Understanding Chromosomes, Mitosis, and Meiosis: A Guide to Genetic Information and Cell Reproduction

1. Chromosomes Where Are the Genes Found?

2. What Cellular Structure Holds the Genetic Information? • Chromosomes • Contain the genetic material: DNA, RNA • Chromatin • Is the chromosomal material in its decondensed, threadlike state.

3. Mitosis • Form of asexual reproduction. • Occurs when organism grows or replaces damaged cells. • Prior to mitosis, cell undergoes replication. • Process in which chromatin is copied. • Produces diploid cells.

4. Prophase • Start of mitosis • Chromatin condenses into rod-like chromosomes • Each chromosome consists of sister chromatids, connected at the centromere • Nuclear membrane disappears

5. Metaphase • Chromosomes align themselves in flat plane at cell equator.

6. Anaphase • Centromeres split. • Sister chromatids-now chromosomes- are pulled to opposite poles of the cell.

7. Telophase • Chromosomes unravel, returning the chromatin to its non-dividing threadlike state. • Nuclear membrane assembles.

8. Cytokinesis • Division of the cytoplasm. • Begins during anaphase and telophase.

10. Cytokinesis • Differs in animals and plant cells. • Plant cells form a cell plate. • membranous vesicles congregate at center of cell. • Vesicles contain cell wall material.

11. Cytokinesis • Animal cells form a cleavage furrow. • Forms around the periphery of the dividing cell. • Furrow becomes deeper and deeper until membrane pinches off forming two cells.

12. Chromosomes Come in Matched Pairs • Homologous pairs: chromosomes that are closely matched in size and shape • Determine the same traits • Sex chromosomes: Those that determine the gender of the organism.

13. Chromosomal Theory of Inheritance • The two members of each pair of a homologous pair of chromosomes carry alleles for the same genes and, therefore, affect the same traits. • Proposed in 1903.

14. Mapping genes • Locus: location of a gene on a chromosome. • Currently trying to map all human genes to the appropriate chromosome. • Example: gene responsible for sickle cell anemia is located on chromosome 11.

15. Sexual reproduction • Type of reproduction in which genetic information from female combines with male. • Requires fertilization • Fusion of gametes (egg and sperm) • These cells need to have half the amount of genetic information.

16. Meiosis • Produces haploid cells (gametes-eggs and sperm) • Posses only one member of each pair of homologous chromosomes • Chromosomes replicate before meiosis. • Requires two rounds of division.

17. Prophase I • Chromatin condenses into compact chromosomes. • Nuclear envelope disappears. • Synapsis occurs. • Homologous pairs of chromosomes closely align allowing exchange of chromosome segments

18. Metaphase I • Aligned pairs of replicated chromosomes move to the equator of the dividing cell.

19. Anaphase I • Members of homologous pairs of chromosomes separate from each other • They move to opposite poles of the cell. • Chromosomes experience independent assortment.

20. Telophase I • Chromosomes cluster at opposite poles of cell and begin to decondense • Nuclear envelope may reform. • Cytokinesis occurs • Interkinesis: phase between meiosis I and and II. • Daughter cells now haploid.

21. Prophase II • Partially unraveled chromosomes condense again.

22. Metaphase II • Chromosomes move to cell equator. • No longer homologous pairs, so chromosomes line up singly in middle of cell.

23. Anaphase II • Centromeres divide and chromosomes separate. • Move to opposite poles of the cell.

24. Telophase II • Clustered chromosomes at cell pole begin to decondense. • Nuclear membrane develops. • Cytokinesis occurs. • Produces four non-identical haploid cells.

26. Mitosis vs. Meiosis

27. Cell Cycle • Repetitive sequence of events that characterizes life of cell. • Consists of two main phases: • Interphase • Period that cells are in when not dividing. • 90% of cell cycle • M phase • Includes mitosis and cytokinesis

28. Cell Cycle

29. Cell Cycle • Interphase has three subphases • G1 (first gap) • Cell makes copies of organelles and grows larger • S (synthesis) • Genetic material is copied • End of this phase, cells chromosomes are doubled • Copies are attached; thus total number of chromosomes remains the same • G2 (second gap) • Cell prepares upcoming M phase

30. Regulation of Cell Cycle • Cell cycle must be regulated or can result in cancer • Uncontrolled cell growth • Metastasize: cell breaks free from original cancerous mass and resides in new area in the body

31. Regulation of Cell Cycle • Two checkpoints • First between G1 and S • Second between G2 and M • To pass checkpoints, cell must possess appropriate amount of protein in cytoplasm. • These proteins activate other proteins necessary for production of genetic material and mitosis

32. Regulation of Cell Cycle • To pass checkpoints, cell must possess appropriate amount of regulating protein in cytoplasm. • When regulator concentration is high, cell cycle progresses. • When low, cell cycle is suspended at that stage. • External and internal regulatory agents also influence passage through checkpoints.

33. Why So Some Genetic Traits Tend to Travel Together? • Chromosomes contain genes • Chromosomes follow law of independent assortment, not genes • If two genes are on the same chromosome, • the two genes are inherited together or are said to be linked or in linkage groups

34. Why So Some Genetic Traits Tend to Travel Together? • Example: sex linkage • Sex chromosomes • Contain other genes aside from those to determine gender. • Example: eye color and gender in fruit flies

35. Chromosomes Can Exchange Segments During Meiosis • Crossing over • Exchange of genetic material between chromatids of homologous chromosomes. • Occurs at the chiasmata.

36. Chromosomes Can Exchange Segments During Meiosis • Crossing over • Important mechanism for creating new combinations of genes. • Disrupts linkage groups.

37. What Is the Chemical Nature of the Gene? • 1860s • Frederich Meisner studied fundamental constituents of life • Discovered unknown substance contains carbon, nitrogen, oxygen, and phosphorus. • Found it came from nucleus of cell. • Named it nucelin • His students renamed substance nucleic acid after finding it was acidic.

38. What Is the Chemical Nature of the Gene? • 1881 • Discovered nucleic acids were contained in chromatin. • Question: Was the genetic material made of proteins or nucleic acids?

39. DNA is the Genetic Material • 1928 • Fred Griffiths, medical officer for British Ministry of Health • Studied the bacteria pneumoncoccus • Two kinds • Smooth: Virulent form that appears smooth and shiny when grown on agar plate • Rough: harmless form that appears rough when grown on agar plate. • Experimented by injecting the two types in mice.

40. DNA is the Genetic Material

41. DNA is the Genetic Material • Griffiths identified the material as the transforming principle • Avery, MacLeod and McCartney • Study transforming principle for 20 years. • They determined that the transforming agent was DNA.

42. DNA is the Genetic Material • Alfred Hershey and Martha Chase • Studied viruses that infect bacteria • Viruses are called bacteriophages

43. DNA is the Genetic Material • Viruses are made of • Protein coat and nucleic acid • Viruses mix their genes with host genes • hijack cell machinery and use it to produces new viruses • Usually kills host cell

44. DNA is the Genetic Material • Hershey and Chase labeled protein and DNA differently with isotopes • Variants of elements that share same chemical properties but differ in number of neutrons • Label DNA with 32P and protein with 35S

45. DNA is the Genetic Material