Meiosis Chapter 10 Sexual Reproduction l Chromosomes are

Meiosis Chapter 10

Sexual Reproduction l Chromosomes are duplicated in germ cells l Germ cells undergo meiosis and cytoplasmic division l Cellular descendents of germ cells become gametes l Gametes meet at fertilization

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

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

Homologous Chromosomes Carry Different Alleles l Cell has two of each chromosome l One chromosome in each pair from mother, other from father l Paternal and maternal chromosomes carry different alleles

Sexual Reproduction Shuffles Alleles l Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits l This variation in traits is the basis for evolutionary change

Gamete Formation l Gametes are sex cells (sperm, eggs) l Arise from germ cells ovaries testes anther ovary Figure 10. 2 a Page 164

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

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

Meiosis I Each homologue in the cell pairs with its partner, then the partners separate In-text figure Page 165

Meiosis II l The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated) In-text figure Page 165

Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I Figure 10. 4 Page 167

Prophase I l Each duplicated chromosome pairs with homologue l Homologues swap segments l Each chromosome becomes attached to spindle Figure 10. 4 Page 167

Metaphase I l Chromosomes are pushed and pulled into the middle of cell l The spindle is fully formed Figure 10. 4 Page 167

Anaphase I l Homologous chromosomes segregate l The sister chromatids remain attached Figure 10. 4 Page 167

Telophase I l The chromosomes arrive at opposite poles l Usually followed by cytoplasmic division Figure 10. 4 Page 167

Prophase II l Microtubules attach to the kinetochores of the duplicated chromosomes Figure 10. 4 Page 167

Metaphase II l Duplicated chromosomes line up at the spindle equator, midway between the poles Figure 10. 4 Page 167

Anaphase II l Sister chromatids separate to become independent chromosomes Figure 10. 4 Page 167

Telophase II l The chromosomes arrive at opposite ends of the cell l A nuclear envelope forms around each set of chromosomes l Four haploid cells Figure 10. 4 Page 167

Crossing Over • Each chromosome becomes zippered to its homologue • All four chromatids are closely aligned • Nonsister chromosomes exchange segments Figure 10. 5 Page 168

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

Random Alignment l During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes l Initial contacts between microtubules and chromosomes are random

Random Alignment l Either the maternal or paternal member of a homologous pair can end up at either pole l 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: 2 n (n is number of chromosome types)

1 Possible Chromosome Combinations or or or Figure 10. 6 Page 169 2 3

Plant Life Cycle mitosis multicelled sporophyte zygote fertilization Diploid meiosis Haploid spores gametes multicelled gametophytes mitosis Figure 10. 7 Page 170

Animal Life Cycle mitosis multicelled body zygote fertilization Diploid Haploid gametes meiosis Figure 10. 7 Page 170

Oogenesis oogonium (diploid) first polar body (haploid) primary oocyte (diploid) secondary oocyte (haploid) Growth three polar bodies (haploid) Meiosis I, Cytoplasmic Division ovum (haploid) Meiosis II, Cytoplasmic Division Figure 10. 8 Page 171

spermatogonium (diploid ) primary spermatocyte (diploid) secondary spermatocytes (haploid) spermatids (haploid) sperm (mature, haploid male gametes) Spermatogenesis Growth Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division cell differentiation, sperm formation Figure 10. 9 Page 171

Fertilization l Male and female gametes unite and nuclei fuse l Fusion of two haploid nuclei produces diploid nucleus in the zygote l Which ¡ Adds two gametes unite is random to variation among offspring

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

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

Prophase vs. Prophase I l Prophase (Mitosis) ¡ Homologous pairs do not interact with each other l Prophase I (Meiosis) ¡ Homologous pairs become zippered together and crossing over occurs

Anaphase, Anaphase I, and Anaphase II l Anaphase I (Meiosis) ¡ Homologous chromosomes separate from each other l Anaphase/Anaphase ¡ Sister II (Mitosis/Meiosis) chromatids of a chromosome separate from each other

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