1. Meiosis and Sexual Reproduction
Chapter 10

2. Impacts, Issues Video
Why Sex

3. Why Sex
In nature, the main function of sex is to perpetuate one’s genes
Perpetuation of life includes reproduction and development

4. Why Sex Asexual reproduction is easier and faster
Single parent produces offspring
All offspring are genetically identical to one another and to parent
Sexual reproduction can be an alternative adaption in changing environments
chromosomes are duplicated in germ cells
Involves
Meiosis
Gamete production
Fertilization
Produces genetic variation among offspring

5. Cost of Sexual Reproduction
Specialized cells and structures must be formed Special courtship, and parental behaviors can be costly Nurturing developing offspring, either in egg or body, requires resources from mother

6. Homologous Chromosomes Carry Different Alleles
Cell has two of each chromosome One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles

7. Homologous Chromosomes

8. Sexual Reproduction Shuffles Alleles
Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for evolutionary change

9. Gametes are sex cells (sperm, eggs) Arise from germ cells
Gamete Formation
Gametes are sex cells (sperm, eggs) Arise from germ cells
ovaries
anther
testes
ovary

10. Gamete Formation
Reproductive organs

11. Chromosome Number
Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

12. Human Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
XX (or XY)

13. Meiosis: Two Divisions
Two consecutive nuclear divisions
Meiosis I
Meiosis II
DNA is not duplicated between divisions
Four haploid nuclei form

14. Meiosis I Each homologue in the cell pairs with its partner,
then the partners
separate

15. Meiosis II
The two sister chromatids of each duplicated chromosome are separated from each other
two chromosomes
(unduplicated)
one chromosome
(duplicated)

16. Prophase I
Each duplicated chromosome pairs with homologue Homologues swap segments Each chromosome becomes attached to spindle

17. Metaphase I
Random alignment of homologous chromosomes Chromosomes are pushed and pulled into the middle of cell The spindle is fully formed

18. Anaphase I
Homologous chromosomes segregate The sister chromatids remain attached

19. Telophase I
The chromosomes arrive at opposite poles Usually followed by cytoplasmic division

20. Prophase II
Microtubules attach to the kinetochores of the duplicated chromosomes

21. Metaphase II
Duplicated chromosomes line up at the spindle equator, midway between the poles

22. Anaphase II
Sister chromatids separate to become independent chromosomes

23. Telophase II
The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells

24. Anaphase II
Telophase II
Prophase II
Metaphase II
Meiosis II

25. Mitosis & Meiosis Compared
Functions
Asexual reproduction
Growth, repair
Occurs in somatic cells
Produces 2 diploid cells
Produces clones
Meiosis
Function
Sexual reproduction
Occurs in germ cells
Produces 4 haploid cells
Produces variable offspring

26. Prophase vs. Prophase I Prophase (Mitosis)
Homologous pairs do not interact with each other
Prophase I (Meiosis)
Homologous pairs become zippered together and crossing over occurs

27. Anaphase, Anaphase I, and Anaphase II
Anaphase I (Meiosis)
Homologous chromosomes separate from each other
Anaphase/Anaphase II (Mitosis/Meiosis)
Sister chromatids of a chromosome separate from each other

28. each chromosome duplicated during interphase
germ cell
germ cell
each chromosome duplicated during interphase n
MEIOSIS I
separation of homologues
MEIOSIS II
separation of sister chromatids
gametes
gametes 2n
diploid number restored at fertilization
zygote

29. Factors Contributing to Variation among Offspring
Crossing over during prophase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization

30. Crossing Over
Crossing over

31. Crossing Over Each chromosome becomes zippered to its homologue
All four chromatids are closely aligned
Nonsister chromosomes exchange segments
Crossing-Over

32. Effect of Crossing Over
After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring

33. CROSSOVER FREQUENCY Proportional to the distance that separates genes
Crossing over will disrupt linkage between A and B more often than C and D A B C D 33

34. Animal Life Cycle
Random alignment

35. Random Alignment
During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes
Initial contacts between microtubules and chromosomes are random
Either the maternal or paternal member of a homologous pair can end up at either pole
The chromosomes in a gamete are a mix of chromosomes from the two parents

36. combinations possible1 2 3
combinations possible
Alignment at
metaphase I or or or

37. Possible Chromosome Combinations
As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2n (n is number of chromosome types)

38. Fertilization Male and female gametes unite and nuclei fuse
Fusion of two haploid nuclei produces diploid nucleus in the zygote
Which two gametes unite is random
Adds to variation among offspring 38

39. Animal Egg Formation
Egg formation 39

40. Oocytes Arrested in Meiosis I
Girl is born with primary oocytes already in ovaries
Each oocyte has entered meiosis I and stopped
Meiosis resumes, one oocyte at a time, with the first menstrual cycle 40

41. Animal Sperm Formation41

42. Spermatogenesis
Spermatogonium (2n) divides by mitosis to form primary spermatocyte (2n)
Meiosis produces haploid spermatids
Spermatids mature to become sperm 42

43. Fertilization
Fertilization 43

44. Fertilization
Sperm penetrates to egg cytoplasm Secondary oocyte undergoes meiosis II; forms mature egg Egg nucleus and sperm nucleus fuse to form diploid zygote

45. Life Cycle of a Leopard Frog
Leopard frog life cycle

46. midsectional views top view side view
Gamete Formation
Fertilization
Cleavage
Gastrulation
midsectional views
top view
side view
Organ Formation
Growth, Tissue
Specialization

47. Experimental Evidence of Localized Differences
Cytoplasmic localization 47

48. Cleavage begins within 24 hours of fertilization blastula forms Cell secretions produce a fluid-filled cavity in center of ball of cells called the blastocoel
blastoceol
blastula

49. Gastrulation - Day 15
Primitive streak forms along one axis of the inner cell mass
Cells migrate inward here to produce a 3-layered embryo
Endoderm
digestive tract
respiratory tract
Mesoderm
muscle, bone, circulatory
Ectoderm
skin, nervous tissue,

50. Stages of Human Development