Chapters 12 and 13: The Cell Cycle, Mitosis and Meiosis

1. Chapters 12 and 13: The Cell Cycle, Mitosis and Meiosis

2. Systems of Cell Reproduction • Four events occur before and during cell division: • A signal to reproduce must be received. • Replication of DNA and vital cell components must occur. • DNA must be distributed to the new cells. • The cell membrane or cell wall must separate the two new cells.

3. Systems of Cell Reproduction • Prokaryotes divide by fission. • Most prokaryotes have one circular chromosome. • As DNA replicates, each of the two resulting DNA molecules attaches to the plasma membrane. • As the cell grows, new plasma membrane is added between the attachment points, and the DNA molecules are moved apart. • Cytokinesis separates the one cell into two, each with a complete chromosome.

4. Figure 12.2 Prokaryotic Cell Division

5. Systems of Cell Reproduction • Eukaryotic cells divide by mitosis or meiosis. • Eukaryotes usually have many chromosomes. • Eukaryotes have a nucleus, which must replicate and, with few exceptions, divide during cell division.

6. Cell Division: Key Roles • Genome: cell’s genetic information • Somatic (body cells) cells • Gametes (reproductive cells): sperm and egg cells • Chromosomes: DNA molecules • Diploid (2n): 2 sets of chromosomes • Haploid (1n): 1 set of chromosomes • Chromatin: DNA-protein complex • Chromatids: replicated strands of a chromosome • Centromere: narrowing “waist” of sister chromatids • Mitosis: nuclear division • Cytokinesis: cytoplasm division • Meiosis: gamete cell division

7. The Cell Cycle • Interphase (90% of cycle) • G1 phase~ growth • S phase~ synthesis of DNA • G2 phase~ preparation for cell division • Mitotic phase • Mitosis~ nuclear division & Cytokinesis~ cytoplasm division

8. Mitosis • Prophase • Prometaphase • Metaphase • Anaphase • Telophase

9. Prophase • Chromosomes visible • Nucleoli disappear • Sister chromatids • Mitotic spindle forms • Centrosomes move

10. Prometaphase • Nuclear membrane fragments • Spindle interaction with chromosomes • Kinetochore develops • prometaphase, the nuclear lamina disintegrates and the nuclear envelope breaks into small vesicles, permitting the fibers of the spindle to “invade” the nuclear region

11. Metaphase • Centrosomes at opposite poles • Centromeres are aligned • Kinetochores of sister chromatids attached to microtubules (spindle)

12. Anaphase • Paired centromeres separate; sister chromatids liberated • Chromosomes move to opposite poles • Each pole now has a complete set of chromosomes

13. Telophase • Daughter nuclei form • Nuclear envelopes arise • Chromatin becomes less coiled, “decondenses” • Two new nuclei complete mitosis

14. Cytokinesis • Cytoplasmic division • Animals: cleavage furrow • Plants: cell plate

15. Cell Cycle regulation • Growth factors • Density-dependent inhibition • Anchorage dependence

16. Interphase and the Control of Cell Division • Transitions from G1 to S and G2 to M depend on activation of a protein called cyclin-dependent kinase, or Cdk. • A kinase catalyzes phosphorylation of a protein. • Cdk is activated by binding to a second type of protein called cyclin. • Several different cyclins exist, which, when bound to Cdk, phosphorylate different target proteins.

17. Figure 12.4 Cyclin-Dependent Kinases and Cyclins Trigger Transisions in the Cell Cycle

18. Interphase and the Control of Cell Division • Cyclin-Cdk complexes act as checkpoints. When functioning properly, they allow or prevent the passage to the next cell cycle stage, depending on the extra- and intracellular conditions. • In cancer cells, these cyclin-Cdk controls are often disrupted.

19. Interphase and the Control of Cell Division • Some cells which no longer go through the cell cycle may respond to growth factors provided by other cells. • Examples include platelet-derived growth factor, interleukins, and erythropoietin. • Growth factors act by binding to target cells, and triggering events within the target cell that initiate the cell cycle. • Cancer cells cycle inappropriately because they either make their own growth factors or no longer require them to start cycling.

