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Unit 7: Cell Division Vocabulary to Define: checkpoint tumor malignant diploid haploid homologous chromosomes crossing over benign What you should know… Cell cycle: a repeated pattern of growth and division that occurs in eukaryotic cells; consists of three phases. Interphase Purpose is for cell growth; divided into three phases. ○ During the G1 (gap 1) phase, the cell grows and synthesizes proteins. ○ During the S (synthesis) phase, chromosomes replicate and divide to form identical sister chromatids held together by a centromere. ○ During the G2 (gap 2) phase, cells continue to grow Mitosis Purpose is cell division: making two cells out of one; divided into four phases. ○ Prophase: chromosomes condense and are more visible; nuclear membrane (envelope) disappears; centrioles separate and move to opposite sides of the cell; spindle fibers form ○ Metaphase: the shortest phase of mitosis; chromosomes line up across the middle of the cell; spindle fibers connect the centromere of each sister chromatid to the centrioles ○ Anaphase: centromeres split; sister chromatids separate becoming individual chromosomes, and move to opposite poles of the cell ○ Telophase: the last phase of mitosis; chromosomes uncoil; nuclear envelope reforms around the chromosomes; spindle fibers break down and dissolve; cytokinesis begins. Cytokinesis Division of the cytoplasm into two individual cells, diploid, genetically identical Animal cells form a cleavage furrow that eventually pinches the cell into two nearly equal parts Plant cells form a cell plate midway between the divided nuclei; eventually forms the cell wall Cancer Cell cycle is driven by a chemical control system that regulates the cell cycle at certain checkpoints. Signals are involved in turning the process of cell division off and on. ○ Internal signal: cell senses chemicals produced inside the cell ○ External signal: cell senses chemicals produced in other cells. Cells can also respond to physical signals from their environment: division is turned off when cells are closely packed; division is turned on when cells are not in contact with a surface Cancer cells continue to divide even when they are very packed and/or there is no growth factor present. Stem Cells In multicellular organisms, a fertilized egg gives rise to many different types of cells, each with a different structure and function, through process of cell division ○ As cell division proceeds, the cells undergo differentiation to become specialized ○ The various types of cells (such as blood, muscle, or epithelial cells) arrange into tissues which are organized into organs, and which are organized into organ systems. ○ During differentiation, only specific parts of the DNA are activated; the parts of the DNA that are activated determine the function and specialized structure of a cell. Stem cells are unspecialized cells that continually reproduce themselves and have the ability to differentiate into one or more types of specialized cells. ○ 2 types: embryonic (taken from embryos) and adult stem cells 1/18 Unit 7: Cell Division Meiosis occurs in two steps: Meiosis I: chromosome pairs replicate, results in two haploid daughter cells with duplicated chromosomes different from the chromosomes in the original diploid cell. Meiosis II: haploid daughter cells from Meiosis I divide, results in four haploid daughter cells called gametes, or sex cells (eggs and sperm), with single chromosomes. Meiosis I A prophase I B metaphase I C anaphase I D telophase I Meiosis I begins with interphase in which cells: (1) increase in size (2) synthesize proteins (3) replicate DNA Prophase I: nuclear membrane breaks down; centrioles move to opposite sides of the nucleus and begin to produce spindle fibers; chromosomes pair up and become visible as a cluster of four chromatids called a tetrad (a pair of homologous chromosomes – one from mom and one from dad) Metaphase I: chromosomes are pulled into the mid-line (or equator) of the cell in pairs. Anaphase I: homologous chromosome pairs separate; sister chromatids remain attached to each other Telophase I & Cytokinesis: chromosomes gather at the poles, nuclear membrane reforms, cytoplasm divides Each daughter cell contains only one chromosome (consisting of two sister chromatids) Meiosis II Prophase II: same as prophase I, except crossing over does not take place Metaphase II: sister chromatids line up across the center of the cell. Anaphase II: sister chromatids separate, and individual chromosomes move to opposite poles Telophase II & Cytokinesis: ends with 4 daughter cells that are haploid and genetically different 2/18