Meiosis and Sexual Reproduction Chapter 10 Meiosis and Sexual Reproduction

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

Why sex? Asexual Sexual

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

Life Cycles Plant Animal

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

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

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

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

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

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

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

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

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

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

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

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

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

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