with some slides by Stephen Taylor

1. 2.5 Cell Division IB Biology I

2. Questions Why are you bigger now than you were born? Answer: Cells divide and take up more space Why do cells divide instead of growing? Answer: Large cells have reduced SA:VOL ratio = less efficient than smaller cells What needs to happen for you to grow? Answer: More cells need to be produced Each cell needs a copy of the organism’s DNA Each cell needs copies of organelles to perform all functions

3. Prokaryotes x Eukaryotes How would cell division work? Think about each cell’s structure.

4. Prokaryotes x Eukaryotes Prokaryotes divide by binary fission Eukaryotes have chromosomes carrying their genes in the nucleus. A complete set is necessary for the cell to function properly

5. What is the purpose of cell division? Growth (example: baby  adult) Reproduction (asexual reproduction in single celled organisms) Replacement of dead/damaged/infected (example: skin/red blood cells/bone cells) Gamete formation in multi-cellular organisms (meiosis)

6. Chromosome Structure In eukaryotes: made up of DNA and proteins At different times, proteins cause the DNA to: be spread out like spaghetti in a bowl be tightly condensed into the X-shaped (these we can see in the microscope) Central constricted region called centromere that serves as an attachment point for the spindle fibers during mitosis.

8. after DNA replication, chromosomes have 2 sister chromatids, held together at the centromere. Each chromatid is one piece of DNA with its supporting proteins. Why is DNA replication necessary?

9. Chromosomes

11. Cycle: Interphase – Mitosis – Interphase – Mitosis

12. Interphase = period between 2 cell divisions. Cell increases in size, but the chromosomes are invisible (long and thin)

14. Mitosis Mitosis is the division of the eukaryote nucleus, which goes on throughout life in all parts of the body. Organelles can be randomly separated into the daughter cells but chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA. Every human cell has the same 46 chromosomes Mitosis is usually divided into 4 phases: Prophase (P) Metaphase (M) Anaphase (A) Telophase (T) PMAT

15. Prophase Important events: The chromosomes condense (the proteins attached to the DNA cause the chromosomes to go from long thin structures to short fat one, which makes them easier to pull apart) The nuclear envelope disappears (the double membrane that surround the nucleus dissolves into a collection of small vesicles, freeing the chromosomes to use the whole cell for division) Pair of centrioles separate and move to opposite ends of the cell (except in plants) The spindle fibers (microtubules) start to form, growing out of the centrioles towards the chromosomes.

20. Cytokinesis The organelles get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in. Plant x Animal cells: Plants: a new cell wall made of cellulose forms between the 2 new nuclei (cell plate) Animals: a ring of actin fibers (microfilaments) forms around the cell equator, pinching the cell in half.

21. Summary of Mitosis Prophase: Chromosomes condense Nuclear envelope disappears Centrosomes move to opposite sides of the cell Spindle fibers form and attach to centromeres on the chromosomes Metaphase Chromosomes are lined up on equator of cell Centrioles are at opposite ends of cell Anaphase Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes Chromatids pulled to opposite poles by the spindle fibers Telophase Chromosomes decondense Nuclear envelope reappears Cytokinesis: the cytoplasm is divided into 2 cells http://biology.uoregon.edu/reference/ort_mitosis/CellCycle.html

23. Important:The four diagrams must have the name of the phase, otherwise award [3 max].The four stages must be included to receive [5]. If correct number of chromosomes is not shown award [4 max].

25. How does mitosis ensure the daughter cells are genetically identical?

26. What happens when there is no control of cell division? Tumors = result of uncontrolled cell division The genetic checks that stop cells from reproducing fail to work and they grow out of control Oncogene = gene that turns a normal cell into a cancer cell Tumors can occur in any organ or tissue, though are most common after exposure to carcinogens (e.g. tobacco smoke) or in particularly active tissues (e.g. breast, skin) Angiogenesis: tumor recruits blood vessels and grows larger Metastasis: part of the tumor invades the blood vessel, travels through the blood and starts to forma a tumor in another part of the body

28. Normal cells stay in the G1 stage of the cell cycle until they are given a specific signal. Cancer cells enter the S phase without waiting for a signal.

29. Normal cells are mortal. This means that they can divide about 50 times and then they lose the ability to die. This “clock” gets re-set during the formation of the gametes. Cancer cells escape this process of mortality: they are immortal and can divide endlessly.

33. It is hard to remove all the tumor cells. Tumors often lack sharp boundaries for easy removal, and metastatic tumors can be very small and anywhere in the body.

34. Radiation and chemotherapy are aimed at killing actively dividing cells, but killing all dividing cells is lethal: you must make new blood cells, skin cells, etc. So treatment must be carefully balanced to avoid killing the patient.

35. Chemotherapy also has the problem of natural selection within the tumor. If any of the tumor cells are resistant to the chemical, they will survive and multiply. The cancer seems to have disappeared, but it comes back a few years later in a form that is resistant to chemotherapy. Using multiple drugs can decrease the risk of relapse: it’s hard for a cell to develop resistance to several drugs at the same time.