1. Meiosis and Sexual Reproduction
Chapter 10
Meiosis and Sexual Reproduction

2. Why Sex
Fig. 10-1b, p.154

3. Why sex?
Asexual
Sexual

4. Why sex? Asexual Easier, faster Sexual Changing env Big population
Indentical
Bits can make whole indv.
No new combos
All inherit the same info
Clones
parthogenesis
Sexual
Changing env
More variety
New combos
Involves meiosis (gametes) and fertilization
allele

5. Modes of Reproduction Sexual reproduction
Meiosis, gamete formation, and fertilization
Offspring show genetic variation
Asexual reproduction
Single parent produces offspring
Offspring are genetically identical

6. Cost of Sexual Reproduction
Fig. 43-2c, p.756

7. 43.1 (p. 756) Cost of Sexual Reproduction
Specialized cells and structures must be formed
Special courtship, and parental behaviors can be costly
Timing of gamete formation and mating
Nurturing developing offspring, either in egg or body, requires resources from mother

8. 10.2 What Meiosis Does
Meiosis – nuclear division that divides parental c-some # by half in specialized reproductive cells
Ex: anther, ovules
anther
ovary

9. Homologous Chromosomes Carry Different Alleles
Homologous c-some – same shape, length and assortment of genes, line up with each other
Paternal and maternal chromosomes can carry different alleles

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

11. Meiosis: Two Divisions
Two consecutive nuclear divisions
Meiosis I – aligns with partner
Meiosis II – sister chromatids separate
DNA is not duplicated between divisions
Four haploid nuclei form

12. 10.3 Tour of Meiosis Prophase I
Each duplicated chromosome pairs with homologue (synapse)
Homologues swap segments (crossing over)
Each chromosome becomes attached to spindle
Fig. 10-5, p. 158

13. Metaphase I Chromosomes are pushed and pulled into the middle of cell
The spindle is fully formed
Fig. 10-5, p. 158

14. Anaphase I Homologous chromosomes segregate
The sister chromatids remain attached
Fig. 10-5, p. 158

15. Telophase I The chromosomes arrive at opposite poles
Usually followed by cytoplasmic division
Fig. 10-5, p. 158

16. Meosis II: Prophase II
Microtubules attach to the kinetochores of the duplicated chromosomes
Attach to one chromatid of each chromosome
Fig. 10-5, p. 158

17. Metaphase II
Duplicated chromosomes line up at the spindle equator, midway between the poles
Fig. 10-5, p. 158

18. Anaphase II
Sister chromatids separate to become independent chromosomes
Attachments break
Fig. 10-5, p. 158

19. Telophase II The chromosomes arrive at opposite ends of the cell
A nuclear envelope forms around each set of chromosomes
Four haploid cells
Fig. 10-5, p. 158

20. 10.4 Factors Contributing to Variation among Offspring
Crossing over during prophase I
Independent assortment
Random alignment of chromosomes at metaphase I
Random combination of gametes at fertilization

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

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

23. Independent Assortment
Microtubules from spindle poles attach to kinetochores of chromosomes randomly, between Prophase I and Metaphase I

24. Randomness cont.
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

25. 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)

26. Fertilization Which two gametes unite is random
Adds to variation among offspring

27. Life Cycles
Plant
Animal

28. Plant Life Cycle sporophyte zygote diploid fertilization meiosis
haploid
gametes
spores
gametophytes
Fig. 10-8a, p.162

29. Animal Life Cycle multicelled body zygote diploid fertilization
meiosis
haploid
gametes
Fig. 10-8b, p.162

30. 44.2 Spermatogenesis
Spermatogonium (2n) divides by mitosis to form primary spermatocyte (2n)
Meiosis produces haploid spermatids
Spermatids mature to become sperm
movie
Figure 44.4
Page 775

31. Other Testicular Cells
Sertoli cells
Line the seminiferous tubules
Nourish the developing sperm
Leydig cells
Lie between the seminiferous tubules
Secrete testosterone

32. Male Hormonal Control Hypothalamus GnRH Inhibin Anterior Pituitary FSH
Leydig Cells
Sertoli Cells
Testes
Testosterone
Formation and Development of Sperm

33. 44.1 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

34. Ovarian Cycle Follicle grows and matures Ovulation occurs
secondary oocyte
first polar body
Follicle grows and matures
Ovulation occurs
Corpus luteum forms
antrum
corpus luteum
primordial follicle
Figure 44.8
Page 778

35. Primary oocyte, not yet released from meiosis I
Primary oocyte, not yet released from meiosis I. A cell layer is forming around it. A follicle consists of the cell layer and the oocyte.
A transparent and somewhat elastic layer, the zona pellucida, starts forming around the primary oocyte.
A fluid-filled cavity (antrum) starts forming in the follicle’s cell layer.
Mature follicle. Meiosis I is over. The secondary oocyte and first polar body are now formed.
primordial follicle
first polar body
secondary oocyte
The corpus luteum breaks down when the woman doesn’t get pregnant.
A corpus luteum forms from remnants of the ruptured follicle.
Ovulation. Mature follicle ruptures and releases the secondary oocyte and the first polar body.
Fig. 44-8b, p.778

36. Female Hormonal Control
Hypothalamus
GnRH
Rising estrogen stimulates surge in LH
Anterior pituitary
Progesterone,
estrogens LH
FSH
Ovary
follicle growth,
oocyte maturation
Estrogen
Corpus luteum
forms

37. Mitosis & Meiosis Compared
Functions
Asexual reproduction
Growth, repair
Occurs in somatic cells
Produces clones
Meiosis
Function
Sexual reproduction
Occurs in germ cells
Produces variable offspring

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

39. 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

40. Results of Mitosis and Meiosis
Two diploid cells produced
Each identical to parent
Meiosis
Four haploid cells produced
Differ from parent and one another

41. An Ancestral Connection
Was sexual reproduction a giant evolutionary step from aseuxal reproduction?
Giardia intestinalis
Chlamydomonas
Recombination mechanisms are vital for reproduction of euk cells may have evolved from DNA repair mechanisms in prok ancestors