1. Chapter 12The Cell Cycle

2. Cell division creates duplicate offspring in unicellular organisms (Asexual Reproduction) • Provides for growth, development, and repair in multicellular organisms (Sexual Reproduction) • The cell cycle extends from the creation of a new cell by the division of its parent cell to its own division into two cells

3. Cell division results in genetically identical daughter cells • The process of recreating a cell necessitates the EXACT duplication and equal division of the DNA

4. Each diploid eukaryote has a characteristic number of chromosomes in each somatic cell • Reproductive cells, or gametes, have half that number of chromosomes. Diploid: Haploid: Mosquito 6 Frog 26 Corn 20

5. Chromosomes • Each c’some is a very long DNA molecule with associated proteins. This DNA-protein complex is called chromatin.

6. Distribution of Chromosomes During Eukaryotic Cell Division • Before cell division, a cell copies its DNA and each c’some densely coils and shortens. • Duplicated c’somes consist of 2 identical sister chromatids. • A duplicated c’some has a centromere where the chromatids are most closely attached.

7. The sister chromatids separated during mitosis (division of the nucleus) • Then the cytoplasm divides during cytokinesis, producing 2 separate, genetically equivalent daughter cells.

8. Phases of the cell cycle • Cell cycle (or life of the cell) lasts for about 20 hours • There are 2 main stages in the cycle of the eukaryotic organism • The cell cycle consists of the mitotic (M) phase, which includes mitosis and cytokinesis, and interphase, during with the cell grows and duplicates its c’somes

9. interphase • Usually lasting 90% of the cell cycle • Includes the G1 phase, the S phase, and the G2 phase • Gap 1 (G1) phase- growth period of the cell • S (synthesis) phase- period when DNA replication occurs • Gap 2 (G2) phase- cell prepares for division

10. mitosis • Mitosis- equal division of genetic information (chromosomes) between the two new cells (division of the nucleus) • 4 phases of M: prophase, metaphase, anaphase, and telophase

11. prophase 1. Prophase- nuclear envelope disintegrates; nucleolus disappears; and, the chromatin fibers coil and fold into visible

12. In cells that have centrioles, such as animals’ cells, the centrioles replicate • Filaments called asters develop & radiate from the centrioles • The asters lengthen and become connected to the centromeres • The asters are now called spindle fibers

13. Metaphase • Chromatids become aligned at the center of the cell or at its equator • Centrioles have migrated to opposite sides of the cell, called the poles

14. anaphase • The centromeres separate and the daughter chromosomes begin to migrate to opposite poles • This migration occurs as a result of a shortening of the spindle fibers.

15. telophase • Begins when all chromosomes reach the 2 poles • Spindle fibers begin to disappear and the nuclear membranes begin to reappear • Chromosomes begin to disappear as the chromatin material disperses through the new nuclei

16. cytokinesis • Cytokinesis begins during telophase • In animal cells the cytoplasm begins to pinch inward • In plant cells, a partition called a cell plate begins to form and divides the cytoplasm

17. Meiosis • One special type of cell reproduction is for organisms that produce offspring by sexual reproduction • Special reproductive cells must be produced by each parent. These are the cells that unite to form the new cell.

18. Sex cells • Gamete- special type of reproductive cell that contains only one set of chromosomes (sperm & egg) • A special reproductive process that cells use to produce gametes is called meiosis (reduction division)

19. First cell division: meiosis I • Prophase I- nuclear membrane disappears. The homologous chromosomes (made up of 2 chromatids) pair up next to each other. • At this time, genetic material can be exchanged by a process called crossing over.

20. Metaphase I • Homologous chromosomes move to the equator of the cell

21. Anaphase I • Homologs of each pair go to opposite poles (chromatids remain attached at the centromeres)

22. Telophase I • 2 daughter cells results but the chromosome number has not been halved which results in a second division…Meiosis II