1. Meiosis and Sexual Reproduction
Chapter 10, 36.4

2. Why Sex
In nature, the main function of sex is to perpetuate one’s genes
Perpetuation of life includes reproduction and development
Asexual reproduction
easier and faster
all offspring are genetically identical to one another and to parent

3. Why Sex Sexual reproduction
alternative adaption in changing environments
chromosomes are duplicated in germ cells
Involves
Meiosis
Gamete production
Fertilization
Produces genetic variation among offspring

4. 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

5. Homologous Chromosomes Carry Different Alleles
Cell has two of each chromosome one chromosome from mother, the other from father Paternal and maternal chromosomes carry different alleles

6. Homologous Chromosomes

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

8. 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

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

10. 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)

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

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

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

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

15. 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 15

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

17. 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

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

19. 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)

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

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

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

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

24. Anaphase II
Sister chromatids separate to become independent chromosomes

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

26. Anaphase II
Telophase II
Prophase II
Metaphase II
Meiosis II

27. 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

28. 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

29. 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

30. 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

31. 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 31

32. Animal Egg Formation 32

33. 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 33

34. Animal Sperm Formation34

35. Spermatogenesis
Spermatogonium (2n) divides by mitosis to form primary spermatocyte (2n)
Meiosis produces haploid spermatids
Spermatids mature to become sperm 35

36. Fertilization
Fertilization 36

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

38. Life Cycle of a Leopard Frog

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

40. Cytoplasmic localization40

41. 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

42. Gastrulation - Day 15
Primitive streak forms along one axis of the inner cell mass
Cells migrate inward here to produce a 3-layered embryo
Cell Differentiation
Certain groups of genes are activated in some cells but not in other
Genes are not lost, just inactivated

43. ENDODERM
MESODERM
ECTODERM
epithelial lining
digestive
respiratory tract
excretory
reproductive
muscle system
bone (skeletal) system
circulatory (lymphatic)
system
excretory system
reproductive system
skin
nervous tissue
jaw, teeth
germ cells

44. Stages of Human Development

45. Impacts, Issues Video
Why Sex