Cell Division & Mitosis

1. Cell Division&Mitosis December 2nd/3rd, 2008

2. Cell Division in Eukaryotes • In eukaryotes, cell division is complex and occurs in two main stages: • Stage 1: division of the cell nucleus, called mitosis. • Stage 2: division of the cytoplasm, called cytokinesis. • Reproduction by mitosis is considered asexual since the cells produced are genetically identical to the parent cell. • Mitosis is the source of new cells when multicellular organisms (such as humans) grow and develop.

3. Chromosomes • Chromosomes are made up of DNA, which carries the genetic information that is passed on from one generation of cells to the next.

4. Chromosomes are only visible during cell division. All other times, the DNA is too spread out in the nucleus to be seen. • Chromosomes consist of two parts: • Chromatid: one of two identical “sister” parts of a duplicated chromosome. • Centromere: area where the chromatids of a chromosome are attached. Sister Chromatids Sister Chromatids

5. The Cell Cycle • The cell cycle is the series of events that cells go through as they grow and divide. There are two main stages: • Interphase: the period of the cell cycle between cell divisions, characterized by cell growth and DNA replication. • M-phase: the period of the cell cycle where cell division occurs, also known as mitosis and cytokinesis.

6. The Cell Cycle (6)

7. Interphase • Interphase is the longest phase of cell division and is characterized by growth and DNA replication. It is divided into three phases: • G1: period of activity in which cells do most of their growing. • S: chromosomes are replicated and the replication of DNA molecules take place. • G2: many of the organelles and molecules required for cell division are produced.

8. Mitosis • Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase.

9. Prophase • Prophase is the first and longest phase of mitosis. During this phase, the chromosomes become visible and centrioles and spindle fibers form in the cytoplasm. • Centrioles move to opposite sides of the cell. • Spindle fibers connect to the centrioles and help to separate the chromosomes during anaphase. • Additionally, the nuclear envelope around the chromosomes begins to break down.

10. Metaphase • During metaphase, the chromosomes line up across the center of the cell. • Microtubules connect the centromere of each chromosome to the two poles of the spindle.

11. Anaphase • During anaphase, the centromeres that join the sister chromatids split, allowing the sister chromatids to separate and become individual chromosomes once again. • The chromosomes continue to move apart until they have separated into two groups near the poles of the spindle.

12. Telophase • In telophase, the chromosomes begin to disappear into a tangle of dense material. • A nuclear envelope reforms around each cluster of chromosomes. • The spindle begins to break apart and disappear. • A cleavage furrow begins to form.

13. Cytokinesis • Cytokinesis is the division of the cytoplasm and the other organelles in the cell. Cytokinesis usually occurs at the same time as telophase. • In most animal cells, the cell membrane is drawn inward (cleavage furrow) until the cytoplasm is pinched into two nearly equal parts, resulting in two identical daughter cells. • In plant cells, a cell plate begins to form between the two new cells.

14. Cell Cycle Diagram Directions: • Grab one of each supply at the middle table. • Cut out the shapes from the diagram papers and organize them on to your large paper to correctly show mitosis. Do NOT glue until you are absolutely sure the diagram is correct. • Use the supplies provided by the teacher and in your supply bin to complete each cell. • Chromosomes = String • Centrioles = Straws • Nuclear Envelope = Draw with markers or colored pencils. • Spindle Fibers = Draw with markers or colored pencils.

15. Warm up – 12/5/08 A E C B D

16. Regulating the Cell Cycle • One of the most amazing things about cells is how carefully cell growth and division is controlled. • Not all cells move through the cell cycle at the same pace. • For example, in the human body… • Muscle and nerve cells do not divide at all once they have developed. • However, cells from your skin, digestive tracts, and bone marrow pass through the complete cycle every few hours.

17. Controls on Cell Division • Cells have their own built in control for cell division. • If you place cells apart from one another in a petri dish, they will grow and divide until they come into contact with another cell or the sides of the dish. • If you remove cells from the middle of the dish, the cells bordering the open space will begin dividing until they have filled the empty space.

18. Something similar happens within the human body. • When you cut your skin or break your bone, cells at the edges of the injury are stimulated to divide rapidly.

19. This action produces new cells, starting the process of healing. • When the healing process nears completion, the rate of cell division slows down. • Eventually, the cells stop dividing when they run into one another and the gap in the cut or break is closed.

20. Cell Cycle Regulators • Cyclin • The family of proteins that help regulate the timing of the cell cycle in eukaryotic cells. • There are two types of regulatory proteins: those that occur inside the cell and those that occur outside the cell.

21. Internal Regulators • Proteins that respond to events inside the cell. • Allow the cell cycle to proceed only when certain processes have happened inside the cell. • For example, they make sure the cell does not enter mitosis until all the chromosomes have been replicated. • External Regulators • Proteins that respond to events outside the cell. • Direct cells to speed up or slow down the cell cycle. • Stimulate growth and cell division and prevent excessive cell growth.

22. Uncontrolled Cell Growth • Consequences of uncontrolled cell growth in multicellular organisms are very severe. • For Example: Cancer • Cancer is a disease of the cell cycle. It is a disorder in which some of the body’s own cells lose the ability to control growth. • Cancer cells do not respond to the signals that regulate the growth of most cells. • They divide uncontrollably and form masses of cells called tumors, that can damage the surrounding tissues. • Most cancer cells have a defect in the gene called p53, which normally halts the cell cycle until all chromosomes have been properly replicated.