PowerLecture: Chapter 10 Meiosis and Sexual Reproduction

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

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

Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells become gametes Gametes meet at fertilization

Asexual Reproduction Single parent produces offspring All offspring are genetically identical to one another and to parent

Sexual Reproduction Involves Meiosis Gamete production Fertilization Produces genetic variation among offspring

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

Homologous Chromosomes Fig. 10-2, p.156

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

Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells ovaries anther testes ovary Figure 10-3 Page 156

anther (where cells that give rise to male gametes originate) FLOWERING PLANT anther (where cells that give rise to male gametes originate) ovules, inside an ovary (where cells that give rise to female gametes originate) Fig. 10-3a, p.156

Chromosome Number Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

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

PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I MEIOSIS I newly forming microtubules in the cytoplasm spindle equator (midway between the two poles) one pair of homologous chromosomes plasma membrane PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I Fig. 10-5, p.158

Prophase I Metaphase I Anaphase I Telophase I newly forming microtubules Prophase I Metaphase I spindle equator one pair of homologous chromosomes Anaphase I Telophase I Meiosis I Stepped Art Fig. 10-5a, p.158

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II there is no DNA replication between the two divisions PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II MEIOSIS II Fig. 10-5b, p.159

Anaphase II Telophase II Prophase II Metaphase II Meiosis II Stepped Art Fig. 10-5b, p.159

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

Possible Chromosome Combinations As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2n (and this is without crossing over)

Possible Chromosome Combinations 2n for humans would be … 223 or… 8,388,608

combinations possible Possible Chromosome Combinations 1 2 3 combinations possible or or or Fig. 10-7, p.161

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

Oogenesis three polar bodies (haploid) first polar body (haploid) oogonium (diploid) primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division Growth Figure 10-10 Page 163

Spermatogenesis Figure 10-9 Page 163 spermato- gonium (diploid ) primary spermatocyte (diploid) secondary spermatocytes (haploid) sperm (mature, haploid male gametes) spermatids (haploid) Spermatogenesis Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division Growth cell differentiation, sperm formation Figure 10-9 Page 163

Fig. 10-10, p.163

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 adding to variation

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 Fig. 10-12, p.166

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

An Ancestral Connection Was sexual reproduction a giant evolutionary step from aseuxal reproduction? Giardia intestinalis Chlamydomonas