Mitosis and Meiosis. Chapters 8 & 10

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1 Mitosis and Meiosis Chapters 8 & 10

2 O Quiz #6: December 6th Learning Goals O Describe what happens during interphase O Identify steps of mitosis/meiosis by picture and function O Explain the diseases that occur when mitosis and meiosis go wrong, such as cancer and nondisjunction

3 Cell Reproduction O The life-cycle of the cell is broken into two sections: Interphase and Mitosis O In interphase, the cell is living its life and performing it s normal functions. O In mitosis, the cell is dividing and creating a daughter cell. O At the end of mitosis are two 100% identical daughter cells O The parent no longer exists

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5 Interphase Interphase is the majority of a cell s life Interphase is three separate stages During the first stage the cell has just been born. It is growing and developing proteins which will build the organelles in the cell During the second stage, the cell copies all of it s DNA (which it will pass to it s daughter cell) During the third stage, the cell performs its necessary functions and prepares for mitosis

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7 Prophase First Stage of Mitosis During prophase, the nucleus and nuclear envelope seem to disappear What is the chromatin inside the nucleus? (The DNA of the cell) The chromatin coils up into unique structures called chromosomes Each chromosome is attached to its identical copy of DNA made during interphase, and held together by a centromere Each organism has chromosomes of specific shapes and shades

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12 Second Stage of Mitosis O Metaphase O In metaphase, the centromeres of the chromosomes line up in the center of the cell and attach to spindle fibers O Spindle are cytoskeleton that are shaped like a football O The spindle are attached to an organelle called a centriole

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14 Second Stage of Mitosis O The centrioles are located where the two daughter cells will eventually form O Each chromosome lines up in the very center of the cell O Each chromosome is attached to two spindle fibers, which will pull the chromatids to each centriole

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16 O Anaphase Third Stage of Mitosis O In anaphase, the centromeres split and each identical chromatid begins to move to one of the centrioles O The chromatids reach each centriole by walking along the spindle fibers

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19 Fourth Stage of Mitosis O Telophase O In telophase, chromatids have reached each centriole. O The nucleus and nuclear envelope begin to reappear O A new membrane begins to form and the two daughter cells become visible

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22 Cytokinesis O The last step, which is separate from the rest of mitosis, is cytokinesis. O The two daughter cells have formed, but they are still attached to each other like conjoined twins O In animal cells, the two cells get pinched by the plasma membrane until the two cells are forced apart O In plant cells, the cell wall and the plasma membrane has to be built like a brick wall

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25 Apoptosis A cell can only undergo so many divisions before it becomes too risky to reproduce Apoptosis is also known as programmed cell death, because the cell will undergo reactions to cause its own destruction First, the DNA will fragment into pieces and all envelopes and membranes will blister Then, enzymes that have been dormant in the cell since the beginning of interphase first division are activated These enzymes destroy the organelles and membrane of the cells and deposit the cell parts into the blood stream for recycling

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28 Cancer O Cancer is a disorder that disrupts the process of cell reproduction O Carcinogenesis (development of cancer) is gradual and can occur for years before a cell becomes cancerous O Some of the following are characteristics that identify a cancerous cell from non-cancerous O 1. No differentiation. O Cancerous cells have lost function. They are simply excess tissue.

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30 Cancer O 2. No contact inhibition. O Normal cells do not add press against neighboring cells. Cancerous cells expand no matter how much room they have, causing neighboring cells stress O 3. Formation of tumors O Cancer cells pile on top of each other, constantly dividing without end. They never undergo apoptosis O 4. Cancer cells invade O Cancer can produce enzymes that seep into the blood stream and invade other cells, causing the cancer to spread

31 O Why does cancer spread? Cancer O Sometimes cancer is benign which means your body located the cancer and contained it inside a capsule so it can t spread from one cell to the next. O Even this is rare. You and I get cancer all the time. Our body fights it off and we never notice. O If a tumor does grow, it typically damages the capillaries between the cell and the blood vessels, so the cell can t receive any nutrients. O You d think this would kill the cancer O In order to survive, the cancer cell has learned to either spread its proteins to other cells (metastasizing) or rebuild new blood vessels into the tumor (angiogenesis).

