Chapter 2: Cells and Cell Division

1. Chapter 2: Cells and Cell Division By the completion of this chapter, you should have an understanding of: The structure and organization of cells in higher organisms Chromosomes as cellular structures that carry genetic information The parts of the cell cycle The stages of mitosis The significance of mitosis and the genetic control of the cell cycle The process of meiosis and it’s involvement in the formation of gametes

2. 2.1 Cell Structure Reflects Function • Cells vary widely in size, shape, function, and life cycle • All ours have a plasma membrane, cytoplasm, membrane-bound organelles, and a membrane-bound nucleus • Many genetic disorders alter cellular structure or function

4. Plasma Membrane • Double-layered plasma membrane separates the cell from external environment • Controls what enters and exits the cell • Small molecules, gases, and water can pass easily through; larger molecules require energy • Molecules on the surface of the P.M. give the cell identity

6. Cytoplasm • Jelly-like mixture that contains specialized structures called organelles

7. Organelles • In eukaryotes (contain a nucleus), organelles divide the cell into function compartments

8. Endoplasmic Reticulum • Network of membranes that form channels in the cytoplasm • The space inside the ER is called the lumen – proteins are folded, modified, and readied for transport to other location.. Or are tagged for export from the cell

9. Endoplasmic Reticulum, cont’ • The outside of the ER is often covered with RIBOSOMES • Ribosomes are the most numerous cellular structures and can also be found in the cytoplasm • Ribosomes are the site of protein synthesis

10. Golgi Apparatus (Golgi Body) • Flattened membrane sacs • Receives proteins from the ER and distributes them to their destinations inside and outside cell • Sorts, modifies, and packages proteins

11. Lysosomes • Membrane-enclosed vesicles that contain digestive enzymes • Enzymes are made in the ER, transported to Golgi, packaged into vesicles that bud off the Golgi and become lysosomes • Proteins, fats, carbs, and viruses that are brought into the cell are broken down and recycled, or exported for disposal in lysosomes

12. Mitochondria • Energy transformation (food  energy) • Carries own genetic information in form of a circular DNA molecule

13. Nucleus • Largest organelle • Enclosed by a double membrane called nuclear envelope with pores that allow direct communication between nucleus and cytoplasm • Inside: Nucleolus (Nucleoli plural) makes ribosomes, and Chromatin (dark strands of DNA) eventually condense to form chromosomes

14. Chromosome Review • DNA + Proteins = Chromosomes • In humans, chromosomes exist in pairs • Body cells (somatic) carry 23 pairs, or 46 total chromosomes • Egg and sperm (gametes) carry only 23 UNPAIRED chromosomes • Sex chromosomes – X and Y – males have one of each; females have 2 X’s • All other 22 pairs of chromosomes are called AUTOSOMES

16. 2.2 The Cell Cycle • Two stages – Division and Non-division • Cell Cycle: the sequence of events from division to division • 3 parts= Interphase, Mitosis, Cytokinesis • Interphase = time between division • Mitosis = division of chromosomes • Cytokinesis = division of cytoplasm

17. Interphase • 3 stages: • G1 (Gap 1) = cytoplasmic components are constructed; cell size doubles • S (Synthesis) = each chromatid is copied; chromosomes become double stranded • G2 (Gap 2) = a smaller period of cell growth

18. Mitosis • Division and distribution of chromosomes • End result = 2 identical daughter cells with 46 chromosomes each • 4 phases: Prophase, Metaphase, Anaphase, Telophase

19. Prophase • Chromatin coils to become shorter, thicker chromosomes • At the end of prophase, chromosomes appear to be 2 strands of identical chromatids • Chromatids are held together by centromere • Two chromatids held together by a centromere are called SISTER CHROMATIDS • Near the end of Pro., nuclear membrane breaks down and spindle fibers form

21. Prophase

22. Metaphase • Chromosomes are free in cytoplasm • Spindle fibers attach to centromeres and pull chromosomes to equator of cell • META = MIDDLE

23. Anaphase • Centromeres divide • Sister chromatids become singular chromosomes • Chromosomes move to opposite ends of the cell

24. Telophase and Cytokinesis • Chromosomes uncoil, spindle fibers break down, nuclear membrane reforms • Cleavage furrow down equator of cell causes constriction and eventually the cell divides in two (Cytokinesis)

25. 2.3 Mitosis is Essential for Growth and Cell Replacement • Bone Marrow – continuously producing 2 million red blood cells each second • Skin Cells – divide to replace dead cells • Many muscle cells – enter a state of G0 and do not divide (spinal cord cells do same) • Occasionally cells escape control of cell cycle = CANCER

27. Metaphase Prophase Spindle Fiber Centriole Nuclear Membrane Chromosome Anaphase Interphase Cell Membrane Nucleolus Centriole Telophase Chromatids Chromatin Animation Cytokinesis

28. 2.4 Cell Division by Meiosis • Cells with 2 copies of each chromosome are called DIPLOID (2n) = body cells • 2n = 46 in humans • In meiosis, pairs of chromosomes separate so each cell receives only 1 of each chromosome • This is called HAPLOID (n) • In humans, egg and sperm are haploid • n= 23 • n (sperm) + n (egg) = 2n (baby)

29. Meiosis 1 • Meiosis 1: Chromosome # Reduced • Interphase = same as interphase in mitosis • Prophase 1 = chromosomes find their matches (homologous chromosomes) and form a tetrad • Metaphase 1 = hom. chrom. line up together in middle of cell • Anaphase 1 = hom. chrom. separate • Telophase 1 = cell divides into 2 containing 1 of each chromosome

31. Meiosis 2 • **NO INTERPHASE AGAIN!!! • Prophase 2 = spindle forms • Metaphase 2 = sister chromatids line up on equator attached @ centromere • Anaphase 2 = sister chromatids separate at centromere and move to opposite ends of cell • Telophase 2 = chromosomes uncoil, nuclear membrane reforms, cells divide into 4 cells total with half the number of original chromosomes

33. Summary of Meiosis Comparing Mitosis and Meiosis

34. Meiosis Produces Variation • Random Assortment of Chromosomes: • eggs/sperm can get combination of maternal and paternal chromosomes • Crossing Over: • genes on maternal chromosome can switch with genes on paternal chromosome during Prophase 1.

35. Random Assortment

36. Crossing Over