuous process with several subphases (prophase, prometaphase, metaphase, anaphase, telophase) based on specific events. Prophase - Chromosomes condense and become visible in the light microscope during prophase. - Also during this phase, the mitotic spindle forms from a microtubule-organizing center. The spindle is made up of groups of microtubules called spindle fibers that attach to the chromosomes. This spindle is the structure that later in mitosis pulls the chromosomes into the two daughter cells. - In animal cells, the microtubule-organizing center is a centrosome—a structure that contains a pair of centrioles. Prometaphase - During prometaphase, the nuclear envelope breaks down. - In addition, spindle fibers from each mitotic spindle attach to one of the two sister chromatids of each chromosome. They attach at a structure called the kinetochore, which is located in the centromere region. Metaphase - During metaphase, the spindle fibers attached to the kinetochores of each chromatid line up the chromosomes in the middle of the cell. Notes From Reading CHAPTER 11:T HE C ELLCYCLE (PGS .234-256) - The imaginary plane formed by this is called the metaphase plate. Anaphase - During anaphase, centromeres split and sister chromatids are pulled by the spindle fibers toward opposite poles of the cell. - This causes the replicated chromosomes to split into two identical sets of unreplicated chromosomes, one for each daughter cell. Telophase - During telophase, a new nuclear envelope begins to form around each set of chromosomes. In addition, the mitotic spindle disintegrates, and the chromosomes begin to de-condense. When two independent nuclei have formed, mitosis is complete. Cytokinesis - Immediately after mitosis, cytokinesis typically occurs. During this process, the cytoplasm divides to form two daughter cells, each with its own nucleus and complete set of organelles. - Cytokinesis in animals, fungi, and slime molds occurs when a ring of actin and myosin filaments contracts inside the cell membrane, causing it to pinch inward in a cleavage furrow. - Cytokinesi
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