Mitosis and Meiosis TWO TYPES OF CELL DIVISION

 Growth  Repair  Replace  Reproduce WHAT IS THE PURPOSE OF CELL DIVISION?

 Asexual by mitosis  One parent  Offspring identical to parent  Advantages?  Disadvantages? Examples in: Prokaryotes Protist/Fungi Plants Sponges etc TWO TYPES OF 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! SEXUAL REPRODUCTION

HOW ARE CHROMOSOME NUMBERS MAINTAINED FROM ONE GENERATION TO THE NEXT?

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. CHROMOSOME NUMBERS:

ID of Paired chromosomes (homologous)  One from each parent  Same Centromere position  Similar size  Similar genes or alleles in the same locus  Stain the same HOMOLOGOUS CHROMOSOMES p.138ap.138a

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? WHAT NEEDS TO HAPPEN BEFORE FERTILIZATION?

CYCLE OF MEIOSIS AND FERTILIZATION sporophyte meiosis diploid fertilization zygote gametes gametophytes spores haploid Fig. 9-8a, p.146

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

OVERVIEW OF MEIOSIS

 Common to mitosis  Nuclear membrane disappears  Centrioles divide and migrate to poles  Spindle fibers form  Different  Synapsis (homologous chrom. come together  Called Tetrads  Crossing over PROPHASE I

 Segments between homologous chromosome are exchanged (swapped)  Results in new combinations of genes CROSSING OVER

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 MEIOSIS II

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

1.Crossing over between homologous chromosomes HOW DOES MEIOSIS PRODUCE VARIATION?

2. INDEPENDENT ASSORTMENT OF DIFFERENT CHROMOSOMES DURING METAPHASE I

 Nondisjunction of homologous chromosomes  Downs, Trisomy 21  Klinefelters, XXY  Turners Syndrome XO Turner’s Syndrome – Linda Hunt CHROMOSOMAL ABNORMALITIES

Klinefelter’s KaryotypeDown’s Syndrome CHROMOSOMAL ABNORMALITIES

CHROMOSOMAL BREAKAGE DURING CROSSING OVER THAT RESULTS IN SEGMENTS OF CHROMOSOMES SHOWING ….