chapter09 - Chapter 9 CHROMOSOMES MITOSIS AND MEIOSIS...

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Chapter 9 CHROMOSOMES, MITOSIS AND MEIOSIS. Chromosomes get their name from the Greek words chroma, color and soma, body. A chromosome is made of highly coiled and condensed chromatin. Chromatin is made of DNA and associated proteins. In non-dividing cells, the chromatin is extended and uncoiled. Chromosomes are or organized into informational units called genes . Every individual of a species has the characteristic number of chromosomes of the species, e.g. all humans have 46 chromosomes. The number varies from species to species. It is possible for an individual to have an abnormal number of chromosomes. The information in the chromosomes is what makes the uniqueness of the species and not the number of chromosomes. Chromosomes exist in pairs, half of which was been contributed by each parent of the individual. Body cells have the full complement of chromosomes and they are called diploid cells. Gametes have only half of the species chromosomes and they are called haploid . THE CELL CYCLE The cell cycle is the period of time from the beginning of one cell division to the beginning of the next division. The time it takes for the cycle to be completed is called the generation time . Generation time can vary widely. Cell division involves two processes, 1. Mitosis : it involves the nucleus and it insures that each daughter cell will receive the same number and type of chromosome as were present in the original nucleus. 2. Cytokinesis : refers to the division of the cytoplasm of the cell to form two daughter cells. When mitosis is not followed by cytokinesis, the cell becomes multinucleated or coenocytic. A. Interphase : it can be divided into the first gap phase (G 1 ) , chromosomal synthesis phase (S) , and the second gap phase (G 2 ) .
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First gap phase (G 1 ): the cell grows and prepares itself for the S phase. Chromosomal synthesis phase (S): DNA and chromosomal proteins are synthesized. Second gap phase (G 2 ) : protein synthesis increases in preparation for cell division. Cells that are not dividing become arrested at the G 1 stage, which is not part of the cell cycle. Cells at this state are called to be at the G o state. B. Mitosis ensures orderly distribution of chromosomes. 1. Prophase : duplicated chromosomes are made of two sister chromatids held together at the centromere; the nucleolus disappear; nuclear envelope breaks down; spindle of microtubules begins to form. Dividing cells have a microtubule-organizing center at each pole, from which the microtubules grow outward.
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