1. Chapter 2 Mitosis & Meiosis

2. Cell Types Prokaryotes (bacteria) Eukaryotes Single cell
No organelles, no nucleus (nucleoid – circular DNA)
Eukaryotes
Multicellular (generally)
Organelles, organized nucleus

3. Bacteria
Cheek cells

5. Cell Organization Nucleus Rough/Smooth ER Ribosomes
Cell membrane/cell wall
Cell coat (animal cells)
Cytoplasm, cytoskeleton
Mitochondria, chloroplasts, etc.

6. Nucleus Chromatin/chromosomes Nucleolus (RNA Synthesis)
Nucleolus Organization Region (NOR)
Sections of DNA that code for rRNA

7. DNA in the nucleus
Chromatin – loosely coiled DNA. DNA exists in this form until ready to divide.

8. Chromsomes Chromatin condenses before mitosis
Chromosome – tightly packed DNA, 2 “sister chromatids”
P ARM
Q ARM

10. Karyotypes
Karyotype: “chart” of all an organism’s chromosomes

11. Homologous Chromosomes
Most organisms have 2 copies of each chromosome (homologous)
Diploid (2n) – somatic cells
Haploid (n) – gametes. Haploid cells contain only 1 copy of each chromosome

13. Homologous Chromosomes
2 copies of each chromosome, 2 copies/version of each gene (ALLELE)
Genes are located at identical sites on sister chromatids (LOCI)

15. Cell Cycle A sequence of cell growth and division
Numerous factors control when cells divide

17. Chrom duplicate during INTERPHASE (90% of cell’s life)
G1 phase - cells grow and synthesize biological molecules
S phase - DNA replication
G2 phase - gap of time between S phase and mitosis (preparation for division)
G0 phase

18. Mitosis Why do cells need to divide? Zygote
How much time did you spend as a single egg?

19. Mitosis Purpose is to ensure the orderly distribution of chromosomes
Four Stages:
Prophase
Metaphase
Anaphase
Telophase

20. y

21. Interphase Chromosomes are extended & uncoiled, forming chromatin
Centrioles duplicate

22. Prophase (early)
Chromosomes condense, centrioles divide and move apart

23. Prophase (middle) Also called “prometaphase”
Chromosomes doubled, clearly visible
Centrioles at poles, spindle fibers form
Nucleolus dissapears
Nuclear membrane dissolves

25. Metaphase Chromosomes line up in the middle of the cell
Spindle fibers extend from the poles, attached to the chromosomes

26. Anaphase
Centromeres split and sister chromatids (now referred to as chromosomes) migrate to the poles

27. Telophase Two separate nuclei form
Cell returns to conditions similar to interphase
Nuclear envelope reforms; nucleoli reappear
Cytokinesis occurs

29. Cytokinesis
In animals cells, a furrow develops caused by filaments that encircle the equatorial region
In plant cells, a cell plate forms originating from the Golgi complex

31. Meiosis Why must cells undergo meiosis? Reduce chromosome number
Production of gametes (gametogenesis)
Sexual reproduction involves the union of gametes to form a zygote
Sexual reproduction results in greater variation amongst offspring – offspring are not clones of their parents

32. How many combinations?
Number of combinations possible 2n, where n is the haploid number of the organism
If n = 3, there are 8 possible combos
Humans: n = 23, there are 223 ~ 8 million possible combinations of chromosomes

33. A few notes about chromosomes….
Autosomes: non-sex chromosomes
How many do humans have?
Sex Chromosomes
Females: XX
Males: XY
Only small parts of these have the same genes, most of their genes have no counterpart on the other chromosome

36. Meiosis – an overview
Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice
The first division, meiosis I, separates homologous chromosomes
The second, meiosis II, separates sister chromatids

37. Chromosomes separate
Chromatids separate

39. Meiosis contains 2 stages, Meiosis I and Meiosis II
Meiosis I and II each include prophase, metaphase, anaphase, and telophase

40. Meiosis I - Prophase I
Homologous chromosomes pair and undergo synapsis (special proteins)
Synapsis is the association of four chromatids (two copies of each homologous chromosome)
The resulting complex is called a bivalent or tetrad
In humans, there are 23 tetrads and 92 chromatids in this phase

41. Prophase I
At several sites, the chromatids of tetrads are crossed (chiasmata) and segments of chromosomes are traded (crossing over)

43. Synaptonemal complex forms between members of the tetrad & genetic material is exchanged by crossing over
Crossing over ensures greater genetic variation

44. Crossing Over

45. Meiosis I - Metaphase I
Tetrads line up at the equator of the cell

46. Meiosis I - Anaphase I
The homologous chromosome separate and move to the poles
Each pole receives a mixture of maternal and paternal chromosomes

48. Meiosis I - Telophase I Chromosomes decondense
The nuclear membrane may reform
Cytokinesis usually occurs

50. Meiosis II - Prophase II
Brief
Recondensation of the chromosomes
Very similar to conditions in prophase of mitosis

51. Metaphase II & Anaphase II
Metaphase II- Chromosomes line up at the equator
Anaphase II- the chromatids separate and are now called chromosomes

52. Telophase II
There is one copy of each homologous chromosome at each pole
Nuclei form around chromatids
Cytokinesis separates the cytoplasm
At the end of meiosis, there are (typically) 4 haploid daughter cells

55. Mitosis vs. Meiosis
In mitosis a single division results in two genetically identical daughter cells and there is no crossing over
In meiosis, two sets of divisions occur resulting in four genetically different cells. A great deal of genetic diversity occurs caused by synapsis and independent assortment

56. Meiosis & Genetic Variation
The events of meiosis & fertilization are responsible for the variation in each new generation
Independent assortment
Crossing over
Random fertilization

57. Independent Assortment
Contributes to genetic variability due to random orientation of tetrads at metaphase plate
50-50 chance that a particular daughter cell of meiosis I will get the maternal chromosome, chance that it will receive paternal chromosome

59. Crossing Over
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
Begins very early in prophase I - homologous chromosomes pair up gene by gene
Homologous portions of two nonsister chromatids trade places.
Humans - occurs 2 – 3x/chromosome pair

60. Crossing Over
One sister chromatid may undergo different patterns of crossing over than its “mate”
Once these undergo independent assortment in meiosis II, variation of gametes increases even more!

62. Fertilization
An ovum is one of ~ 8 million possible chromosome combinations
Successful sperm represents one of 8 million different possibilities
Resulting zygote - 1 in 70 trillion (223 x 223) possible combinations
Crossing over adds even more variation

63. Taken together…
All three mechanisms reshuffle the various genes carried by individual members of a population
Mutations, still to be discussed, are what ultimately create a population’s diversity of genes