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Meiosis and Sexual Reproduction

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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

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