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122 Lec 04 Mult All Mitosis Meiosis

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122 Lec 04 Mult All Mitosis Meiosis Objectives Announcements • Understand inheritance of blood type in humans. • Problem sets due this week at the • Know how beginning of lab. – organisms vary in chromosome number among life stages and among species. • Show your work! – to tell chromosomes apart • Write out solutions, helps to think things – to figure out which chromosomes are homologues. through. • Understand the role of mitosis and meiosis in asexual and sexual reproduction. • Review study CD that came with text for lab • Distinguish between stages of cell division. this week (especially mitosis and meiosis). • Recognize how chromosome movement during meiosis results in Mendel's laws of Segregation and Independent Assortment. Human blood type example Blood type phenotypes • Four phenotypes: A, B, AB, O • One locus determines blood type • A, B or no substance coats blood cells • Three alleles are common • Blood type determined by whether • Two alleles are dominant to the third antibodies react to substance and codominant with each other Blood type genotypes Eukaryote chromosome structure • Three alleles present- Ia, Ib, i • Chromosomes contain DNA & proteins – Ia and Ib are codominant • made of – i is recessive – centromere- region to which spindle fibers • Possible genotypes attach during mitosis and meiosis – arms – IaIa homozygote, Iai heterozygote, • region that extends from centromere b b b – I I homozygote, I i heterozygote, • contain genes – ii homozygote, IaIb heterozygote 1 Figure 12.3 Chromosome duplication and distribution during mitosis Figure 13.x4 Human male chromosomes Figure 13.x5 Chromosomes differ in length and position of centromere Life cycle stages differ in ploidy • Ploidy = number of copies of each homologue • Common ploidy levels – Haploid- one copy (1N) – Diploid- two copies (2N) – Polyploid- multiple (>2) copies (3N, 4N, etc) Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization (syngamy) How do organisms differ in chromosome number? • Ploidy- how does organism spend most of life? – Haploid (protists, algae & fungi, moss) – Diploid (ferns, flowering plants, insects, vertebrates) – Polyploid (many plants, few animals) • ‘haploid’ chromosome number – Humans: 23 – Fruit flies: 4 – Ferns: thousands 2 Figure 13.1 The asexual reproduction of a hydra Reproduction in eukaryotes • asexual – budding or vegetative reproduction – offspring genetically identical to parent • sexual – fusion of two haploid gametes – different combinations of genes than in parents Figure 33.7 The life cycle of the hydrozoan Obelia (Layer 3) Cell cycles in eukaryotes • Interphase – 90% of cell life – cell growth occurs – chromosomes are copied but not visible • Mitotic phase – nucleus 'dissolves' – chromosomes condense – cell divides – two daughter cells genetically identical to parent Figure 12.4 The cell cycle Figure 12-09x Mitosis in an onion root 3 Mitosis Prophase • Occurs throughout life of a multicellular • Nuclear membrane disappears organism • Mitotic spindle forms – development – Microtubule organizing centers – growth (centrosomes) form and migrate to opposite ends of cell – maintenance – Microtubules attach to centromere at • Involved with asexual reproduction kinetochore • We define stages (prophase, metaphase, • Duplicated chromosomes (sister anaphase, telophase) chromatids) move toward center of cell Figure 12.5 The stages of mitotic cell division: G2 phase; prophase Figure 12.5 The stages of mitotic cell division : prophase; prometaphase Figure 12.6 The mitotic spindle at metaphase Metaphase • Chromosomes align along metaphase plate • Centromeres aligned with each other • Kinetochores face centrosomes away from center of the cell 4 Figure 12.5 stages of mitotic cell division: metaphase; anaphase Anaphase Sister chromatids are pulled toward opposite ends of cell Stages of mitotic cell division : anaphase; telophase and cytokinesis Telophase • Nuclear membrane in each daughter cell starts to form • Chromosomes elongate • Cell division occurs Significance of Meiosis • Gamete cells formed with one copy of Prophase I (longest phase) each chromosome • pairing of homologous chromosomes • Meiotic events cause Mendel's laws • paired homologues consist of 4 – Segregation- Homologues separate chromatids (tetrads) – Independent assortment- independent orientation of chromosomes • X-shaped configurations form (chiasmata) • Recombination occurs (independent • Crossing over occurs orientation, crossing over) • chiasmata move to end of chromosome arms 5 What happens during crossing over? Metaphase & Anaphase I • exchange of • tetrads moved to metaphase plate segments at identical • homologues pulled apart into different positions along daughter cells homologues • no loss or • sister chromatids remain attached at addition of centromeres genetic material • chromosome number is now reduced Figure 13.7 The stages of meiotic cell division: Meiosis I Telophase I and Prophase II • Chromosomes may uncoil as cell division occurs • No chromosome duplication occurs Figure 13.7 The stages of meiotic cell division: Meiosis II Metaphase II, Anaphase II, Telophase II • mitotic division repeated, but now sister chromatids separate • genetic material reduced, but not chromosome number 6.
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