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