1 Cellular Division

2 When do Cells Divide? Cells obtain nutrients and eliminate wastes through the cell membrane. There must be enough surface area for all of the exchanges to take place with the environment. It is more difficult for cells with a larger volume to transport, so cells remain relatively small to survive. When a cell reaches its maximum size, the nucleus initiates cell division.

3 Why do Cells Divide? Cell division accounts for 3 essential life processes: Growth - increase in size Differentiation - cells develop specialized shapes and functions.

4 Why do Cells Divide? Cell division accounts for 3 essential life processes: Repair - heal injuries by cell regeneration at the injury site

5 Why do Cells Divide? Cell division accounts for 3 essential life processes: Reproduction - making new organisms Asexual - offspring produced by one parent; the result of cell division Sexual - offspring have a combination of genetic material from two parent organisms; requires meiosis, a specialized form of cell division

6 How do Cells Divide? Phase - a defined period within a cycle of change Cell cycle - the sequence of phases in the life cycle of a cell

7 The Cell Cycle

8 Interphase Interphase - growth and preparation –Occurs between divisions 3 stages –G1 (gap 1) - growth and development –S (synthesis - chromosomes replicate; the cell commits to cell division –G2 (gap 2) - cell synthesizes other organelles and other materials needed for division

9 Interphase During interphase, the chromosomes are in chromatin form - a tangled bundle During S phase, each chromosome is replicated, creating sister chromatids As cell division begins (during prophase), the chromatin coils and condenses to chromosomes and the sister chromatids are clearly visible, joined at the centromere.

10 Chromatin - uncoiled DNA Chromosome - neatly coiled DNA

11 Cell Division - Mitosis and Cytokinesis There are 2 stages in cell division: –1. Mitosis »The cell’s nucleus divides into 2 nuclei. –2. Cytokinesis »The cell’s cytoplasm divides to make 2 daughter cells

12 Mitosis Mitosis is the splitting of the cell’s nucleus It has 4 parts: –Prophase –Metaphase –Anaphase –Telophase

13 Mitosis - Prophase Chromosomes condense In animal cells, a pair of centrioles migrates to each side of the cell The mitotic spindle forms from the centrioles The nuclear membrane and nucleolus dissappear

14 Mitosis - Prophase

15 Mitosis - Metaphase Spindle arranges chromosomes along “metaphase plate”

16 Mitosis - Anaphase Centromeres divide and sister chromatids are pulled apart, so that each daughter cell has an identical set of chromosomes

17 Mitosis - Telophase Nuclear membrane reforms around each set of chromosomes Two nucleoli form Chromosomes uncoil into chromatin Cleavage furrow starts to form (for cytokinesis)

18 Cytokinesis The cell’s cytoplasm divides to make 2 daughter cells The cleavage furrow pinches cell in half

19 Four Mitotic Stages Prophase Prophase Metaphase Metaphase Anaphase Anaphase Telophase Telophase

20 Identical Daughter Cells Parent Cell Two identical daughter cells

21 Mitosis in Animal Cells Rat epithelial cells

22 Mitosis in Plant Cells Plant cells have a cell wall. During mitosis, a cell plate forms a new cell membrane and cell wall between the two daughter cells.

23 History Mitosis was first observed microscopically in the 1870s by the German biologist Walter Flemming –He coined the term mitosis –from the Greek word mitos, –meaning thread

24 The Cell Cycle Interphase may be up to 90% of the cell cycle. The time varies for different cell types.

25 The Cell Cycle

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28 Meiosis Formation of Gametes (Eggs & Sperm)

29 Facts About Meiosis Produces sex cells called gametes (eggs & sperm) Produces sex cells called gametes (eggs & sperm) Occurs in the testes in males (Spermatogenesis) Occurs in the testes in males (Spermatogenesis) Occurs in the ovaries in females (Oogenesis) Occurs in the ovaries in females (Oogenesis)

30 Facts About Meiosis The gametes (eggs and sperm) are haploid - which means they have 1 set of chromosomes, not 2 like all of our diploid body cells. The gametes (eggs and sperm) are haploid - which means they have 1 set of chromosomes, not 2 like all of our diploid body cells. Why we need meiosis --> this is critical for sexual reproduction. Why we need meiosis --> this is critical for sexual reproduction.

31 It starts with a diploid cell that has 46 chromosomes (2N). It starts with a diploid cell that has 46 chromosomes (2N). The DNA is replicated, so there are 92 chromosomes. There are 4 copies of each, making a tetrad. The DNA is replicated, so there are 92 chromosomes. There are 4 copies of each, making a tetrad. The Process of Meiosis

32 The first division, very similar to mitosis, occurs, producing two cells with 46 chromosomes each. The first division, very similar to mitosis, occurs, producing two cells with 46 chromosomes each. A second division occurs, producing 4 haploid daughter cells that each have 23 chromosomes (N) A second division occurs, producing 4 haploid daughter cells that each have 23 chromosomes (N) The Process of Meiosis

33 Meiosis: Two Part Cell Division Diploid Meiosis I Meiosis II Diploid at start Haploid

34 Meiosis must reduce the chromosome number by half Meiosis must reduce the chromosome number by half Fertilization then restores the 2n number Fertilization then restores the 2n number from momfrom dadchild meiosis reduces genetic content too much! The right number! Meiosis Forms Haploid Gametes

35 Pieces of chromosomes are exchanged, which produces genetic recombination in the offpsring Crossing-Over

36 Crossing-over multiplies the number of different gamete types Crossing-Over

37Fertilization 1N = 3 2N = 6

38 Humans –Haploid number (1N) = 23 –Diploid number (2N) = 46 Somatic cells are diploid. Gametes are haploid.

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