uq.au

1. Cellular Basis of Reproduction and InheritanceLecture 8, Mitosis http://www.uq.edu.au

2. Much of the text material in the lecture notes is from our textbook, “Essential Biology with Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship. Other sources were sometimes used, and are noted.

3. Where We Are Cells Genetics http://www.robslade.demon.co.uk Evolution and Diversity Ecology Animal Structure and Function

4. Outline • Overview • Asexual and sexual reproduction • Chromosomes • Mitosis and the cell cycle • Cytokinesis • Cancer cells • Words and terms to know • Possible test items

5. Reproduction • Reproduction is often associated with the formation of new offspring. • Reproduction, however, is not only about forming new organisms—it also occurs in many types of cells. • Our skin has an outer layer of dead epithelial cells—underneath are layers of epithelial cells dividing and undergoing chemical reactions.

6. Human Skin http://publications.nigms.nih.gov http://www.web-books.com

7. Cell Division • New epithelial cells move toward the skin surface to replace cast-off dead cells. • New cells are formed in our tissues to help heal wounds when we are injured. • This form of cellular reproduction, or cell division, is a lifelong process for growth to tissue repair.

8. Genetic Transmission • When a cell divides, the two daughter cells are identical to each other and the parent cell. • In this context, daughter—the term used by biologists—does not imply gender. • The parent cell duplicates its set of chromosomes before it splits into two daughter cells. • During cell division, the two sets of chromosomes are distributed to the daughter cells. • Each daughter cell receives identical sets of chromosomes containing identical genes.

9. Asexual Reproduction • Some single-cell organisms, such as amoeba, reproduce through simple cell division. • The offspring are genetic replicas of the one parent. • The process is known as asexual reproduction since it does not involve fertilization of an egg by a sperm. • Growing clippings from houseplants often involves asexual reproduction. • In asexual reproduction, the parent and its offspring have identical genetic material. • The process is known as mitosis.

10. Amoeba Amoeba reproduce asexually. Cell division http://biophysik.uni-bremen.de Microscopic view http://www.gutenburg.org

11. Sea Stars and Crab http://jacobtanenbaum.com Two sea stars and a crab—sea stars and crabs can replace their appendages through mitosis.

12. Sexual Reproduction • Sexual reproduction requires fertilization of an egg by a single sperm. • The production of egg and sperm cells involves a form of cell division known as meiosis. • The two types of cell division, mitosis and meiosis, are involved in the lives of all sexually-reproducing organisms. • Next week, we will cover meiosis and how the chromosomal material is transmitted in sexual reproduction. Egg and sperm Computer-generated image http://neurophilosophy.files.wordpress.com

13. Genes • A genome is a complete set of an organism’s genes—about 35,000 in humans. • The vast majority of the genome is located in the chromosomes in the cell nucleus of each eukaryotic cell. • The genes are formed from the four nucleotide pairs in the cell’s DNA. • Genes are also found on small DNA molecules in the mitochondria and chloroplasts. http://www.scfbio-iitd.res.in

14. DNA http://www.csulb.edu Two different molecular representations of DNA—modified ball-and stick and space-filling models.

15. DNA Nucleotide Structure Nitrogenous bases— thymine (T) and cytosine (C) have single-ring structures Adenine (A) and guanine (G) have double-ring structures CH2 Phosphate group Sugar (deoxyribose) Deoxyribose + phosphate group = sugar-phosphate backbone DNA base pairing rules: A with T, and G with C

16. Eukaryotic Chromosomes • A eukaryotic chromosome contains a single long DNA molecule bearing most of the organism’s genes. • The number of chromosomes varies by species—human cells usually have 46, dog cells have 78, and mouse cells have 40. • Chromosomes consist of chromatin, made-up of DNA and a ‘wrapping’ of protein molecules. • These proteins help organize the chromosomes and control gene activity. Chromatin in packed form, computer-generated image http://www.cgl.ucsf.edu

