Meiosis and Sexual Reproduction Chapter 9

Fig. 9-1a, p.138

Fig. 9-10b, p.147

Plant Life Cycle multicelled sporophyte multicelled gametophytes zygote gametes spores meiosisfertilization mitosis Diploid Haploid

Animal Life Cycle multicelled body gametes zygote meiosisfertilization mitosis Diploid Haploid

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

Fig. 9-3, p.140

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

Fig. 9-2, p.140

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

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

one chromatid its sister chromatid centromere one chromosome in the duplicated state p.141a

Stages of Meiosis Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

p.141b

Meiosis I Each homologue in the cell pairs with its partner, then the partners separate

Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other one chromosome (duplicated) two chromosomes (unduplicated)

plasma membrane newly forming microtubules in the cytoplasm spindle equator (midway between the two poles) one pair of homologous chromosomes PROPHASE IMETAPHASE IANAPHASE ITELOPHASE I MEIOSIS I Fig. 9-5a, p.142

PROPHASE IIMETAPHASE IIANAPHASE IITELOPHASE II MEIOSIS II there is no DNA replication between the two divisions Fig. 9-5b, p.142

no interphase between nuclear divisions TELOPHASEPROPHASEANAPHASEMETAPHASETELOPHASE IIPROPHASE IIANAPHASE IIMETAPHASE II typically two nuclei (n) Crossing over occurs between homologues. Homologues separate from their partner. Homologous pairs align randomly. four nuclei (4n) Sister chromatids of chromo-somes seperate. Chromosomes align at spindle equator. Fig. 9-11b, p.148

a Both chromosomes shown here were duplicated during interphase, before meiosis. When prophase I is under way, sister chromatids of each chromosome are positioned so close together that they look like a single thread. Fig. 9-6a, p.144

b Each chromosome becomes zippered to its homologue, so all four chromatids are tightly aligned. If the two sex chromosomes have different forms, such as X paired with Y, they still get zippered together, but only in a tiny region at their ends. Fig. 9-6b, p.144

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

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

c We show the pair of chromosomes as if they already condensed only to give you an idea of what goes on. They really are in a tightly aligned, threadlike form during prophase I. d The intimate contact encourages one crossover (and usually more) to happen at various intervals along the length of nonsister chromatids. e Nonsister chromatids exchange segments at the crossover sites. They continue to condense into thicker, rodlike forms. By the start of metaphase I, they will be unzippered from each other. f Crossing over breaks up old combinations of alleles and puts new ones together in the cell’s pairs of homologous chromosomes. Fig. 9-6c, p.144

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

123 combinations possible or Alignment at metaphase I Fig. 9-7, p.145

Spermatogenesis Growth Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) spermato- gonium (diploid male germ cell)

Oogenesis Growth Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division ovum (haploid) primary oocyte (diploid) oogonium (diploid reproductive cell) secondary oocyte (haploid) first polar body (haploid) three polar bodies (haploid)

Corn (Zea mays) meiosis, Anaphase I. Credit: © Carolina Biological

(Zea mays) meiosis, metaphase I Credit: © Carolina Biological

Corn (Zea mays) meiosis, Telophase I. Credit: © Carolina Biological

Corn (Zea mays) meiosis, Prophase II. Credit: © Carolina Biological

Corn (Zea mays) meiosis, Metaphase II. Credit: © Carolina Biological

Corn (Zea mays) meiosis, Anaphase II. Credit: © Carolina Biological

Corn (Zea mays) meiosis, Telophase II. Credit: © Carolina Biological