PowerLecture: Chapter 10 Meiosis and Sexual Reproduction

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

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

Asexual Reproduction Single parent All offspring genetically identical

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

Homologous Chromosomes Carry Different Alleles Chromosomes in homologous pairs One maternal, one paternal May carry different alleles

Homologous Chromosomes Fig. 10-2, p.156

Sexual Reproduction Shuffles Alleles Offspring inherit new combinations of alleles, produces variations in traits This variation is the basis for evolutionary change

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 - diploid (2n) Gametes - haploid (n) Meiosis halves chromosome number

Meiosis: Two Divisions Two consecutive nuclear divisions Meiosis I Meiosis II DNA not replicated between divisions 4 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 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

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

Effect of Crossing Over Crossing over mixes 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

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 Haploid Male and female gametes unite - nuclei fuse Produces diploid nucleus in zygote Which 2 gametes unite 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 Random alignment of chromosomes at meiosis I Random combination of gametes at fertilization

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

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

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

Crossing Over… a preview taken from Chapter 11

Linkage Groups Genes on one type of chromosome travel together Degree of linkage may vary

Full Linkage A B a b x Parents: A B a b AB ab F1 offspring: All AaBb meiosis, gamete formation Equal ratios of two types of gametes: A B a b Figure 11.15 Page 178 50% AB 50% ab

Incomplete Linkage AC ac A C a c x Parents: A C a c F1 offspring: All AaCc meiosis, gamete formation A a A a Unequal ratios of four types of gametes: C c c C parental genotypes recombinant genotypes Figure 11.15 Page 178

Crossover Frequency Proportional to the distance that separates genes B C D Crossing over will disrupt linkage between A and B more often than C and D In-text figure Page 178

Linkage Mapping in Humans Linkage maps based on pedigree analysis through generations Color blindness and hemophilia are very closely linked on X chromosome