1. Unit 9 - Cell Reproduction In this unit, we look at the mechanics of cells multiplying for both growth and reproductive processes.

2. Cell Division Cells divide into two for three main purposes: Enlargement of the organism – cells will divide in order for the total number of cells, an thus the total size of the whole organism, to increase. In children, cells such as bone cells continuously divide in order to create larger bones. Maintenance – as cells die off, they need to be replaced. In order to maintain a constant state, dead cells must be continuously replaced. An example of this occurs in the skin.

3. Cell Division The third purpose of cell division is to create new organisms. This may mean one cell splitting into two identical clones, or may mean the create of sperm/egg cells to create a new organism. All of these processes where new cells are made are called reproduction. Growth specifically refers to the process of an individual cell increasing in size.

4. Cell Division Cells must divide because once they reach a certain size, they hit the limit of their surface-to- volume ratio and can’t grow any more. They cannot grow because they don’t have enough surface to bring in food/oxygen and to pump out wastes. Therefore, organisms decide instead to split into two entire cells, solving this problem.

5. Reproduction There are two types of cellular reproduction. Sexual reproduction creates new, genetically- unique organisms from specialized cells from two organisms. Asexual reproduction creates identical organisms (clones) to the original. This requires only one organism and does not create generic variety.

6. Asexual Reproduction Asexual reproduction comes in four types: Binary Fission – used by prokaryotes only. It is simply the duplication of the DNA, and then the cell splitting into two, each one with a copy of the DNA.

7. Asexual Reproduction Fragmentation – when an organism is broken apart and each piece will regrow into a full organism. Most plants and sea stars can do this. This requires the organism to be simple enough that parts can live on without the whole organ system.

8. Asexual Reproduction Budding – this is a process where a portion of an organism is purposely released to act independently. The hydra creates a second entire organism on its stalk which eventually buds off as a whole. This is NOT the same as releasing spores, however.

9. Asexual Reproduction Mitosis – this is the eukaryotic process of making two identical cells from one original. In some ways it resembles binary fission. However, the multiple chromosomes of a eukaryote make it more complex than diving the single DNA piece in prokaryotes.

10. Mitosis Mitosis is broken down into four steps. o Prophase o Metaphase o Anaphase o Telophase

11. Mitosis - Interphase Between cell divisions, the cell exists in interphase. During this time, the DNA is loose in the nucleus so that it can be accessed. This is the “normal” state of a cell, growing, eating, etc. When it comes time to divide, the cells leaves interphase and goes into the mitotic phase.

12. Mitosis - Prophase The first phase of mitosis is prophase. In this step, the DNA condenses into visible chromosomes. A chromosome has an X-shape because it is made of two pieces of DNA (each called a chromatid ) attached together at the centre at the centromere.

13. Mitosis - Prophase The nuclear membrane begins to break down. This will allow the DNA to be carried into each of the two cells to be created. A special organelle called the centrosome starts to activate. This organelle has two “cores” which control a number of microtubules called the mitotic spindle.

14. Mitosis - Metaphase By metaphase, the nuclear membrane has completely dissolved and the chromosomes have all aligned at the “equator” of the cell.

15. Mitosis - Metaphase The centrosomes have fully set up the spindle network. Each centromere has tubules attached to it. These help align the chromosomes and make sure neither of the new cells gets too many or too few chromosome.

16. Mitosis - Anaphase Once anaphase begins, the spindle fibers pull the chromosomes apart, dragging one chromatid of each chromosome towards the ends of the cell. This way, each of the new cells will have half of every chromosome and will be able to make the other by copying the first.

17. Mitosis - Telophase Telophase sees the DNA fully reach the poles of the cell. In each end, the nuclear membrane begins to reform. The spindle fibers begin to break down and the centromeres deactivate. Telophase is similar to prophase in reverse.

18. Mitosis - Telophase The final process is to separate into two cells. This process is called cytokinesis and involves the center of the cell’s membrane pinching in to create a cleavage furrow that ultimately completely separates the membranes.

19. Mitosis - Interphase Once complete, the two cells will each enter interphase. During this time they will grow and duplicate their DNA (remember that they only have 1 chromatid of each chromosome at first). Later on, the cell may undergo mitosis again and again.

