Cell Cycle

I. Eukaryotic Chromosome structure: Chromosomes are small packages of genetic information that contain the genes (informational units of DNA). Each species has a characteristic number of chromosomes. 1. Chromosomes are made up of DNA and protein. This DNA-protein complex is called chromatin. 2. Chromosomes come in pairs 3. Members of a given pair of chromosomes are called homologous chromosomes. 4. A cell that has 2 of each chromosome is diploid (2n), one of each is haploid (1n). II. Cell Cycle This occurs in somatic cells. The time between divisions is called the generation time. 1. Interphase: A. G1 Phase (Gap 1): the growth phase of the cell. It is during this phase that the decision to divide is made B. S Phase: DNA synthesis phase. When a cell is about to divide, each chromosome replicates itself, forming sister chromatids. Each is still one chromosome but with twice as much DNA. They remain attached to each other at the centromere. A disk of protein called the kinetochore holds them together. C. G2 Phase (Gap 2): Further preparations cell division. a single centrosome found near the nucleus divides to provide two centrosomes. Microtubule formation is initiated at these sites. Each centrosome may contain a pair of centrioles, each pair made up of one "parent" and one "daughter" centriole 2. M Phase: Mitosis is the phase in which the nucleus divides. A. Prophase a. Condensation of chromosomes continues throughout prophase.

b. The nuclear envelope breaks down into small vesicles c. The mitotic fibers form: The centrosomes that contain the centrioles move to opposite ends of the cell. Microtubules form from the centrioles, growing out in all directions to form a star shaped structure called the aster. The microtubules attach to the kinetochore in the centromere of each sister chromatid to separate poles of the cell. d. Chromosomes start to move toward the center of the cell Top of page B. Metaphase: a. The chromosomes line up in the center of the cell, equidistant from the two poles on the metaphase plate. C. Anaphase: a. Each centromere splits in two, freeing the two sister chromatids to be drawn to the opposite poles of the spindle in the next phase. b. Daughter chromosomes move rapidly to opposite poles of the cell. D. Telophase: a. The spindle apparatus is disassembled b. A nuclear envelope begins to form around each group of chromosomes. c. Chromosomes begin to uncoil. 3. Cytokinesis: Division of the Cytoplasm: The division of the cytoplasm of the cell, and the cleavage of the cell into roughly two equal halves is called cytokinesis. A. Animals: the cell is pinched in two with a constricting belt of microfilaments. This produces a cleavage furrow, or indentation around the cell.

B. Plants: Plant cells assemble membrane components in their interior, at right angles to the spindle. This partition is called a cell plate. It continues to grow outward until it reaches the interior surface of the plasma membrane and fuses with it. Cellulose is then laid down on the new membranes, creating two new cell walls. Top of page III. Control of the Cell Cycle a. Internal control: The Cdk-cyclin complex is able to phosphorylate other proteins that can then bind to DNA and start the replication process. b. External control: the signal to divide may come from outside a cell and are generally called growth factors. These bind to target cells and tell them to divide — enter the S phase. Several hormones are also growth factors. IV. MEIOSIS — the other M Phase Meiosis is nuclear division that decreases the ploidy level of the cells by 1/2 1. There are 2 successive nuclear and cell divisions potentially producing 4 cells. 2. DNA replication occurs only one time even though there are two divisions. 3. Each of the 4 cells is haploid. 4. The homologous chromosomes containing genetic information form each parent is independently distributed to each new cell. 5. There is some exchange of parts between homologous chromosomes Meiosis I: Reduces the chromosome number A. Prophase I: All of the same thing happen as in prophase of mitosis with the following additions: 1. Synapsis: pairing of homologous chromosomes

2. Crossing-over: DNA is exchanged between homologous chromosomes. Top of page B. Metaphase I: Very similar to metaphase of mitosis except the spindle fibers are connected to only one side of each chromosome. When the chromosomes line up on the metaphase plate, there is a independent mixing of maternal and paternal chromosomes (independent assortment of chromosomes). C. Anaphase I: Because the microtubules are attached to only one of the centromeres, the centromeres are not pulled apart and whole chromosomes are pulled to either end of the cell. D. Telophase I: The stage at which the two compliments of chromosomes gather together at their respective poles is called telophase I. Cytokinesis will occur in some organisms. Haploid number is reached upon finishing Meiosis I. Meiosis II: Meiosis II occurs to separate the sister chromatids. Meiosis II is very similar to mitosis. E. Prophase II: spindle fibers form F. Metaphase II: same things happen as in metaphase of mitosis. G. Anaphase II: sister chromatids are separated. H. Telophase II: the spindle fibers are dismantled and nuclear envelopes develop around the groups of chromosomes Chromosomes are haploid and unduplicated. I. Genetic variability: a. crossing over b. independent assortment of chromosomes Top of page