20. Genetic Control of Cell Cycle • Oncogenes – This is the “growth” gene, it must be “on” for cell division to occur. • Tumor Suppressor Genes – These genes are present to prevent uncontrolled cell division. They inhibit production of CDK’s or kinases. Most cells have three or four active ones. • Apotosis genes – example of this gene is p53, a protein that causes a cancerous cell to commit “suicide” • Many cancers involve mutations in all of these genese.

21. Cancer • Transformation • Tumor: benign or malignant • Metastasis

22. Meiosis and Sexual Life Cycles

23. Heredity • Heredity: the transmission of traits from one generation to the next • Asexual reproduction: clones • Sexual reproduction: variation • Human life cycle: • 23 pairs of homologous chromosomes (46); • 1 pair of sex and 22 pairs of autosomes; • karyotype; • gametes are haploid (1N)/ all other cells are diploid (2N); •fertilization (syngamy) results in a zygote • Meiosis: cell division to produce haploid gametes

24. Reproduction: Asexual and Sexual • Cells in metaphase can be killed and prepared in a way that spreads the chromosomes around a region on a glass slide. • A photograph of the slide can be taken, and images of each chromosome can be organized based on size, number, and shape. This spread is called a karyotype.

25. Figure 13.13 Human Cells Have 46 Chromosomes

26. Alternative life cycles • Fungi/some algae •meiosis produces 1N cells that divide by mitosis to produce 1N adults (gametes by mitosis) • Plants/some algae •Alternation of generations: 2N sporophyte, by meiosis, produces 1N spores; spore divides by mitosis to generate a 1N gametophyte; gametes then made by mitosis which then fertilize into 2N sporophyte

27. Meiosis • Preceded by chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II) • 4 daughter cells; 1/2 chromosome number (1N); variation

28. Meiosis vs. mitosis • Synapsis/tetrad/chiasmata(prophase I) • Homologous vs. individual chromosomes (metaphase I) • Sister chromatids do not separate (anaphase I) • REDUCTION STEP: Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

29. Figure 13.16 Crossing Over Forms Genetically Diverse Chromosomes

30. Origins of Genetic Variation, I • Independent assortment: homologous pair of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II • Combinations possible: 2 ; with n the haploid number of the organism n

31. Origins of Genetic Variation, II • Crossing over (prophase I): • the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes) • Random fertilization: • 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!

32. Figure 13.14 Meiosis (Part 1)

33. Figure 13.14 Meiosis (Part 2)

34. Figure 13.14 Meiosis (Part 3)

35. Meiosis: A Pair of Nuclear Divisions • The second meiotic division separates the chromatids. • Meiosis II is similar to mitosis but one difference is that DNA does not replicate before meiosis II. • The number of chromosomes in the resulting cells is therefore half that found in diploid mitotic cells. • In meiosis II, sister chromatids are not identical and there is no crossing-over.

36. Figure 9.14 Meiosis (Part 4)

37. Figure 9.14 Meiosis (Part 5)

38. Figure 9.14 Meiosis (Part 6)

39. Meiotic Errors • Nondisjunction occurs when homologous chromosomes fail to separate during anaphase I, or sister chromatids fail to separate during anaphase II. • The result is a condition called aneuploidy.

40. Figure 13.18 Nondisjunction Leads to Aneuploidy

41. Meiotic Errors • One reason for aneuploidy may be a lack of cohesins. • Failure of chromosome 21 to separate in humans results in trisomy 21—Down syndrome. • Translocation, a process in which part of a chromosome attaches to another, can also cause abnormality.

42. Meiotic Errors • Polyploids have extra whole sets of chromosomes, and this abnormality in itself does not prevent mitosis. • Triploids are 3n; tetraploids are 4n. • Although mitosis usually is unimpaired, meiosis is problematic, especially for odd numbers of sets, as in triploidy.

43. Cell Death • Cells die in one of two ways: necrosis and apoptosis. • Necrosis occurs when cells either are damaged by poisons or are starved of essential nutrients. These cells swell and burst.

44. Cell Death • Genetically programmed cell death is called apoptosis: • The cell may no longer be needed, e.g., cells of the weblike tissue between the fingers of a developing human fetus. • Cells that are old or damaged may need to be replaced. • The cell death cycle is controlled by signals. • The cell becomes isolated, chops up its own chromatin, and gets ingested by surrounding living cells.

45. Apoptosis