1. Cell Division L3 Biology

2. Why do cells divide? Growth Growth Repair/regeneration Repair/regeneration Reproduction Reproduction  asexual

3. The Cell Cycle – life cycle of a cell that will divide:

4. Cells go through a life cycle First growth phase 9 hr Synthesis phase – DNA is copied – 10 hr Second growth phase – 2 hr Cell division – mitosis – 1 hr – followed by division of the cytoplasm interphase

5. Chromosomes Invisible most of the time - Only visible during cell division (mitosis or meiosis) Invisible most of the time - Only visible during cell division (mitosis or meiosis) During S-phase – the DNA replicates (makes an exact copy of itself) During S-phase – the DNA replicates (makes an exact copy of itself) This means the cell has twice as much DNA in it after replication This means the cell has twice as much DNA in it after replication Once a chromosome has replicated, it shortens and thickens and can now be seen in our microscopes. Once a chromosome has replicated, it shortens and thickens and can now be seen in our microscopes.

6. One Chromosome Sister SisterChromatids Each strand is an identical copy of the other one Centromere Where the two chromatids Are attached to each other – This is different for each chromosome DNA

7. Chromosome Number Each species has the same number of chromosomes in all their cells that are made by mitosis. This is the diploid number (2n). In humans this number is 46. So cells of your skin and muscle and liver each have 46 chromosomes in them. Look how many chromosomes are in the cells of these creatures: Each species has the same number of chromosomes in all their cells that are made by mitosis. This is the diploid number (2n). In humans this number is 46. So cells of your skin and muscle and liver each have 46 chromosomes in them. Look how many chromosomes are in the cells of these creatures: 2n = 38 2n = 78 2n = 94 2n = 42

8. When cells divide by mitosis, each daughter cell receives the same number of chromosomes as its mother cell has. When cells divide by mitosis, each daughter cell receives the same number of chromosomes as its mother cell has. In order to do this, the chromosomes must be copied first, then one of each copy is placed in the new cells. In order to do this, the chromosomes must be copied first, then one of each copy is placed in the new cells. 46 92

9. Humans have 22 pairs of similar chromosomes; one pair of sex chromosomes

10. Sex Chromosomes Similar in females: XX Similar in females: XX Not similar in males: XY Not similar in males: XY

11. Mitosis Cell division resulting in nuclei identical to parent cell – asexual reproduction for some organisms. Cell division resulting in nuclei identical to parent cell – asexual reproduction for some organisms. Four phases: Prophase Four phases: Prophase Metaphase Metaphase Anaphase Anaphase Telophase Telophase

12. Prophase Mother cell Chromosomes become visible, nucleus disintegrates, spindle fibers form, centrioles move to poles Chromosomes line up in the middle Chromatids separate and move to opposite poles Chromosomes become invisible, nuclear membranes form around new nuclei, spindle fibers disappear.

14. Prophase Chromosomes become visible Chromosomes become visible Spindle fibers form Spindle fibers form Nuclear membrane disintegrates Nuclear membrane disintegrates Nucleolus disintegrates Nucleolus disintegrates In animal cells, centrioles move to opposite ends of the cell (poles) and spindle fibers attach to them. In animal cells, centrioles move to opposite ends of the cell (poles) and spindle fibers attach to them.

15. Metaphase Chromosomes line up single file at the middle of the cell Chromosomes line up single file at the middle of the cell

16. Anaphase Sister chromatids are pulled apart toward opposite poles Sister chromatids are pulled apart toward opposite poles

17. Telophase Nuclear membrane forms around Nuclear membrane forms around each group of chromosomes Nucleolus reappears in each nucleus Nucleolus reappears in each nucleus Spindle fibers disappear Spindle fibers disappear Chromosomes become invisible again as chromatin Chromosomes become invisible again as chromatin Cytokinesis begins in plant cell by formation of cell plate; cleavage furrow in animal cell completely separates the two nuclei into two different cells. Cytokinesis begins in plant cell by formation of cell plate; cleavage furrow in animal cell completely separates the two nuclei into two different cells. Cell plate

18. Occurs after nucleus has Occurs after nucleus has been duplicated Begins in anaphaseby the Begins in anaphaseby the formation of a cleavage furrow Cytokinesis in an animal cell

19. Begins in telophase in plant cells Begins in telophase in plant cells by the formation of a cell plate. by the formation of a cell plate.

21. Find the different stages of mitosis in these onion root cells: C B A D E

22. Meiosis Cell division producing cells that have half the number of chromosomes of the mother cell Cell division producing cells that have half the number of chromosomes of the mother cell Produces gametes – eggs and sperm Produces gametes – eggs and sperm Occurs so that fertilization doesn’t increase the number of chromosomes in each generation. Occurs so that fertilization doesn’t increase the number of chromosomes in each generation.

23. Why is cell division important to understand? Cancer is uncontrolled cell division – cells then spread to other parts of the body. Cancer is uncontrolled cell division – cells then spread to other parts of the body.

24. Gametes = sex cells Eggs or sperm Eggs or sperm Have half the normal number of chromosomes = 23 in humans Have half the normal number of chromosomes = 23 in humans Combine DNA from two different parents, producing offspring that is different from each parent Combine DNA from two different parents, producing offspring that is different from each parent

25. Skin cancer cells This is how skin cancer looks – a change in a mole is the first symptom. If you know how your moles usually look, you can identify any changes. This is the worst kind of skin cancer – Malignant melanoma – it has often metastesized by the time it is diagnosed.

26. Meiosis = Gametogenesis – the formation of eggs and sperm Oogenesis Oogenesis Production of an egg One mother cell produces one egg cell and three polar bodies that die Spermatogenesis Production of sperm One mother cell produces 4 equally sized sperm cells

27. One egg 4 sperm Sperm formation Egg formation

28. Fertilization The sperm unites with the egg forming a zygote (fertilized egg). The zygote then divides by mitosis to produce the trillions of cells that make up a multicellular body like yours.

29. The End