20. Mitosis

21. Ploidy Normally in a cell, most organisms carry two copies of every chromosome. This is called an diploid (2n) organism. One of each chromosome was inherited from each parent. Cells that are intended to create a new organism (such as sperm/egg cells) are called gametes. Gametes only carry one of each chromosome and are therefore haploid (n) cells. One gamete will fuse with another to give the new organism a full set.

22. Ploidy Normally in a cell, most organisms carry two copies of every chromosome. This is called an diploid (2n) organism. One of each chromosome was inherited from each parent. Cells that are intended to create a new organism (such as sperm/egg cells) are called gametes. Gametes only carry one of each chromosome and are therefore haploid (n) cells. One gamete will fuse with another to give the new organism a full set.

23. Ploidy In diploid organisms, we have two of each chromosomes. As they code for the same genes, they are called homologous chromosomes or homologs. Each homolog is made of two chromatids, for a total of four. In mitosis, the new cells get one chromatid from every chromosome, in sexual reproduction, each of the gametes created will only get one chromatid from each pair of homologs, giving them only half of the DNA of a normal cell, making them haploid.

24. Meiosis To create gametes, which are needed for sexual reproduction, a diploid cell undergoes two cell divisions in a row to create four haploid cells. Essentially, mitosis is happening twice in a row. So instead of leaving a cell with one chromatid of every chromosome, each cell at the end of two divisions will only have one chromatid from each pair of chromosomes. This process is called meiosis.

25. Meiosis Meiosis works mechanically very similarly to mitosis done twice. There are, however a few key differences. For each division, there are prophase, metaphase, anaphase, telophase, and cytokinesis. One key difference is that instead of entering interphase after the cell division, in meiosis, the cell will divide again immediately.

26. Meiosis I For the first cell division of the cell, meiosis I, prophase I occurs virtually the same as prophase in mitosis except for one major difference. Instead of each chromosome lining up individually at the equator of the cell, each pair will now line up in a structure called a tetrad (tetra=four, as there are four chromatids in a pair of chromosomes).

27. Meiosis I A special process called crossing over occurs in prophase I. The two sister chromosomes will trade pieces DNA with each other. This creates a new combination of traits not seen in the parent, leading to more diversity.

28. Meiosis I The crossing-over site is called a chiasmata. On any one pair of homologs, multiple chiasmata may be present. The final DNA is considered to be recombinant as it has been broken apart and recombined.

29. Meiosis I At metaphase I, all the tetrads will be aligned at the equator, and one chromosome will go to each of the cells to be made. In mitosis, one chromatid of every chromosome went to both cells, here, both chromatids of only a single of each chromosome go to each of the new cells.

30. Meiosis I Anaphase I will separate the DNA just like in mitosis. Telophase will reform the cells as two separate cells, just like in mitosis. Cytokinesis will occur and we are left with 2 cells.

31. Meiosis II Instead of entering interphase, meiosis II will occur right away. This is important because this means the cells had no time to duplicate their DNA, and thus the final product will be cells with only half the needed DNA.

32. Meiosis II Meiosis II has all the usual steps of mitosis. Prophase and metaphase happen normally, and there is NOT crossing over in meiosis II. At metaphase II, each chromosome lines up on the equator, and a lone chromatid is pulled to each of the new cells to be made.

33. Meiosis II Telophase and cytokinesis leave us with a total of four haploid cells from our original single diploid cell. Each of these gametes only has one chromatid for each pair of chromosomes in the original.

34. Diversity Due to crossing over, the diversity of traits passed on to offspring is increased dramatically. This is how multiple siblings can all look very different from one another and from their parents. Another factor that increases diversity is that no particular homolog of one chromosome has to be paired with a particular homolog of another chromosome. This means that every chromosome/chromatid inherited is completely independent of each other, meaning certain traits on different chromosomes are not linked.

35. Diversity In this example, note that a light green chromosome may end up with either a light blue chromatid or a dark green chromatid at the final step. The traits on one chromosome are not linked to ones on other chromosomes.