Cellular Reproduction: Cells from Cells

1. Cellular Reproduction:Cells from Cells Chapter 8

3. Reproduction at the cellular level • Within your body, millions of cells must divide every second to maintain a total of ~60 trillion cells

4. Biology and Society:A $50,000 Egg! • Ad in ivy league college newspaper: “Egg Donor Needed – Large Financial Incentive” - tall, athletic, healthy, high SAT scores • Tabloid: “Sterile Couple Seeks Ivy Dream Girls Eggs” • Infertility affects 1 in 10 American couples • In vitro fertilization (IVF) can help infertile couples - a sperm and an egg are joined in a petri dish - resultant embryo is implanted into the mother’s uterus

5. Human embryo developed through in vitro fertilization • Original cell divided 3 times to form 8 cells which will continue to divide and then specialize Figure 8.1

6. What Cell Reproduction Accomplishes • Reproduction – birth of new organisms - occurs much more often at the cellular level • Within your body million of cells must divide every second to maintain your total of ~60 trillion cells • Cell division plays a role in - the replacement of lost or damaged cells - cell reproduction and growth

7. Passing on Genes from Cell to Cell • Cell division results in 2 genetically identical ‘daughter’ cells and to the original ‘parent’ cell • Before a parent cell divides, it must duplicate its chromosomes (1 set into 2 sets) - DNA-containing structures that contain the cell’s genetic blueprint - during cell division one set of chromosomes is distributed to each daughter cell

8. The Reproduction of Organisms • Asexual reproduction – offspring are genetic replicas of the parent - single-celled organisms reproduce by simple cell division • Many multicellular organisms can reproduce asexually - some sea star species can grow new individuals from fragmented pieces - a new plant from a houseplant clipping • One simple principle of inheritance in asexual reproduction - lone parent and each offspring have identical genes • Mitosis – asexual reproduction

9. 1 set of chromosomes have been duplicated  2 sets • 2 identical sets allocated to opposite sides of the parent • Division - 2 daughter amoebas will be genetically identical Play Hydra Budding Figure 8.2a

10. Split into 2 pieces, each piece may be able to regrow into new organisms • 1 arm is in the process of regenerating the rest of the body Figure 8.2b

11. African violet can be grown asexually from a clipping • Resulting plant will be genetically identical to the plant from which the clipping was taken Figure 8.2c

12. Sexual Reproduction • Requires fertilization of an egg by a sperm • Meiosis – a special type of cell division occurs only in reproductive organs - such as testes and ovaries in humans - results in production of egg (oocyte) and sperm cells - germ cells have only half as many chromosomes as the parent cell • Sexually reproductive organisms use meiosis for reproduction and mitosis for growth and maintenance

13. The Cell Cycle and Mitosis • Almost all of the genes of a eukaryotic cell are located on chromosomes in the cell nucleus - exception are mitochondrial and chloroplasts genes • Eukaryotic chromosomes contain one very long DNA molecule - typically with 1000s of genes • Number of chromosomes in a eukaryotic cell depends on the species

14. Figure 8.3

15. Eukaryotic Chromosomes • Chromatin – a combination of DNA and proteins - proteins help organize the chromatin and control activity of its genes • Chromosomes are not visible in a cell until cell division occurs - exist as a diffuse mass of fibers longer than the nucleus its stored in until a cell prepares to divide - total DNA in a human cell’s 46 chromosomes could stretch for over 2 meters

16. A plant cell just before division: purple threads are the chromosomes • Thinner red threads in the surrounding cytoplasm are the cytoskeleton Figure 8.4

17. DNA Packing • How can long DNA molecules fit into a tiny nucleus - DNA in a cell is packed into an elaborate, multilevel system of coiling and folding • Histones - association of the DNA with histones allows for this system of coiling and folding

18. DNA packing in a eukaryotic chromosome • Successive levels of DNA coiling results in highly condensed chromosomes • When not dividing DNA of active genes only lightly packed - ‘beads on a string’ arrangement • Highly compacted chromosome from a cell preparing to divide – restricted region is the centromere Play DNA Packing Figure 8.5

19. Sister Chromatids • Before a cell divides, each chromosome is duplicates all of its chromosomes - DNA molecule of each chromosome is copied - the 2 copies are called sister chromatids and are joined together at the centromere • When the cell divides, the sister chromatids separate from each other - once separated each chromatid is a full-fledged chromosome that is identical to the original chromosome - one new chromosome goes to one daughter cell, and the other goes to the other daughter cell

20. Chromosome duplication & distribution During cell reproduction - A cell duplicates each of its chromosomes and distributes a copy to each of the daughter cells Figure 8.6

21. The Cell Cycle • The rate at which a cell divides depends on its role within the organism - some cells divide once a day, other less, and highly specialized cells (muscles and neurons) do not divide • Eukaryotic cells that divide undergo an orderly sequence of events called the cell cycle • The cell cycle consists of 2 distinct phases: - Interphase - Mitotic phase (mitosis and cytokinesis)

22. Interphase • G0 - time when a cell performs its normal functions - example: a cell in your stomach lining might make and release enzymes that aid in digestion • During G1 (gap or growth), S (DNA syntheis) and G2 of interphase a cell roughly doubles everything in its cytoplasm - increases its supply of proteins, the number of many of its organelles, and grows in size - S phase: chromosome duplication - in G2 each chromosome consists of 2 identical sister chromatids and the cell is preparing to divide

23. Figure 8.7

24. Mitotic Phase (M Phase) • Part of the cell cycle when the cell is dividing - includes 2 overlapping processes, mitosis and cytokinesis • Mitosis - nucleus and its contents (duplicated chromosomes) divide - evenly distributed forming 2 daughter nuclei - consists of 4 phases: prophase; metaphase; anaphase; and telophase • Cytokinesis – cytoplasm is divided in two - usually begins before mitosis is completed • Combination of mitosis and cytokinesis produces 2 identical daughter cells

25. Play Mitosis Overview Figure 8.8.1

26. Figure 8.8.2

27. Figure 8.8.3

28. Cytokinesis • Typically occurs during telophase • Is the division of the cytoplasm • Is different in plant and animal cells Play Cytokinesis Figure 8.9a