1. Reduction-Division Genetic Recombination
MEIOSIS
Reduction-Division
Genetic Recombination

2. Meiosis
The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are produced.
DIPLOID (2n)  HAPLOID (n)
Meiosis is SEXUAL reproduction.
TWO divisions (MEIOSIS I and MEIOSIS II).

3. Meiosis Sex cells divide to produce GAMETES (sperm or egg).
Gametes have HALF the # of chromosomes.
Occurs only in GONADS (testes or ovaries).
Male: SPERMATOGENESIS -sperm
Female: OOGENESIS - egg or ova

4. Involves 2 cell divisions
Meiosis
Similar in many ways to mitosis
Several differences however
Involves 2 cell divisions
Results in 4 cells with 1/2 the normal genetic information.
Vocabulary:
Diploid (2N) - Normal amount of genetic material
Haploid (N) - 1/2 the genetic material. 4 4

5. In Humans, these are the Ova (egg) and sperm.
Meiosis
Meiosis results in the formation of haploid cells.
In Humans, these are the Ova (egg) and sperm.
Ova are produced in the ovaries in females
Process is called oogenesis
Sperm are produced in the testes of males.
Process is called spermatogenesis
Meiosis occurs in 2 phases: Meiosis I & Meiosis II 5 5

6. Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) Meiosis II human
sex cell
diploid (2n)
n=23
Meiosis I

7. Oogenesis n=23 n=23 2n=46 Haploid (1n) human egg sex cell
Meiosis II
2n=46
human
sex cell
diploid (2n)
n=23
Meiosis I
Polar Bodies (die)

8. Spermatogenesis & Oogenesis
Sperm formation
Egg formation 8 8

9. Interphase I Similar to mitosis interphase.
CHROMOSOMES (DNA) replicate in the S phase
Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES.
CENTRIOLE pairs also replicate.

10. Meiosis I
Prior to division (S phase), amount of DNA doubles 10 10

11. Interphase I Nucleus and nucleolus visible. Nucleus chromatin
cell membrane
nucleolus

12. Meiosis I (four phases)
Cell division that reduces the chromosome number by one- half.
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
Prophase I

13. Prophase I Longest and most complex phase (90%). Chromosomes condense.
Synapsis occurs - Homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids).

14. Non-Sister Chromatids-HOMOLOGS
Homologs contain DNA that codes for the same genes, but different versions of those genes
Genes occur at the same loci

15. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

16. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes:
a. First 22 pairs of autosomes
b. Last pair of sex chromosomes
LOCI

17. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

18. Crossing Over
Crossing over may occur between non-sister chromatids at sites called chiasmata.
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.)
Causes Genetic Recombination

19. Genetic Recombination
Tetrad
nonsister chromatids
chiasmata: site of crossing over
variation

21. Sex Chromosomes
XX chromosome - female
XY chromosome - male

22. MEIOSIS I
Meiosis I
Homologs separate

23. Prophase I Nucleus & Nucleolus disappear Spindle forms
Chromosomes coil & Synapsis (pairing) occurs
Tetrads form & Crossing over Occurs
centrioles
spindle fiber
aster
fibers
TETRAD

24. Metaphase I Shortest phase Tetrads align on the equator.
Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION

25. Homologs line up at equator or metaphase plate
Metaphase I
Homologs line up at equator or metaphase plate OR

26. Formula: 2n
Example: 2n = 4
then 1n = 2
thus 22 = combinations

27. Metaphase I Genes are exchanged at these connections
During Metaphase I homologous chromosomes line- up along the metaphase plate or EQUATOR
Areas of homologous chromosomes connect at areas called CHIASMATA
Genes are exchanged at these connections 27 27

28. Crossing Over
Segments of homologous chromosomes break and reattach at similar locations.
Results in new genetic combinations of offspring.
This is the main advantage of sexual reproduction 28 28

29. Chiasmata 29 29

30. Question:
In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

31. Answer Formula: 2n Human chromosomes: 2n = 46 n = 23
223 = ~8 million combinations

32. Anaphase I
During Anaphase I, each HOMOLOGOUS CHROMOSOME is pulled to opposite sides of the cell.
Unlike mitosis, the CENTROMERES DO NOT BREAK.
Nuclei MAY OR MAY NOT reform following division.
CYTOKENESIS may or may not occur. 32 32

33. Anaphase I Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.

34. Anaphase I
Homologs separate

35. Telophase I Each pole now has haploid (1n) set of chromosomes.
Cytokinesis occurs and two haploid daughter cells are formed.

36. Telophase I
cytokinesis

37. Sister Chromatids Separate
MEIOSIS II
Sister Chromatids Separate
Meiosis II

38. Meiosis II No Interphase II or very short No DNA Replication
Remember: Meiosis II is similar to mitosis

39. Meiosis II DNA DOES NOT double
Chromosomes randomly line-up along metaphase plate like regular mitosis.
During Anaphase II, CENTROMERES BREAK and each SISTER CHROMATID is pulled to opposite sides of the cell.
Nuclei reform and cytokinesis usually occurs (although it is often unequal).
Sister chromatids 39 39

40. Prophase II Same as Prophase in mitosis Nucleus & nucleolus disappear
Chromosomes condense
Spindle forms

41. Metaphase II Same as Metaphase in mitosis
Chromosomes (not homologs) line up at equator

42. Anaphase II
Same as Anaphase in mitosis
SISTER CHROMATIDS separate

43. 1n Sperm cell fertilizes 1n egg to form 2n zygote
Telophase II
Same as Telophase in mitosis.
Nuclei and Nucleoli reform, spindle disappears
CYTOKINESIS occurs.
Remember: FOUR HAPLOID DAUGHTER cells are produced.
Called GAMETES (eggs and sperm)
1n Sperm cell fertilizes 1n egg to form 2n zygote

44. Telophase II

46. Variation Also known as GENETIC RECOMBINATION
Important to population as the raw material for NATURAL SELECTION.
All organisms are NOT alike
Strongest “most fit” survive to reproduce & pass on traits

47. Question:
What are the 3 sources of genetic recombination or variation?

48. Answer: CROSSING OVER (prophase I)
INDEPENDENT ASSORTMENT (metaphase I)
RANDOM FERTILIZATION

49. Question:
A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

50. Answer:
10 chromosomes (haploid or 1n)

51. Karyotype
An organized picture of the chromosomes of a human arranged in pairs by size from largest to smallest.
Pairs 1-22 called AUTOSOMES
Last pair are SEX CHROMOSOMES
Male - XY

52. Karyotype
Female - XX

53. Karyotype
Down Syndrome – Trisomy 21
Female - XX

54. Fertilization The fusion of a sperm and egg to form a zygote.
A zygote is a FERTILIZED EGG
n=23
egg
sperm
n=23
2n=46
zygote

55. Question:
A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

56. Answer:
10 chromosomes

57. Meiosis
Gamete Production 57 57

58. Overview of Meiosis 58 58

59. 59 59