Two types of Cell Division Mitosis and Meiosis

What is the purpose of cell division? Growth Repair Replace Reproduce

Two types of Reproduction Asexual by mitosis One parent Offspring identical to parent Advantages? Disadvantages? Examples in: Prokaryotes Protist/Fungi Plants Sponges etc

Sexual Reproduction Process by which two cells (egg and sperm) - from two parents join together to form one cell (zygote) which then develops into the offspring Advantage Variation Disadvantage Look for mates, loss of gametes, (energy investment) Chromosome number incompatibility!

Chromosome numbers: All eukaryotic cells have pairs of chromosomes where one of each pair are form each parent. Haploid (N) = Half the number of chromosomes in an eukaryotic cell. Usually one of each pair Diploid (2N) = total of number chromosomes in an eukaryotic cell.

Homologous Chromosomes ID of Paired chromosomes (homologous) Centromere position Similar size Similar genes or alleles in the same locus Stain the same p.138ap.138a

What needs to happen before fertilization? Meiosis: The egg/sperm cell will have N chromosomes (or one of each pair). Fertilization: When egg and sperm unite – parental chromosome number is restored (N + N  2N) How is meiosis different from mitosis?

multicelled body meiosis diploid fertilization zygote gametes haploid Fig. 9-8b, p.146

Overview of Meiosis Overview of Meiosis

Prophase I Common to mitosis Nuclear membrane disappears Centrioles divide and migrate to poles Spindle fibers form Different Synapsis (homologous chromosomes come together Called Tetrads Crossing over

Crossing Over Segments between homologous chromosome are exchanged (swapped) Results in new combinations of genes

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

Metaphase I Tetrads line up on the equator Anaphase I Homologous chromosomes separate and go to the poles Telophase I Chromosomes may become diffuse and nuclear membrane forms Cytokinesis I 2 cells formed Meiosis I cont….

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

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

Meiosis II Prophase II: Chromosomes condense – have the X shape Metaphase II: Chromosomes line up on the equator Anaphase II: Centromeres split and chromosomes migrate to the poles Telophase II: Two nucleii form (cell plate or cleavage furrow forms) Cytokinesis: Two new cells form – each containing _____ chromosomes

a Growthb Meiosis I and cytoplasmic division c Meiosis II and cytoplasmic division ovum (haploid) primary oocyte (diploid) oogonium (diploid female reproductive cell) secondary oocyte (haploid) first polar body (haploid) three polar bodies (haploid) Fig. 9-10a, p.147

a Growth b Meiosis I and cytoplasmic division c Meiosis II and cytoplasmic division cell differentiation, sperm formation (mature, haploid male gametes) spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) spermato- gonium (diploid male reproductive cell) Fig. 9-9, p.147

How does meiosis produce variation? 1. Crossing over between homologous chromosomes 2. Independent assortment of different chromosomes during Metaphase I 2 23 = 8,388,608 combinations! 3. Random Fertilization of gametes

Chromosomal Abnormalities Nondisjunction of homologous chromosomes Downs, Klinefelters, Turners Syndrome Chromosomal breakage during crossing over that results in segments of chromosomes showing Deletion Inversion Translocation Duplication