32 O Back to genetics Meiosis O How do you end up with only one allele for each gene from your parents? O The sex cells of your body, called gametes, (eggs or sperm, for us) have a process that ensures which alleles offspring receive are completely random. O Their division is different than every other cell in your body O The division of gametes in organisms is called meiosis

33 Homologous Chromosomes O Homologous Chromosomes are two similar chromosomes O They are similar because they contain the same GENES but not necessarily the same ALLELES (versions of a gene) O Humans have 23 pairs of homologous chromosomes for a total of 46 chromosomes. O We get one chromosome of each pair from our mom and one chromosome from our dad

34 Homologous Chromosome Pair From Mom From Dad

35 Diploid vs Haploid O Most cells in our body are diploid (2n), meaning they have two sets of each chromosome O A set from mom and a set from dad. O The gametes in our body, however, are haploid (n), meaning they have only one set of each chromosome O **Two haploid gametes fertilize to produce one diploid zygote** O Meiosis is the process of creating these haploid gametes.

36 Meiosis I O Meiosis is broken into two sections: meiosis I and meiosis II O At the start of meiosis there is one diploid (2n) cell O By the end of meiosis there will be four haploid (n) cells O These four haploid cells will be the gametes for the organism

37 Prophase I O Prophase I looks like prophase, for the most part O Organelles disappear, chromosomes appear O One difference though is a process called crossing over O Crossing over is when homologous chromosomes actually trade sections of chromosomes

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39 Prophase I O Why would cells do this? O Increases randomness of which alleles are found on each chromosome O In prophase I the organelles disappear, the chromosomes for each cell appear, and each pair of homologous chromosomes undergoes crossing over

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41 Metaphase I O Just like in mitosis metaphase, the chromosomes line up in the center of the cell in metaphase I of meiosis. O One difference: Each homologous chromosome lines up next to each other in the cell

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43 Anaphase I O In anaphase I, instead of chromatids separating as in mitosis, the homologous chromosome pairs separate from each other O Whole chromosomes move to each centriole O This step ensures each gamete will hold only one copy of each chromosome

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45 Telophase I O Unlike in mitosis, the cell will not undergo a full division O Instead, a small membrane will be built to ensure the chromosomes do not go between each new cell O The cell is now ready to enter Meiosis II

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47 Meiosis II (Similar to Mitosis) O Prophase II O Chromosomes appear and nuclear envelopes disappear again O Metaphase II O Chromosomes line up at the center of the cell at the metaphase plate O Anaphase II O Individual chromatids separate and move toward separate centrioles O Telophase II O Cell parts reform, and cytokinesis occurs O Four new daughter cells have been produced

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49 Genetic Variation O How does meiosis account for the randomness of which gene the offspring will receive? O In Prophase I, crossing over mixes which genes are found on which chromosome O In Anaphase I, separating chromosomes ensures each cell will only receive 1 allele/gene O In Meiosis II, the separation of chromosomes into chromatids ensures each cell only has ½ of the necessary DNA for an organism O This will have to mix with the DNA of the other parent, further adding to the randomness O Because of this, can you guess the number of possible genetic combinations you can have?

50 4,951,760,200,000,000,000,000,000,000

51 Nondisjunction O Nondisjuction is when homologous chromosomes fail to properly separate O Nondisjunction occurs if the homologous chromosomes or chromatids do not separate. O The results can be gametes with an extra set of chromosomes or a missing set. O Sometimes the gametes can still fertilize. Sometimes they cannot. If they can, however, the resulting zygote will have too many or too few chromosomes.

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53 O Trisomy 21 Nondisjunction O An extra #21 chromosome O Result: Down Syndrome O XXY O XO O The male has an extra X chromosome O Result: Klinefelter s syndrome O The female is missing an extra sex chromosome O Result: Turner s Syndrome

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