17. Chromosomes Prior to Cell Division • For much of a cell’s lifecycle, the chromosomes are a mass of long fibers much longer than the diameter of the cell nucleus. • As a cell prepares to divide the chromatin fibers coil up and form compact chromosomes. • The chromosomes are visible under a light microscope as shown below. • When a cell is not dividing, the chromosomes are too thin to be visible in a light micrograph. Chrysanthemum (plant) cell http:/z.about.com

18. DNA Packing • The DNA in each chromosome is packed into a multi-level system of coils and folds. • At the first level, the DNA has the appearance of beads on a string. • Each bead is called a nucleosome, consisting of DNA wound around eight histone molecules. • At the second level, the beaded chain is wrapped into a tight helix-like fiber. • At the third level, the helical fibers are arranged into a much thicker coil. • The coils and folds enable large amounts of DNA to be packed into a small cell nucleus.

19. Nucleosome http://www.bio.davidson.edu First level of packing—the DNA is the red cord-like material.

20. DNA Packing http://www.mun.ca Second and third levels of packing. ‘Recording’ the Golden Gate Bridge http://www.sounddesign.unimelb.edu.au

21. Chromatids • A cell duplicates all of its chromosomes through the process of replication before cell division begins. • Each chromosome now has two copies called sister chromatids (the term does not imply gender). • The sister chromatids are joined at their waists at a junction known as the centromere.

22. Sister Chromatids Sister chromatids http://www.cbs.dtu.dk

23. Chromatid Separation • The sister chromatids separate from each other during cell division to form an identical chromosome in each daughter cell. • Each daughter cell receives a complete, identical set of chromosomes. • A dividing human skin cell has 46 duplicated chromosomes. • Each of its two daughter cells will have 46 single chromosomes to form 23 pairs. • Millions of cells must divide every second to maintain over 60 trillion cells in the human body.

24. Cell Cycle • The rate at which cells divide depends on their role within the organism. • Some cells divide as often once a day, others less often, and others (such as muscle cells and neurons) usually not at all. • The cell cycle is a sequence of events from the time a new cell is formed until it divides and forms two new cells. • The cell division phases are the process of mitosis, which occurs after an interphase.

25. Cell Cycle—Interphase http://bhs.smuhsd.org

26. Interphase—G1 Stage • About 90 percent of the cell cycle is spent in interphase when the cell performs its normal metabolic functions. • In the first growth stage (G1) the cell doubles the number of organelles in preparation for cell division. • The cell grows in size to accommodate this ongoing doubling process.

27. Interphase—S and G2 Stages • In the synthesis stage (S), the DNA is doubled and the chromosomes are duplicated. • Each chromosome consists of two sister chromatids in preparation for cell division. • The sister chromatids have identical genetic content for the formation of daughter cells. • In the second growth stage (G2), further cell growth occurs.

28. Cell Cycle—Mitotic Phase http://bhs.smuhsd.org

29. Mitotic or M Phase • The part of the cell cycle when the cell divides is called the mitotic or M- phase. • The M-phase has two overlapping components: mitosis and cytokinesis. • In mitosis, the chromosomes are evenly distributed into the two daughter cell nuclei. • In cytokinesis, the cytoplasm of the parent cell is divided in two individual compartments (daughter cells).

30. End of M-Phase • At the end of the M-phase, each daughter cell contains its own nucleus, cytoplasm, organelles, and plasma membrane. • Mitosis and cytokinesis produce two genetically-identical daughter cells.

31. Mitosis • Mitosis is an accurate mechanism for allocating genetic material to two daughter cells. • In yeast cells, errors in chromosomal division occur about once in every 100,000 cell divisions. • Mitosis is unique to eukaryotic cells. • Prokaryotic cells have one chromosome, and use a simpler mechanism for allocating DNA to daughter cells.

