How Cells Divide – Mitosis and Meiosis

1. How Cells Divide – Mitosis and Meiosis Chapters 11&12

2. Cell Division in Prokaryotes • Prokaryotic cell division occurs as binary fission in which cell divides into two halves. • Genetic information exists as a single, circular double-stranded DNA molecule. • Copying begins at replication origin, and proceeds bi-directionally. • One genome ends up in each daughter cell.

3. Binary Fission

4. Discovery of Chromosomes • All eukaryotic cells store genetic information in chromosomes. • Most eukaryotes have between 10 and 50 chromosomes in their body cells. • Human cells have 46 chromosomes. • 23 nearly-identical pairs

5. Structure of Chromosomes • Chromosomes are composed of a complex of DNA and protein, chromatin. • heterochromatin - not expressed • euchromatin - expressed • DNA exists as a single, long, double-stranded fiber extending chromosome’s entire length. • forms nucleosome every 200 nucleotides • DNA coiled around histone proteins

6. Eukaryotic Chromosomal Organization

7. Structure of Chromosomes • Karyotype - Individual’s particular array of chromosomes. • diploid - A cell possessing two copies of each chromosome (human body cells). • Homologous chromosomes are made up of sister chromatids joined at the centromere. • haploid - A cell possessing a single copy of each chromosome (human sex cells).

8. Karyotype & Chromosomes

9. Phases of the Cell Cycle • Five phases of cell division: • G1 - primary growth phase • S - genome replicated • G2 - secondary growth phase • collectively called interphase • M -mitosis • C -cytokinesis

10. Cell Cycle

11. Interphase • G1- cells undergo majority of growth • S - each chromosome replicates to produce sister chromatids • attached at centromere • contains attachment site (kinetochore) • G2 - chromosomes condense • assemble machinery such as centrioles

13. Mitosis • Prophase • spindle apparatus assembled • Microtubules connect kinetochores on each pair of sister chromatids to the spindle poles. • nuclear envelope breaks

15. Mitosis • Metaphase • chromosomes align in cell’s center • metaphase plate • spindle

17. Mitosis • Anaphase • sister chromatids pulled toward poles • poles move apart • centromeres move toward poles • microtubules shorten • Telophase • spindle disassembles • nuclear envelope forms around each set of sister chromatids

19. Cytokinesis • Cleavage of cell into two halves • animal cells • constriction belt of actin filaments • plant cells • cell plate • fungi and protists • mitosis occurs within the nucleus

20. Cytokinesis

22. Cell Cycle Control • Two irreversible points in cell cycle: • replication of genetic material • separation of sister chromatids • Cell can be put on hold at specific checkpoints.

23. Cell Control Cycle • G1 /S - primary division decision point • G2/ M - commitment to mitosis • Spindle checkpoint - all chromosomes are attached to spindle

25. Growth Factors and the Cell Cycle • Each growing cell binds minute amounts of positive regulatory signals (growth factors) that stimulate cell division. • If neighboring cells use up too much growth factor, there is not enough left to trigger cell division. • Growth factors trigger intercellular signaling systems.

27. Sexual Reproduction and Meiosis Chapter 12

28. Reduction Division • In sexual reproduction, gametes fuse (fertilization) to produce a zygote. • Gamete formation involves a mechanism (meiosis) that reduces the number of chromosomes to half that found in other cells. • Adult body cells are diploid. • Gamete cells are haploid. • alternation of generations

29. Sexual Life Cycle • Diploid cells carry chromosomes from two parents • 2 haploid cells join to form diploid cell

30. Sexual Life Cycle • Three types of sexual life cycles. • In sexual reproduction, haploid cells or organisms alternate with diploid cells or organisms

31. Sexual Life Cycle

32. Meiosis • Synapsis • Homologues pair along their length. • Homologous recombination • Genetic exchange (crossing over) occurs between homologous chromosomes. • Reduction division • Meiosis involves two successive divisions, with no replication of genetic material between them.

33. Unique Features of Meiosis

34. Prophase I • Homologous chromosomes become closely associated in synapsis, exchange segments via crossing over, and then separate. • Presence of a chiasma indicates crossing over has occurred.

35. Metaphase I • Terminal chiasmata holds homologous pair together. • Spindle microtubules attach to kinetochore proteins on the outside of each centromere. • Joined pairs of homologues lines up on metaphase plate. • orientation of each pair is random

37. Completing Meiosis • Anaphase I • Spindle fibers begin to shorten and pull whole centromeres toward poles. • Each pole receives a member of each homologous pair. • complete set of haploid chromosomes • random orientation results in independent assortment

39. Completing Meiosis • Telophase I • Chromosomes are segregated into two clusters; one at each pole. • Nuclear membrane re-forms around each daughter cell. • Sister chromatids are no longer identical due to crossing over.

41. Second Meiotic Division • Meiosis II resembles normal mitotic division. • prophase II - nuclear envelope breaks down and second meiotic division begins • metaphase II - spindle fibers bind to both sides of centromere • anaphase II - spindle fibers contract and sister chromatids move to opposite poles • telophase II - nuclear envelope re-forms • Final result - four haploid cells

43. Sex • Asexual reproduction - individual inherits all its chromosomes from a single parent • parthenogenesis - development of an adult from an unfertilized egg • Sexual reproduction - produces genetic variability. • Segregation of chromosomes tends to disrupt advantageous combinations. • Only some progeny maintain advantages.

44. Origin and Maintenance Of Sex • Theories • DNA repair hypothesis • Only diploid cells can effectively repair certain kinds of chromosomal damage. • Contagion hypothesis • A secondary consequence of the infection of eukaryotes by mobile genetic elements.

45. Origin and Maintenance Of Sex • Red Queen hypothesis • Current recessive alleles can be stored in reserve for future use. • Miller’s Ratchet • Sexual reproduction may be a method of keeping the mutational load low.

46. Evolutionary Consequences of Sex • Evolutionary process is revolutionary and conservative. • pace of evolutionary change is accelerated by genetic recombination • evolutionary change not always favored by selection • may act to preserve existing gene combinations

47. Independent Assortment