Cell Division: Understanding Cell Size Limitations and the Cell Cycle

1. Biology

2. Chapter 5 Cell Division I. When Do Cells Divide? A. Cell Size Limitations 1. Surface Area-to-Volume Ratio a. As a cell’s size increases, it’s volume increases much faster. 1) Cell membrane cannot let enough nutrients in for the cell to survive 2) Diffusion cannot spread the nutrients fast enough. 2. Nuclear Control a. If the cell is too big, there is not enough DNA to run the cell.

3. B. Growth – a cell will grow until it reaches its maximum size, then it will divide. C. Repair – cell division is crucial in the repair of tissues. 1. Regeneration – the process of growing back lost body parts. ex – starfish growing back, skin cells replacing damaged cells. D. Reproduction 1. Asexual Reproduction – offspring are produced by only one parent a. Have the same genetics as the parent 2. Sexual Reproduction – involves the combination of reproductive cells. a. Offspring have a combination of genetic material from parents

4. II. Cell Cycle – sequence of growth and development of a cell. A. Interphase – the growth period of a cell 1. Most of the cell’s life is spent in this phase. 2. Cell grows in size and carries out metabolism 3. As the cell nears the period of division, chromosomes duplicate. a. Chromatin – form that chromosomes take during most of the cell’s life. 1) Strands of DNA wrapped around protein molecules.

5. B. Mitosis – cell division in which chromosomes duplicate and divide into identical sets. 1. Prophase – longest phase of mitosis a. Chromatin shortens and thickens and becomes visible as chromosomes. b. Sister Chromatids – the duplicated strands of DNA which are exact copies of each other. 1) Centromere – holds the sister chromatids together. c. Nuclear membrane breaks down and the nucleus disappears d. Centrioles – small dark cylindrical structures that help form the spindle. 1) Spindle – thread-like structure that aids in the movement of chromosomes.

6. 2. Metaphase a. Chromatids attach to the spindle and move toward the center of the cell -- equatorial plane 3. Anaphase a. Centromere splits and chromatids move towards opposite poles of the cell.

7. 4. Telophase – a. Begins as chromatids reach the opposite poles of the cell. b. Chromosomes uncoil and revert back to chromatin. c. Spindle breaks down and the nucleus reappears. d. Cytokinesis – cytoplasm divides 1) In animal cells, plasma membrane pinches together to form two new daughter cells. 2) In plant cells, a cell plate forms in the center of cell.

9. III. Meiosis A. Genes and Chromosomes 1. Diploid Number – cell with two of each kind of chromosome (2n) 2. Haploid Number – cell with one of each kind of chromosome (n) 3. Homologous Chromosomes – paired chromosomes a. Have genes for the same traits. B. Meiosis – cell division which produces gametes that contain ½ number of chromosomes. 1. Interphase – 2. Prophase I a. Each set of homologous chromosomes pair up and form a tetrad.

10. b. Crossing Over - homologous chromosomes in a tetrad sometimes exchange pieces. 3. Metaphase I a. Tetrads line up in the middle of the equator. 4. Anaphase I a. Homologous chromosomes separate and move to opposite poles.

12. 5. Telophase I a. Cell splits and now contain ½ the number of original chromosomes C. Meiosis II 1. Prophase II 2. Metaphase II 3. Anaphase II Same as Mitosis 4. Telophase II a. Four haploid cells are formed. 1) Males -- sperm cells 2) Females -- Eggs b. Zygote – cell that results when the egg is fertilized.

13. D. Genetic Variation 1. Meiosis results in cells that are different from the original cells. 2. Produces organisms that vary from the parent. a. In people, 70 trillion different zygotes are possible.