32. Stages of Mitosis • Although mitosis is a continuum of cell activity, four stages are commonly described: • Prophase • Metaphase • Anaphase • Telophase • During mitosis, the movement of chromosomes is dependent on the the mitotic spindle, a football-shaped structure of microtubules consisting of proteins. • The spindle grows from centrosomes, clouds of material in the cytoplasm that contain centrioles.

33. Mitotic Spindle Centrosome and centrioles http://mcb.berkeley.edu The spindle has two poles and an equator http://www.ornl.gov

34. Onion Field Elba, New York http://content.answers.com Onions are often grown in rich soil that was once lake or river bottom.

35. Onion Root Cell • Onion root cells are commonly used to demonstrate mitosis—it is one of the biological materials we will use in a lab session. • The cells have large chromosomes which take stain well. • Mitosis in onion root cells is readily observed through a light microscope. http://www.mytinyplot.co.uk

36. Mitosis, Onion Cell http://www.sep.alquds.edu Interphase (G2) Prophase Metaphase Anaphase Telophase Mitosis consists of phases 2 through 5

37. Interphase http://www.sep.alquds.edu http://www.microscopy-uk.org.uk

38. Interphase • Interphase is the period of cell growth when the cell makes new molecules and organelles. • In late interphase (G2), the cytoplasm contains two centrosomes and their centriole pairs. • The chromosomes are duplicated, but they cannot be visually distinguished since they are still loosely packed in chromatin fibers. • The nucleolus is visible since it is producing ribosomes needed for protein synthesis during cell division.

39. Prophase http://www.sep.alquds.edu http://www.microscopy-uk.org.uk

40. Early Prophase • Prominent changes occur in the nucleus and cytoplasm during prophase. • The chromatin fibers coil and become thick enough to be seen through a light microscope. • Each chromosome appears as two identical sister chromatids joined at their waists. • The mitotic spindle forms as microtubules grow from the two centrosomes that are moving away from each other.

41. Late Prophase • The nuclear envelope breaks-up, enabling the microtubules of the mitotic spindle to reach the chromosomes. • Some of the microtubules attach to the chromosomes, and place them in an agitated motion. • Other microtubules make contact with the microtubules from the opposite pole to move the chromosomes to the center of the cell.

42. Metaphase http://www.sep.alquds.edu http://www.microscopy-uk.org.uk

43. Metaphase • The mitotic spindle is fully formed, and the chromosomes are positioned on the cell equator. • Microtubules attach to the two sister chromatids of each chromosome to pull them toward the opposite poles. • For now, the tug-of-war keeps the chromosomes positioned between the two poles.

44. Anaphase http://www.sep.alquds.edu http://www.microscopy-uk.org.uk

45. Anaphase • The sister chromatids of each chromosome pair separate suddenly during anaphase. • Each is now a daughter chromosome. • Motor proteins ratchet the daughter chromosomes along the microtubules to the opposite poles of the parent cell. • The microtubules shorten in length to help bring the chromosomes closer to each pole. • Other microtubules, not attached to the chromosomes, lengthen and push the poles apart to elongate the parent cell.

46. Telophase http://www.sep.alquds.edu http://www.microscopy-uk.org.uk

47. Telophase • Telophase begins when the two chromosomes reach the opposite poles. • The nuclear envelopes form, chromosomes uncoil, and the mitotic spindle disappears. • Mitosis is know complete. • Cytokinesis, the division of the parent cell into two separate cells, occurs at the end of telophase.

48. Cytokinesis in Plant Cells http://trc.ucdavis.edu The formation of the cell walls and plasma membranes are visible.

49. Cytokinesis in Plant Cells • Vesicles containing material for the cell walls collect at the equator of the elongated plant cell. • The vesicles fuse to form a cell plate. • The cell plate fuses with the plasma membrane to produce two separate daughter cells.

50. Cytokinesis in Animal Cells http://www.molecularexpressions.com Actin molecules pinch-off the parent cell to form two daughter cells.