The Cell Cycle

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I. Concept 12.1- Cell division results in genetically identical daughter cells

A. Overview
1) The continuity of life is based on the reproduction of cells or cell division. 2) The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two cells.

B. Cellular Organization of the Genetic Material
1) A cell’s endowment of DNA, its genetic information is called its genome. 2) Before the cell can divide to form genetically identical daughter cells, all of the DNA must be copied and then two copies separated so that each daughter cell ends up with a complete genome. 3) The replication and distribution of DNA is manageable because the DNA molecules are packaged into chromosomes. 4) The nuclei of a human somatic cell (all body cells except the reproductive cells) each contain 46 chromosomes made up of two sets of 23, one set inherited from each parent. 5) Reproductive cells or gametes-sperm and eggs-have half as many chromosomes as somatic cells, or only one set of 23 chromosomes. 6) Eukaryotic chromosomes are made of chromatin, a complex of DNA and associated protein molecules.

C. Distribution of Chromosomes During Eukaryotic Cell Division 1) After DNA duplication, the chromosomes condense: Each chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and thick. 2) Each duplicated chromosome has two sister chromatids. The two chromatids, each containing an identical DNA molecule, are initially attached along their lengths by adhesive protein complexes called cohesins. This attachment is known as the sister chromatid cohesion. 3) The duplicated chromosome has a narrow waist at the centromere, a specialized region where the two chromatids are most closely attached. 4) Later in the cell division process, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei, one forming at each end of the cell. 5) Mitosis, the division of the nucleus is usually founded immediately by cytokinesis, the division of the cytoplasm. 6) You produce gametes by a variation of cell division called meiosis, which yields nonidentical daughter cells that have only one set of chromosomes.

II. Concept 12.2- The mitotic phase alternates with interphase in the cell cycle

A. Phases of the Cell Cycle
1) The mitotic phase (M) phase, which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle. 2) Mitotic cell division alternates with a much longer stage called interphase, which often accounts for about 90% of the cell. It is during interphase that the cell grows and copies its chromosomes in preparation for cell division. 3) Interphase can be divided into subphases:

* G1 phase (“first gap”)
* S phase (“synthesis”)
* G2 phase (“second gap”)
4) Mitosis is conventionally broken down into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.

B. The Mitotic Spindle
1) Many of the events in mitosis depend on the mitotic spindle, which begins to form in the cytoplasm during prophase. This structure consists of fibers made from microtubules and associated proteins. 2) In animal cells, the assembly of spindle microtubules starts at the centrosome, a subcellular region containing material that functions throughout the cell cycle to organize the cell’s microtubules. 3) An aster, a radial array of short microtubules. The spindle includes the centrosomes, the spindle microtubules, and the asters. 4) Each of the two sister chromatids of a replicated chromosome has a kinetochore, a structure of proteins associated with specific sections of chromosomal DNA at the centromere. 5) During prometaphase, the spindle microtubules attach to the kinetochores which then moves the chromosomes toward the...
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