Chapter 9, Cellular Growth and Mitosis

1. Chapter 9, Cellular Growth and Mitosis

2. Why are cells so small? • As cells get larger, their surface area to volume ratio keeps getting smaller. • In other words, as cells get bigger, the cell membrane gets proportionately smaller in relation to the cytoplasm.

3. Why is this a problem? • Remember, the cell membrane is responsible for taking in and getting rid of everything that enters and exits the cell. • If the cell membrane is not large enough to transport everything the cell needs in its cytoplasm, then the cell will die. • As a consequence, when a cell gets too big, it must reproduce and divide into another cell or else it will stop growing and eventually die.

4. The Cell Cycle • The life cycle of a cell is known as the Cell Cycle. • The cell cycle is divided into three main stages. • The first stage is called Interphase. • Interphase is the longest stage of the cell cycle. • During interphase, a cell grows, performs normal cellfunctions (like making proteins), makes a copy of its DNA, and then prepares for mitosis.

5. Interphase • Think of interphase as the life of a person. They grow up, eat, drink, sleep, get an education, start a career, buy a house, get married etc. • The three main parts to interphase are • G1 – Cell grows and performs normal functions • S – DNA is replicated • G2 – Cell prepares for mitosis

6. Mitosis • The next phase of the cell cycle is Mitosis. • Mitosis is the stage in which the cell's nucleus and nuclear material divide. • Mitosis is divided into four substages. • The next and final stage of the cell cycle is Cytokinesis. • Cytokinesis is when the cell (cytoplasm and all parts of the cell) divides into two cells.

7. Cell Cycle • G1 – Growth and normal functions • S – DNA is replicated • G2 – Prepares for mitosis • Mitosis – Nucleus divides • Cytokinesis – Cell divides

8. The four substages of Mitosis • Mitosis is a complex series of stages. • The first of the four substages of mitosis is called Prophase. • In prophase, the cell's genetic material (called chromatin) tightens and condenses into x-shaped structures called chromosomes. • Each half of the chromosome is referred to as a Sister Chromatid. • At the center of the chromosome, where the sister chromatids meet is called a Centromere.

9. Prophase and Chromosomes

10. Late Prophase • During late prophase, the nucleolus and the nuclear membrane disappear. • Little structures from the cytoskeleton called centrioles start to appear and go to opposite sides (called the poles) of the cell. • Then a net-like structure called the spindle apparatus starts to form from long strands that attach to the centrioles called spindle fibers. • Small strands also surround the centrioles in a star-like appearance called aster fibers.

11. Late Prophase

12. Metaphase • The second of the four substages of mitosis is called Metaphase. This is the shortest stage of mitosis. • During metaphase the chromosomes line up in a straight line along the equator of the cell. • The centromeres of each chromosome attach to a spindle fiber along the equator of the cell.

13. Metaphase

14. Anaphase • The third of the four substages of mitosis is called Anaphase. • During anaphase, the sister chromatids are separated from the equator of the cell and begin moving towards the poles of the cell. • The spindle fibers pull apart the sister chromatids from the centromeres. • The spindle fibers begin to shorten and move toward the centrioles (located at the poles). • The result is two identical copies of the chromosomes at each pole.

15. Anaphase

16. Telophase • The last of the four substages of mitosis is called Telophase. • Telophase is the stage of mitosis when the chromosomes reach the poles of the cell and begin to relax and decondense. • The nuclear membranes begin to reform around the genetic material called chromatin. • Then the spindle apparatus gets disassembled and the parts become recycled by the cell.

17. Telophase

18. Cytokinesis • The final stage of the cell cycle is cytokinesis. • This is when the cytoplasm and the cell membrane of the cell divides. • The result is two cells with identical nuclei. • In animals, microfilaments (parts of the cytoskeleton) pinch or constrict around the cell, creating a cleavage furrow • In plants, the cell wall divides by creating a new wall in between the two new nuclei. This new wall is called a cell plate

19. Cytokinesis • Cleavage furrow in animals • Cell plate in plants

20. The Cell Cycle

21. Abnormal Cell Cycle: Cancer • Canceris when the cells in your body begin to grow and divide uncontrollably. • Once cancer begins, cancer cells begin to crowd out and kill normal cells. • Cancer cells spend less time in interphase than normal cells do. • Substances and agents that are known to cause cancer are called carcinogens. • Tobacco and too much exposure to the sun are known carcinogens.

22. Stem Cells • Stem cells are cells that are unspecialized, but can become specialized under certain conditions. • This means that they have not developed into a certain type of tissue cell yet. • When embryonic stem cells are dividing in the cleavage phase (100-150 cells) they are still unspecialized stem cells. • Research in stem cells is highly controversial because one common source of stem cells is from human embryos that have been aborted.

23. Stem Cells