Meiosis and Sexual Reproduction Chapter 10, 36.4

Why Sex In nature, the main function of sex is to perpetuate one’s genes Perpetuation of life includes reproduction and development Asexual reproduction easier and faster all offspring are genetically identical to one another and to parent

Why Sex Sexual reproduction alternative adaption in changing environments chromosomes are duplicated in germ cells Involves Meiosis Gamete production Fertilization Produces genetic variation among offspring

Cost of Sexual Reproduction Specialized cells and structures must be formed Special courtship, and parental behaviors can be costly Nurturing developing offspring, either in egg or body, requires resources from mother

Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome one chromosome from mother, the other from father Paternal and maternal chromosomes carry different alleles

Homologous Chromosomes

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

Gametes are sex cells (sperm, eggs) Arise from germ cells Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells ovaries anther testes ovary

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

Human Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY)

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

Prophase I Each duplicated chromosome pairs with homologue Homologues swap segments Each chromosome becomes attached to spindle

Crossing-Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned tetrad 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

CROSSOVER FREQUENCY Proportional to the distance that separates genes Crossing over will disrupt linkage between A and B more often than C and D A B C D 15

Metaphase I Random Alignment of homologous chromosomes Chromosomes are pushed and pulled into the middle of cell The spindle is fully formed

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

combinations possible 1 2 3 combinations possible Alignment at metaphase I or or or

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)

Anaphase I Homologous chromosomes segregate The sister chromatids remain attached

Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division

Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes

Metaphase II Duplicated chromosomes line up at the spindle equator, midway between the poles

Anaphase II Sister chromatids separate to become independent chromosomes

Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells

Anaphase II Telophase II Prophase II Metaphase II Meiosis II

Mitosis & Meiosis Compared Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces 2 diploid cells Produces clones Meiosis Function Sexual reproduction Occurs in germ cells Produces 4 haploid cells Produces variable offspring

Prophase vs. Prophase I Prophase (Mitosis) 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

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

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 Adds to variation among offspring 31

Animal Egg Formation 32

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 33

Animal Sperm Formation 34

Spermatogenesis Spermatogonium (2n) divides by mitosis to form primary spermatocyte (2n) Meiosis produces haploid spermatids Spermatids mature to become sperm 35

Fertilization Fertilization 36

Fertilization Sperm penetrates to egg cytoplasm Secondary oocyte undergoes meiosis II; forms mature egg Egg nucleus and sperm nucleus fuse to form diploid zygote

Life Cycle of a Leopard Frog

midsectional views top view side view Gamete Formation Fertilization Cleavage Gastrulation midsectional views top view side view Organ Formation Growth, Tissue Specialization

Cytoplasmic localization 40

Cleavage begins within 24 hours of fertilization blastula forms Cell secretions produce a fluid-filled cavity in center of ball of cells called the blastocoel blastoceol blastula

Gastrulation - Day 15 Primitive streak forms along one axis of the inner cell mass Cells migrate inward here to produce a 3-layered embryo Cell Differentiation Certain groups of genes are activated in some cells but not in other Genes are not lost, just inactivated

ENDODERM MESODERM ECTODERM epithelial lining digestive respiratory tract excretory reproductive muscle system bone (skeletal) system circulatory (lymphatic) system excretory system reproductive system skin nervous tissue jaw, teeth germ cells

Stages of Human Development

Impacts, Issues Video Why Sex